IET Electrical Measurement Series 8
A Handbook for
EMC
Testing and Measurement
David Morgan
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Contents Foreword 1 Nature and origins of electrom.agnetic com.patibility
1.1 1.2
1.3
1.4
1.5 1.6
2
2.1
2.2
2.3
2.4
2.5
2.6
2.7
2.8
Definitions of electromagnetic compatibility Visualising the EMI problem 1.2.1 Sources of EMI 1.2.2 EMI coupling to victim equipments 1.2.3 Intersystem and intrasystem EMI Historical background 1.3.1 Early EMC problems 1.3.2 Early EMC problems with military equipment 1.3.3 The cost of EMC 1.3.4 Serious EMI problems Technical disciplines and knowledge areas within EMC 1.4.1 Electrical engineering 1.4.2 Physics 1.4.3 Mathematical modelling 1.4.4 Limited chemical knowledge 1.4.5 Systems engineering 1.4.6 Legal aspects of EMC 1.4.7 Test laboratories 1.4.8 Quality assurance: total quality management 1.4.9 Practical skills Philosophy of EMC References
EMC standards and specifications
The need for standards and specifications 2.1.1 Background 2.1.2 Contents of standards 2.1.3 The need to meet EMC standards Civil and military standards 2.2.1 Range of EMC standards in use 2.2.2 Derivation of military standards 2.2.3 Derivation of commercial standards 2.2.4 Generation of CENELEC EMC standards UK/European commercial standards 2.3.1 UK standards relating to commercial equipment 2.3.2 Comparing tests 2.3.3 European commercial standards 2.3.4 German standards US commercial standards 2.4.1 US organisations involved with EMC 2.4.2 FCC requirements 2.4.3 Other US commercial standards Commercial EMC standards inJapan and Canada 2.5.1 Japanese EMC standards 2.5.2 Canadian EMC standards Product safety 2.6.1 Safety of electrical devices 2.6.2 Product safety 2.6.3 Radiation hazards to humans 2.6.4 Hazards of electromagnetic radiation to ordnance ESD and transients 2.7.1 ESD (electrostatic discharge) 2.7.2 Transients and power line disturbances US military EMC standards 2.8.1 MIL STD 461/462/463 2.8.2 MIL-E-6051 D 2.8.3 Other US military standards v
XUl
1 1 1 1 5 7 7 7 8 9 10 10 10 11 11 11 11 11 11 12 12 12 12
14 14 14 14 14 15 15 15 17 18 18 18 19 20 21 23 23 23 24 24 24 25 25 25 26 26 27 27 27
28 28 28 31 31
VI
2.9
2.10 2.11
A HANDBOOK FOR EMC TESTING AND MEASUREMENT UK military standards 2.9.1 Service and establishment-specific standards 2.9.2 Project-specific standards 2.9.3 DEF STAN 59-41 (1.988) Following chapters References
31 31 33 34 36 36
3 Outline of EMC testing 3.1 Types of EM C testing 3.1.1 Development testing 3.1.2 Measurement to verify modelling results 3.1.3 Preconformance test measurements 3.1.4 Conformance testing 3.1.5 Conforrnance test plan 3.2 Repeatability in EMC testing 3.2.1 Need for repeatability and accuracy 3.2.2 Accuracy of EMC measurements 3.2.3 Implications of repeatability of EMC measurements 3.3 Introduction to EMC test sensors, couplers and antennas 3.3.1 EMC sensor groups 3.3.2 Conduction and induction couplers 3.3.3 Radiative coupling EMC antennas 3.4 References
38 38 38 38 39· 39 40 41 41 42 44 44 44 44 45 46
4
Measurem.ent devices for conducted EMI Introduction Measurement by direct connection 4.2.1 Line impedance stabilisation network 4.2.2 10 flF feed through capacitor 4.2.3 RF coupling capacitors 4.2.4 Distributed capacitance couplers 4.2.5 High-impedance RF voltage probes 4.2.6 Directly connected transformers 4.3 Inductively coupled devices 4.3.1 Cable current probes 4.3.2 Current injection probes 4.3.3 Close magnetic field probes 4.3.4 Surface current probes 4.3.5 Cable RF current clamps 4.3.6 Magnetic induction tests 4.4 References
48 48 48 49 51 52 55 56 60
Introduction to antennas EMC antennas EMC antenna basics 5.2.1 Arbitrary antennas 5.2.2 EMC antennas 5.3 Basic antenna parameters 5.3.1 Gain 5.3.2 Aperture 5.3.3 Transmitting antenna factor 5.3.4 Receiving antenna factor 5.3.5 Antenna phase centre 5.3.6 Mutual antenna coupling 5.3.7 Wavefield impedance 5.3.8 Near-field/far-field boundary 5.3.9 Beamwidth 5.3.10 Spot size 5.3.11 Effective length 5.3.12 Polarisation 5.3.13 Bandwidth 5.3.14 Input impedance 5.4 References
72 72 72 72
4.1 4.2
5
5.1 5.2
61 61
65 66 66 68 70 70
73 73 73 74 74 74
75 75 76 76 79 81 82 82 83 84 84
CONTENTS 6
Antennas for radiated emission testing
6.1
6.2
6.3
6.4
6.5
6.6
6.7 6.8 6.9 6.10 6.11 6.12
6.13
Passive monopoles 6.1.1 Construction 6.1.2 Performance Active monopoles 6.2.1 Advantages 6.2.2 Disadvantages Tuned dipoles 6.3.1 Introduction 6.3.2 Practical tuned dipoles 6.3.3 Commercial EMC tuned dipoles 6.3.4 Radiated emission testing Electrically short dipoles 6.4.1 Special short calibration dipoles 6.4.2 Roberts dipoles 6.4.3 Small nonresonant dipoles 6.4.4 Microscopic dipole probes Biconic dipoles 6.5.1 Introduction 6.5.2 Commercial biconic antennas 6.5.3 Use of biconic antennas Wideband antennas 6.6.1 Introduction 6.6.2 Log-periodic antenna Log-periodic dipole antenna Conical log-spiral antenna Horn antennas Ridged guide horn antennas Reflector antennas Magnetic field antennas 6.12.1 Introduction' 6.12.2 Passive loops 6.12.3 Active loops 6.12.4 Loop calibration 6.12.5 Magnetic field susceptibility tests References
7 Use of antennas for radiated susceptibility testing 7.1 Introduction 7.1.1 Types of antennas used in susceptibility testing 7.1.2 Standards requiring immunity tests Free-field antennas 7.2 Tuned halfwave dipoles 7.3 Biconic dipoles 7.4 Log-periodic dipoles 7.5 Conical log-spiral antennas 7.6 Horn antennas 7.7 Parabolic reflector antennas Radiated immunity field strength requirements 7.8 7.8.1 Req uiremen ts for commercial products 7.8.2 Requirements for civil aircraft 7.8.3 Military requirements 7.9 E-field generators 7.9.1 Construction 7.9.2 Practical devices 7.10 Long wire lines 7.10.1 Advantages 7.10.2 Use in testing military equipment Bounded-wave devices 7.11 Parallel-plate line 7.11.1 Properties 7.11.2 Line impedance 7.11.3 Construction
VB
86 86 86 87 88 88 88 89 89 90 91 91 92 92 92 93 93 94 94 94 95 96 96 96 96 98 100 102 103 105 105 105 106 106 107 108 110 110 110 110 III III
112 113 113 114 114 114 114 115 115 115 116 118 118 118 119 119 119 119
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A HANDBOOK FOR EMC TESTING AND MEASUREMENT 7.11.4 Complex lines 7.11.5 Field uniformity and VSWR 7.11.6 Use in screened room
7.12
7.13
7.14·
8
TEM cells 7.12.1 Basic construction 7.12. 2 Crawford cell performance 7.12.3 Wave impedance in TEM cell 7.12.4 Field distortions in TEM cell 7.12.5 Other uses of TEM cells 7.12.6 Asymmetric TEM cells GTEM cells 7.13.1 Description 7.13.2 Typical construction 7.13.3 Power req uiremen ts 7.13.4 GTEM cells used for emission testing 7.13.5 Pulse testing References
Receivers, analysers and measurement equipment Introduction 8.1.1 Outline of equipment 8.1.2 Groups of equipment Instrumentation for emission testing 8.2 EMI receivers 8:2.1 Design requirements 8.2.2 Selectivity and sensitivity 8.2.3 Detectors 8.2.4 Commercially available EMI receivers 8.3 Spectrum analysers 8.3.1 Introduction 8.3.2 Analyser types 8.3.3 Analyser operation 8.4 Preselectors and filters 8.4.1 Preselectors 8.4.2 Bandlimiting filters 8.5 Impulse generators 8.5.1 Description 8.5.2 Design 8.5.3 Use of impulse generators 8.6 Digital storage oscilloscopes 8.6.1 Advan tages of digi tal oscilloscopes 8.6.2 Typical waveforms to be measured 8.6.3 Recording injected pulses for immunity testing 8.6.4 Digital transient recorder architecture 8.7 AF IRF voltmeters 8.8 RF power meters 8.9 Frequency meters Instrumentation for susceptibility testing 8.10 Signal sources 8.10.1 Signal synthesisers 8.10.2 Signal sweepers 8.10.3 Tracking generators 8.11 RF power amplifiers 8.11.1 Introduction 8.11.2 Specifying an amplifier 8.11.3 RF amplifiers - conclusions 8.12 Signal modulators 8.12.1 Modulation requirements 8.12.2 Built-in modulators 8.12.3 Arbitrary waveform generators 8.13 Directional couplers, circulators and isolators 8.13.1 Amplifier protection devices 8.1
121 121 122 123 123 123 124 124 125 126 126 126 126 127 127 128 128
130 130 130 130 130 130 130 132 133 134 134 134 134 135 136 136 136 137 137 137 138 139 139 139 140 140 141 141 142 142 142 142 143 143 144 144 145 147 147 147 147 148 148 148
CONTENTS
8.14
8. 15
9
8.13.2 Directional couplers 8.13.3 Hybrid rings, circulators and isolators conclusion 8.13.4 Protection devices Automatic EMC testing 8.14.1 Introduction 8.14.2 Automated emission testing 8.14.3 Automated susceptibility testing 8.14.4 In the future? References
EMC test regitnes and facilities
9.1
9.2
9.3
9.4
9.5
10
10.1
10.2
10.3
10.4
10.5
Introduction 9.1.1 Main test regimes 9.1.2 Special testing EMC testing in screened chambers 9.2.1 Enclosed test chambers 9.2.2 Standard shielded enclosures 9.2.3 RF anechoic screened chambers 9.2.4 Mode-stirred chambers 9.2.5 Novel facilities Open-range testing 9.3.1 Introduction 9.3.2 Test site 9.3.3 Testing procedures 9.3.4 Site calibration 9.3.5 Measurement repeatability 9.3.6 Comments on open-site testing Low-level swept coupling and bulk current injection testing 9.4.1 Introduction 9.4.2 Low-level swept coupling 9.4.3 Bulk current injection References
Electrotnagnetic transient testing
Introduction 10.1.1 Transient types 10.1.2 Continuous and transient signals Fourier transforms 10.2.1 Introduction 10.2.2 The transform 10.2.3 Introducing phase 10.2.4 Fourier transform expressions 10.2.5 Impulse response 10.2.6 Convolution 10.2.7 Advantages of time-domain manipulation ESD-electrostatic discharge 10.3.1 Introduction 10.3.2 The ESD event 10.3.3 Types ofESD 10.3.4 ESD-induced latent defects 10.3.5 Types of ESD test 10.3.6 Number of discharges per test 10.3.7 ESD test voltage levels 10.3.8 Assessing EDT performance Nuclear electromagnetic pulse 10.4.1 Introduction 10.4.2 Types ofNEMP 10.4.3 Exoatmospheric pulse generation 10.4.4 NEMP induced currents 10.4.5 NEMP testing Lightning impulses
IX
148 150 151 151 151 152 152 152 152
154 154 154 154 154 154 155 159 163 164 165 165 165 165 167 168 171 171 171 172 175 176
179 179 179 179 180 180 180 181 182 182 184 184 185 185 185 187 188 188 191 191 192 192 192 193 193 194 195 201
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A HANDBOOK FOR EMC TESTING AND MEASUREMENT
10.6
10.7
11
10.5.1 Lightning environment 10.5.2 Defining the discharge 10.5.3 Effects on equipment Transients and general power disturbances 10.6.1 Importance of power transients 10.6~2 Examples of power supply immunity standards 10.6.3 Summary References
Uncertainty analysis: quality control and test facility certification
11.1 11.2 11.3 11.4
Introduction Some definitions Measurement factors Random variables 11.4.1 Student's t-distribution 11.5 Systematic uncertainty 11.6 Combining random and systematic uncertainties 11. 7 Uncertainties in EMC measurements 11. 7.1 Contributions to measurement uncertainty 11.7.2 Identification of uncertainty factors 11. 7.3 Estimation of uncertainty values 11.7.4 Estimate of total uncertainty 11.8 Test laboratory measurement uncertainty 11.8.1 NAMAS 11.8.2 NAMAS and measurement uncertainty 11.8.3 Limits and production testing 11.9 NAMAS requirements for laboratory accreditation 11.9.1 Requirements for accreditation 11.9.2 Advantages of laboratory accreditation 11.10 References
12
12.1
12.2
12.3 12.4
13
13.1
13.2
Designing to avoid EMC problem.s
Intrasystem and intersystem EMC 12.1.1 Intrasystem EMC 12.1.2 Design for formal EMC compliance System-level EMC requirements 12.2.1 Top-level requirements 12.2.2 Determining EMC hardening requirement 12.2.3 Simple coupling models 12.2.4 Susceptibility hardening case study 12.2.5 Emission suppression requirement 12.2.6 System hardening flow diagram 12.2.7 Subsystem apportionment and balanced hardening 12.2.8 Staff support for EMC Specific EMC design techniques References
201 202 204 205 205 205 206 207
209 209 209 210 211 213 213 214 214 214 215 216 218 218 218 218 219 219 219 220 221
223 223 223 224 228 228 228 229 231 233 233 233 235 236 236
Achieving product EMC: checklists for product developtnent and testing 238
Introduction 13.1.1 Chapter structure 13.1.2 Example adopted 13.1.3 Personal computers and information technology Information about EMC 13.2.1 Customer sources 13.2.2 Regulatory authorities 13.2.3 Industry sources 13.2.4 Equipment, component and subsystem suppliers 13.2.5 Professional bodies and conferences 13.2.6 EMC consultants and training 13.2.7 Electronics and EMC technical press
238 238 238 238 238 238 239 240 240 240 241 241
CONTENTS 13.3 13.4
13.5
13.6
13.7 13.8
Determining an EMC requirement Developing an approach to EMC design 13.4. 1 Process flow chart 13.4.2 EMC strategy 13.4.3 Immunity first? 13.4.4 Example of EMC design process Technical construction file 13.5.1 Routes to compliance options 13.5.2 Circumstances requiring the generation of a technical file 13.5.3 Contents of a technical file 13.5.4 Report from a competent body 13.5.5 Testing or technical file? Self certification 13.6.1 Need for an in-house facility 13.6.2 Gradual development 13.6.3 Estimates of facility cost 13.6.4 Turnkey facilities Conclusion References
Xl
241 242 242 242 243 243 244 244 245 245 246 246 246 246 247 248 248 248 249
Appendix 1 1.1 Signal bandwid th definitions 1.2 UK EMC legislation (up to 1 January 1996) 1.3 European EMC standards 1.4 German decrees and standards 1.5 US EMC regulations and standards 1.6 German, North American and Japanese EMC standards 1.7 Electrical safety and electromagnetic radiation 1.8 Military EMC standards 1.9 Compendium of EMC and related standards
250 250 252 254 259 261 262 264 266 271
Appendix 2 2.1 Modulation rules
277 277
Appendix 3 3.1 NAMAS-accredited laboratories 3.2 Competent bodies 3.3 EMC consultancy and training 3.4 Useful publications on EMC
278 278 280 282 283
Index
285
Foreword During the latter part of the 1980s the worldwide interest in electromagnetic compatibility has grown significantly in the electronics industry, particularly in the commercial and domestic equipment sectors. The field of electromagnetic compatibility is a dynamic one, with continuous evolution and change. This has accelerated in the 1990s with the introduction of the European Community EMC directive which applies to almost all electrotechnical equipment. The Directive attempts to harmonise regulations in member states concerned with EMC after 1992 (with a transition period up to 1996). Equipment (including imports) must meet the new EMC standards before they can be legally offered for sale within the EC. This book is devised in the form of a handbook concerned with most aspects of electromagnetic compatibility and addresses in a single volume many of the technical and managerial issues which will be of concern to designers and manufacturers of commercial and military electronic equipment. The primary focus of the "book is on EMC testing and discusses the numerous test methods in relation to the EMC standards for which they are required. Understanding EMC is important to produce reliable interference-free products and understanding EMC testing is the key to demonstrating that well designed equipment meets legislative and contractual req uirements, including the new EC regulations. EMC grew in importance in the field of military electronics over a 20-year period from abolit the mid 1960s, and therefore has been a precursor to the developments in the civil sector. Thus many of the design techniq ues and some test methods have been introduced into the field of commercial electronics EMC from this military background. Other test methods have developed entirely in regard to commercial equipment and have no counterpart in military EMC testing. This book contains examples of many commercial and military EMC tests and shows how difficult it can be to compare results from various tests which purport to measure the same quantity. Although the recent surge in interest in EMC has been spurred in the commercial electronics industry by the introduction of the EC Directive on EMC, the interest of military electronics manufacturers in EMC is strong and still growing. Many of the larger test facilities in the UK which carry out testing of commercial and military electronics are operated by military/aerospace companies. For these reasons this book devotes a proper
proportion of space to discussing test methods and specifications which relate to military equipment EMC, and includes a brief section on nuclear electromagnetic pulse effects in a chapter concerned with electromagnetic transients such as electrostatic discharge and lightning. Electromagnetic compatibility engineering is a multidisciplinary activity involving electrical engineers, physicists, chemists, systems and mechanical engineers and has quality, marketing and legal implications in the management of electronic product development. This book has therefore been written with the aim of being accessible to both technical staff and management. For example, it has detailed technical sections on basic antenna theory and EMC testing practice as well as sections oriented towards management/ project control, concerned with regulations and standards, and the EMC planning aspects of product development. 1'he emphasis lies on EMC testing. Within the text, test specifications, test methods and test equipment are discussed in some detail. I t would be cumbersome and impractical to explain each test method in detail for the enormous range of civil and military EMC specifications in use today through the world. Therefore the technical material on testing has been constrained to presenting the details of a number of generic types of test and has been constructed around a description of the physical principles and mechanisms which underlie the measurements. Information is presented for the following generic types of test: Radiated emission Radiated susceptibility Conducted emission Conducted susceptibility Transien t emissions (mainly conducted) Transient susceptibility (including electrostatic discharge) The couplers, sensors and antennas used to make these measurements are treated in groups defined by the physical coupling mechanism they employ. For example, conducted emission devices are divided into direct connection probes, inductive couplers and capacitive couplers. In this way, some physical meaning and order is brought to the construction and operation of the extensive range of antennas and sensors used in EMC testing. Although it contains considerable technical detail the material in this book has also been chosen to give the reader who may be coming to XIU
XIV
A HANDBOOK FOR EMC TESTING AND MEASUREMENT
the subject of electromagnetic compatibility for the first time a broad view of the range of topics covered by the s.ubject. The text therefore includes sections on topics such as The nature of electrical interference Standards and specifications Developing system and subsystem EMC requIrements EMC design techniq.ues The nature of EMC testing Test methods Measurement uncertainty Quality control in test laboratories Laboratory accreditation Achieving EMC in new products While the reader is encouraged to adopt a straightforward reading path *, this material has been organised to facilitate the selection and use of particular chapters in the manner of a handbook for day-to-day use. Chapters 6 and 7 for example contain useful antenna formulas which may need to pe .accessed quite often by a practising EM C test engineer. Where appropriate, small sections of the text have been devoted to reviewing the theory which underlies arguments which are to follow. This has been done in Chapter 5 with some basic concepts of antennas, and in Chapter 10 where an expla*It may be appropriate for readers who are new to EMC to read Chapter 12 on EMC design after Chapter 2 (EMC standards). Chapter 12 contains a number of concepts and insights into the causes and solutions of EMC problems which may help the reader with the rest of the book. The chapter on EMC design has been positioned towards the end of the book so as not to disrupt the progression from the introduction followed by EMC standards directly to the main focus of the book on EMC testing.
nation of the mathematical relationship between the frequency and time domains is given to enable the reader to understand transient testing and to be able to visualise the frequency content of waveforms. For ease of reading, long lists or tables of information have been put into appendices at the end of the book. This is particularly so for Chapter 2 on EMC standards where lists of standards, equipment classes and tests would otherwise break up the flow. Numerous examples of EMC standards from around the world are included in Chapter 2. These range from the earliest historical examples to the recently issued and proposed European Norm standards. EMC standards and test methods are changing rapidly and whilst every attempt has been made to include the most up-todate information, new standards are bound to be issued and supersede existing ones. I t is not possible to cover in a single book the enormous range of information which constitutes the current body of knowledge on EMC. The aim here has been to provide an introduction to EMC and particularly to EMC testing in the form of a useful day-to-day handbook. There are many other published texts, some in up to 12 volumes, which deal in detail with the design aspects of EMC. These are referenced in this text. With the expansion of interest in EMC a number of useful books have been written since 1990 which reflect the dynamic nature of the topic. They are listed in Chapter 13 along with information on the lEE distance learning project on EMC. I t is hoped that readers find this hand book helpful, informative and easy to read and use. If it helps to stimulate an interest in this fascinating subject and direct the reader to sources of more detailed information it will have achieved its objective.
Acknowledgem.ents Extracts from British Standards are reproduced wi th the permission of BS I. Complete copies can be obtained by post from BSI Sales, Linford Wood, Milton Keynes, MK14 6LE. Figures 5.12, 5.14 and 8.34 are from the book 'Reference data for engineers: radio, electronics, computer and communications' edited by Mac
E. Van Valkenburg. Published by SAMS Publishing, a division of Prentice Hall Computer Publishing. Used by permission of the pu blisher. The publishers are grateful to Marconi Instruments Ltd. for help in the cover design of this book.
Chapter 1
Nature and origins of electrom.agnetic com.patibility 1.1 Definitions of electroIllagnetic cOIllpatibility The formal definition of electromagnetic compatibility, as given in the International Electrotechnical Vocabulary (IEC 50) is: 'the ability of a device, equipment or system to function satisfactorily in its electromagnetic environment without introducing intolerable electromagnetic disturbances to anything in that environment' [1 J. A similar definition cited by Duff [2J is given as: 'the ability of equipments and systems to function as intended, without degradation or malfunction in their intended operational electromagnetic environments. Further, the equipment or system should not adversely affect the operation of~ or be affected by, any other equipment or system'. Electromagnetic interference (EMI) can be viewed as a kind of environmental pollution which can have consequences that are comparable to toxic chemical pollution, vehicle exhaust emissions or other discharges into the environmen t. The electromagnetic spectrum is a natural resource which has been progressively tapped by man over the last 100 years. Most of the development has taken place in the last 50 years with the advent of public service broadcasting, point-to-point and mobile communications etc. which has brough t great economic and social benefits. The spectrum is now almost full and it is proving difficult to satisfy the pressures for new uses of this resource. Modern life has come to depend heavily on systems that use the electromagnetic spectrum and its protection is in the interests of us all. For this reason unwarranted electromagnetic interference represents a real economic and social threat which can even result in injury or death. Unfortunately, electromagnetic interference cannot be smelled, tasted or seen by either the lay person who purchases electronic products or by the corporate technical manager who has to supervise the design of the latest electronic product and get it to the marketplace as fast as possible, for the lowest possible cost. There has, therefore, been a tendency to deny that EMI is a problem in the modern world and to argue that
the costs which are associated with achieving electromagnetic compatibility (EMC) need not be borne. Some of these wider issues are explored later, but for now another definition of this fascinating and wide ranging concept is examined. Keiser [3J defines EMC in this way: 'electrical and electronic devices can be said to be electromagnetically compatible when the electrical noise genera ted by each does not interfere wi th the normal performance of any of the others. EMC is that happy situation in which systems work as intended, both within themselves and in their environment' . Electromagnetic in terference ind uces undesirable voltages and curren ts in the circui ts of the victim equipment. This can cause audible noise in radio receivers and spots, snow or loss of frame synchronisation on TV pictures. When vital communications links, computer installations or computer driven industrial process control equipment is the victim equipment, more serious conseq uences can occur. Interference can reach the victim system by two basic routes: conduction along cables, and electromagnetic radiation. This chapter examines typical sources of EMI and discusses the technical basis of electromagnetic compatibility within an equipment, and between the equipment and its environmen t in terms of conducted and radiated interference paths.
1.2 Visualising the EMI probletn 1.2.1 Sources of EM I Any electrical or electronic device that has changing voltages and currents can be a source of EM!. If the culprit equipment has no cables connecting it to the outside world, for example a battery powered electric shaver, then the interfering energy generated by sparking within the electric motor can only travel as an electromagnetic wave. If the shaver is mains powered, both radiated noise and interference conducted along the cable into the mains wiring are possible. This is illustrated in Figure 1.1 where a mains powered shaver and a washing machine are both
2
A HANDBOOK FOR EMC TESTING AND MEASUREMENT HOUSE MAINS WIRING ~,.
EMI CURRENTS FLOW INTO OTHER BUILDINGS
INTERFERENCE NOISE CURRENTS
"
)
,. Motor noise
)
J
Indirect radiation from currents in mains cables
eo
o
Mains power' waveform
Motors, switches relays, etc.
/
~
~,V
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CULPRIT 1 ELECTRIC SHAVER MOTOR
Figure 1.1
\ \
\
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DIRECT RADIATION FROM ELECTRIC SHAVER (ALSO RADIATES OUTSIDE
CULPRIT 2 WASHING MACHINE
TV receiver is victim
Illustration of simple EMI problem
causing interference to a TV picture in an adjacent room. Radiation is emitted not only directly from the shaver or washing machine motor, but also from the mains wiring which is carrying the conducted radio frequency (RF) noise. Generally, the faster the rate of change of voltage or current in the culprit equipment, the wider is the spectrum of RF interference produced. The greater the magnitude of the noise voltages or currents the greater will be the conducted and radiated emissions. Therefore electric motors, which generate high voltages and currents with fast risetimes as the inductive coils in the rotor are switched by the commutator, are good examples of particularly powerful sources of EMI. Other common sources of electromagnetic interference are given in Table 1.1 These sources can be grouped in terms of usage as shown in Figure 1.2. They may be sources of continuous or transient interference as shown in Figure 1.3. Continuous sources include radio transmi tters and emissions from RF heaters for example where the signal is an uninterrupted carrier, but also include pulsed systems such as radars and the emissions from digital compu ters which have wide but stable RF spectra. The emissions from these sources are best measured and analysed in terms of their spectral content using narrow bandwidth scanning receivers or spectrum analysers.
Table 1.1 Common sources of EM! Powerline arcing" and corona discharge Automobile ignition systems Fluorescent lighting Switched-mode power supplies Portable electric generators Static or rotary power converters Any appliance using brush commutator motor Air conditioning equipment Computer equipment and peripherals Equipment with switches and relays Diathermy medical equipment Arc-welding sets High-voltage neon signs Light dimmers Microwave ovens CB radio Radar transmitters Broadcast transmitters Atmospherics (noise from lightning around the world) Whistlers, chorus and hiss from the magnetosphere Nearby lightning storms Precipitation static noise Disturbed and quiet radio noise from the sun Cosmic radio noise
NATURE AND ORIGINS OF ELECTROMAGNETIC COMPATIBILITY
3
SOURCES OF EMI
-BROADCAST STATION -RADAR -CBRADIO -AMATEUR RADIO -ELFNLF NAVIGATION/COMS -MOBILE Tx -REMOTE CONTROL DOOR OPENING TRANSMITTERS -POINT TO POINT HF COMS. -IONOSPHERIC SCATTER RADAR AND COMS.
- PORTABLE POWER GENS. - STATIC & ROTARY CONVERTERS - RECTIFIERS - TRANSMISSION LINE NOISE - POWER FAULTS - CONTACTORS
STATIC NOISE
}'igure 1.2
-AUTOMOBILES & VEHICLES -TRACTION POWER POWER CONVERSION -ELECTRIC MOTIVE POWER -IGNITION SYSTEMS MOBILE Tx.
ATMOSPHERICS
- WORKSHOP MACHINES - COMPRESSORS - ROTARY SAWS - BALL MILLS RF HEATERS - ULTRASONIC CLEANERS - WELDERS -SPARK EROSION - CRANES I FANS - OVENS IKILNS
SOLAR· NOISE
-DIELECTRIC HEATERS -AIR CONDITIONING - COMPUTERS -FLUORESCENT LIGHTS LASER SYSTEMS NEON DISPLAYS - MEDICAL EQUIP. -PROCESS CONTROL X RAY MACHINES
LIGHTNING
- MICROWAVE OVENS - LIGHT DIMMERS - PERSONAL COMPUTERS - MIXERS/BLENDERS - VACUUM CLEANERS - WASHING MACHINES - HAIR DRYERS - ELECTRIC MODELS - FRIDGESIFREEZERS - SHAVERS -THERMOSTATS
COSMIC RADIO BACKGROUND NOISE
(;roups oj EMI sources
Sources of transient emISSIons include lightning, nuclear electromagnetic pulse, powerline faults, switch and relay operation, etc. They are characterised by single or in termi tten t occurrence at unpredictable times with no significant time pattern. Often, the signals are of short duration and hence have a wide signal bandwidth. I t is easier to measure, record and analyse such signals as a waveform in the time domain. rrhe advent of wide-bandwidth transient-capture waveform digitisers and fast transformation algorithms has only recently led to the ability to view easily the spectrum of a single fast transient. The details of EMC test equipment and test techniques that are capable of measuring the wide range of interference signals from these EMI
sources are the main concerns of this book and are to be found in Chapters 4 to 11. ExaInples of the extent of the frequency spectrum, repetition rates and signal amplitudes from some typical EMI sources are given in I'able 1.2 which contains both intended and unintended sources of EM radiation. Examples of approximate field strengths for typical broadcast transmitters in the UK are given in Figure 1.4 and for other sources in Figure 1.5. These data with regard to emission levels and freq uencies for potential sources of ElYlI allow the appreciation of the potential scale of the problem if such sources can couple to sensitive victim equipment which may only be able to tolerate a few microvolts or millivolts of unwanted
SOURCES OF EMI SOURCES OF CONTINUOUS EMI (CONTINUOUS SPECTRUM OF NOISE) • BROADCAST STATIONS HIGH POWER RADAR - ELECTRIC MOTOR NOISE FIXED & MOBILE COMMUNICATIONS COMPUTERS, VDUs & PRINTERS etc. • AC I MULTIPHASE POWER RECTIFIERS HIGH REPETITION RATE IGNITION NOISE SOLAR AND COSMIC RADIO NOISE
BEST MEASURED AND ANALYSED IN THE -FREQUENCY DOMAIN- - (SPECTRUM)
SOURCES OF TRANSIENT EMI (COMPOSED OF SEPARATE PULSES) - LIGHTNING - NUCLEAR EMP - POWER LINE FAULTS (SPARKING) - SWITCHES AND RELAYS ELECTRIC WELDING EQUIPMENT - LOW REPETITION RATE IGNITION NOISE ELECTRIC TRAIN POWER PICK-UP ARCING - HUMAN ELECTROSTATIC DISCHARGE
BEST MEASURED AND ANALYSED IN THE -TIME DOMAIN- - (WAVEFORM)
1.3 Sources oj~ continuous and transient interference
4
A HANDBOOK FOR EMC TESTING AND MEASUREMENT
Table 1.2 Frequencies and noise levels from typical interference sources Source type
Comments
Mains disturbances
Double exponential transients with risetimes of 1 J.1S and fall times of 50 J.1S at approx.10kV 100 kHz ringing waveform with 0.5 J.1S rising edge Power dips up to 100 ms long Power frequency harmonics up to 2 kHz
Unintended radiators Switches and relays
Transients with risetimes of a few ns and levels up to 3 kv producing frequencies into the VHF band
Commutator motors
Produce frequencies up to 300 MHz at repetition rates of up to 10 kHz
Human electrostatic discharge
I-IOns risetime 30-200 ns fall time ampli tudes up to 15 kV
Switching semiconductors
Risetimes from 20 to 1000 ns at rep rate of kHz to 10 MHz for voltages up to 300V
Switched-mode power supplies
Produce continuous spectrum of noise from kHz to 100 MHz
Digital logic
Circuits produce continuous noise up to 500 MHz
Ind ustrial and medical equipment
Metals heating in 1-199 kHz range. Medical equipment operates from 13-40 MHz. Using high power-hundreds of watts
Intended radiators Broadcast stations
See Figure 1.4
Other RF transmitters including radar
See Figure 1.5
1000
180
Figure 1.4 Field strengths in the vicinity of broadcast transmitters
170
160
-r
~
SHORT WAVE
dBuV/m 150
T -r---l..- T TUHF
-
LONG WAVE
130
120 ....
--a.I 10k
Reproduced by permission of' BAe Dynamics Ltd.
...... I 100k
--L .....I
~
1M
TV
VHF
MEDIUM WAVE
TELEGRAPHY 140
-
}VHFl O
RAI
.... I 10M
FREQUENCY OF TRANSMITIER (Hz)
.... I 100M
100
T
Vim
UHF TV
1-
10
.... 1 10
NATURE AND ORIGINS OF ELECTROMAGNETIC COMPATIBILITY 180 100m RADAR 400m
170
CB Tx ON VEHICLES 160
-
AMATEUR TRANSMITIERS @ 10m
_/
150
I
1000
-
/
Figure 1.5 Field strengths in the vicinity oj other sources
100
~_/
dBuV/m
5
VIm
CB Tx (100w @ 10m)
140
f
f
RF HEATERS ~
10
LAND MOBILE BASE STATION TRANSMITIERS @ 10m
130 CBTx 4w @ 10m
120
1M
3M
10M
30M 100M FREQUENCY Hz
1.2.2 EMI coupling to victim equipments An EMI problem can only exist if a source of EMI, a culprit, is able to exchange electromagnetic energy with a receptor or victim equipment. Thus the general EMI problem can be represen ted by these three componen t~ as show.n in Figure 1.6. The coupliI).g can be VIa metalhc cond uctors such as power or signal cables, if they exist, or more generally by electromagnetic radiation from one equipment to the other. Energy exchange will take place between th.e electromagnetic wavefield produced by the culprIt equipment, and metallic conductors attached to the victim equipment which will have RF currents induced in them. If the primary coupling mode is radiative but the receptor has a cable attached to it (but not connected to the culprit) currents can be induced into the cable that will then flow directly into the victim, even if it is protected from direct radiation by a high performance shielded case. See Figure 1.7 a.
RADIATION SOURCE OF EMI
, RECEPTOR OF EMI
(Culprit equipment)
(Victim equipment)
CONDUCTION
SOURCE
Figure 1.6
COUPLING
RECEPTOR
Three components oj an EMC problem
300M
10
Reprod uced by permission of BAe Dynamics Ltd,
Equally, if the primary coupling bet:ve.en culprit and victim is conducted, but the vIctIm has a good filter connected to the input cable, the currents in the cable can still radiate EM energy which may then couple into the victim if it has no electromagnetic screening. See Figure 1.7 b. This situation demons't:ates that both radiative and conducted couphng paths, whether direct or indirect, must be addressed in parallel if the EMI energy is to be preven te? from reaching the victim equipment. ThIs argument leads directly to .the. combi~ed application of shielding and filterIng In practIcal solutions to EMI problems. Common receptors or victim equipments are listed in Table 1.3. The list contains both intended and unintended receivers. Electromagnetic compatibility engineering is composed of four basic topics as shown in Figure 1.8 related to the combinations. of source/ victim and radiated/conducted couphng. EMC standards and specifications often use the following initials to describe these four elements: RE RS CE CS
-
radiated emission from a culprit radiated susceptibility of a victim conducted emission from' a culprit conducted susceptibility of a victim
Because conducted and radiated transient EMI, including electrostatic discharge (ESD) are measured using different techniques, they are sometimes represented as an additional fifth group,
6
A HANDBOOK FOR EMC TESTING AND MEASUREMENT
Shield prevents direct radiative coupling from culprit to victim ~
"Radiated EMI from culprit
/ VICTIM SYSTEM
CULPRIT SYSTEM
Input connector
\
RF CURRENTS CONDUCTED INTO VICTIM
lamps \
~CABLE
ACTS ASAN UNWANTED IlANTENNA" "TO OTHER EQUIPMENT
Induced RF EMI current into cable
""-,
~-.
(aj
///~
POOR or NO SHIELD ~
1--I
Radiation reaches PCB /' Radiation from the Cable
f //l
V~C~\STIEM ( :.L (
L -T
CIRCUITS DIRECTLY EXPOSED TO
RtATE~
-:MI
\
\
\ \~
\~
CULPRIT SYSTEM lamps
(NO DIRECTLY RADIATED EMI)
, , ( s o u r c e of RF currents) High le'el of CONDUCTED EMI
GOOD FILTER STOPS CONDUCTED EMI
(bj Figure 1.7
( aj Radiated to conducted EMI (b j Radiation Jrom cable carrying EM1 currents
Table 1.3 E'xamples
of victims oj EMI
Intended receivers Radio receivers 0.0 1-1 f1 V sensitivity Broadcast receivers TV receivers Mobile communication receivers Microwave relay systems Mobile telephones Aircraft communication receivers N aviga tion aids Radar receivers
[inintended receivers Aircraft engine control systems Aircraft flying surface con troIs Weapons systems, guided missiles Electronic ships systems inc steering gear Video recording/playback equipment Computer equipment Industrial process control systems Signalling systems Medical electronic instruments Heart pacemakers Biological tissue Ordnance and explosive fuses
as in Figure 1.8. RF hazards to fuels, explosives, electrically detonated ordnance, and RF hazards to biological tissue (including humans) can also be encompassed within the general concept of electromagnetic compatibility. EM C testing is carried ou t in all these areas, though the measurement techniques and instrumentation vary widely depending on the particular test. For RE and RS tests the equipment under test (EDT) is placed at some dis tance (usually 1 or 3 m) from a range of antennas that are used either to measure the lowlevel EM emissions from the equipment, or to generate high field strengths at the equipment for susceptibility testing. Cond ucted emission (CE) tests involve the use of special coupling transformers or current probes to sense the level ofRF current being conducted away from the EDT along its cables. The conducted susceptibili ty (CS) tests require RF current to be injected into cables either using coupling transformers and probes or by direct connection. The main body of this text is concerned with explaining these test methods and examining in detail the physical principles and operating practice associated with the types of antennas, current probes and other sensors used.
NATURE AND ORIGINS OF ELECTROMAGNETIC COMPATIBILITY
SAFETY OF ELECTRO-EXPLOSIVE
SAFETY OF NON-IONISING
DEVICES
___________...:------------1--------1
RADIATED EMISSIONS (RE) 10 Hz to 40 GHz E FIELD 14kHz - 400Hz H FIELD 10Hz - 30MHz (LEVELS DOWN TO -110dBV/m)
RADIATED SUSCEPTIBILITY (RS) 10Hz to 40 GHz 1 - 200V/m (CW)
NUCLEAR ELECTROMAGNETIC PULSE 50kV/m RS (10/400ns) 100 A (DAMPED SINUSOID)
Figure 1.8
7
RADIATION ON BIOLOGICAL TISSUE
CONDUCTED EMISSIONS (CE) 20Hz to 100 MHz CABLES 100MHz -4OGHz ANTENNA CABLES (LEVELS DOWN TO -120dBA)
MAINS DISTURBANCES AND LIGHTNING 10kV TRANSIENTS 100ms DROPOUTS HARMONIC DISTORTION
CONDUCTED SUSCEPTIBILITY (CS) 20Hz to 400MHz @ -2OdB VA
HUMAN ESD 15kV TRANSI ENTS DIRECT & RADIATED (5/30ns waveform)
Examples of EMG) and related activities
1.2.3 Intersystem and intrasystem EMI EMC activity can also be differentiated in terms of the level at which it is applied. The widest level concerns the compatibility between a system of interest and all other systems with which it could interact, including the general EM environment. This is called intersystem EMC and can involve for example, frequency planning, equipment siting, antenna sidelobe suppression and the imposition of operating restrictions including timing constraints. In the military world operational restrictions might apply to the minimum space between aircraft or the minimum distance at which they may approach a ground based transmitter. The interaction of commercial civilian systems with broadcast receivers and the general EM environment is regulated largely by voluntary trade agreements or government laws. The advent of the widespread use of desktop digital compu ters has forced the introd uction of legislation to control the radiated and conducted emissions from such equipments in order to protect the broadcast spectrum. Intrasystem EMC is concerned with the selfcompatibility of the system of interest. It relies on the premise that if each individual unit within the system is required to emit less EMI than any of the units would be susceptible to, plus a margin for safety, then when the units are assembled as a whole the system will be electromagnetically compatible. The overall system EMC requirement must be determined and apportioned to each subsystem or unit. The detailed design work is then carried out at the level of circuit and
board design, cable design, choice of I C technologies etc. to meet the unit level EMC requirement. Military equipment designers are required to follow the guidelines given in military EMC design hand books and to meet the unit level EMC testing limits given in MIL STD 461/2/3 or DEF STAN 59-41, for example. The relationship between intersystem and intrasystem EMC is shown in Figure 1.9. A pictorial representation of a typical intersystem EMI situation is given in Figure 1.10 showing possible conducted and radiated coupling paths between various systems. A similar representation, but of intrasystem EMI, is given in Figure 1.11 using a transmitter with a case-mounted antenna as an example.
1.3 Historical background 1.3.1 Early EMC problems EMI problems and EMC solutions are not new. Jolly [4], quoted by Braxton [5], tells of how at around the turn of the century Gugielmo Marconi had been contracted to build demonstration models of his new wireless telegraph sets the British, French and American for governments. When he had installed several equipments on board ships and at land sites, the users complained that they could only operate one station at a time. I t was therefore discovered by acciden t tha t freq uency management was important in communication systems, as the spectrum from the crude transmi tters overlapped. Marconi had to return to the installations and attempt to make them tunable
8
A HANDBOOK FOR EMC TESTING AND MEASUREMENT
Followequipf1!ent specifications - ego MIL STD 461
SUBSYSTEM 2
SUBSYSTEM 1 EMI CONTROL
Figure 1.9
Follow system requirements - ego MIL STD 6051 D IEC 801 FCC part 15/18
SUBSYSTEM n
COMPATIBILITY WITH OTHER KNOWN SYSTEMS IN OPERATIONAL ENVIRONMENT
REGULATIONS ON EMISSIONS TO GENERAL ENVIRONMENT
SUSCEPTIBILITY TO GENERAL CABLE-BORNE INTERFERENCE & TRANSI ENTS
IMMUNITY TO LIGHTNING EFFECTS & POSSIBLY NEMP
Intersystem and intrasystem EMC
1.3.2 Early EMC problems with military equipment
to avoid co-channel interference. This is probably the first example of an EMC 'fix' applied to an existing equipment. Many people have followed Marconi's example in the in tervening years.
While the International Special Committee on Radio Interference (CISPR) has been tackling
-----
NATURAL & COSMIC ______________ -
-----
RADIO & ·STATIC· NOISE
/
RADAR & COMMUNICATIONS TRANSMISSIONS FROM MOVING
LIGHTNING (DIRECT AND INDIRECT EFFECTS)
VE~~ ~
SYSTEM· 1
SYSTEM 2 Susceptibility to and emissions of noise on comms. lines (
Susceptibility to and emission of mains-borne noise { ;
Susceptibility to and emission of mains noise
POWER LINES
DISTURBANCES ON POWER-LINE TELEPHONE LINES
Figure 1.10
Intersystem EMI
PICK-UP ON TELEPHONE LINES
NATURE AND ORIGINS OF ELECTROMAGNETIC COMPATIBILITY
POSSIBLE RF LEAKAGE FROM Tx ANTENNA
------------=M(AoLINC
INTO BOX via CONTROL PANEL
ANTENNA "'-
~
(Example of a transmilfer with a case mounted antenna)
\ FRONT PANEL CONTROLS (Shield apertures if possible)
SUBSYSTEM A SUBSYSTEM B Conducted EMI TRANSMITIER......... OSCILLATOR DOUBLER CONTROL STAGE CIRCUITRY Signal & Data lines
POSSIBLE CONDUCTED NOISE
POWER SUPPLY FILTER EXTERNAL RADIATION INDUCES CONDUCTED EMI INTO MAINS CABLES (DISTURBS POWER SUPPLY REGULATION WITHOUT FI LTERS )
Figure 1.11
9
MAINS POWER LEAD
RF CONTROLLED INTERNAL TO SUBSYSTEM BY SCREENING & FILTERING
~
INTERNAL UNWANTED RADIATION PICKED UP ON POWER & SIGNAL CABLES
~
PA
POWER LINE
~
~I
X:
SLOTS & APERTURES IN SUBSYSTEM & SYSTEM CASES
POTENTIAL RF LEAKAGE IN CASES LEADS TO PICK UP INSIDE SYSTEM
ADEQUATE RF & SAFETY GROUNDS
Example oj'intrasystem EM!
the intersystem problems of interference to broadcast receivers since 1934, the majority of early interest in intrasystem EMC has been with regard to military equipment. Early examples of EMC problems aboard military aircraft have been recorded [6, 7J and illustrate the problems of the day, as follows. 1944 B29: Radiated and power cable conducted noise to HF and VHF communications equipment. 1947 B50: EMI problems due to poor grounding and bonding. 1950 B50, B47, C97: Problems with DC power bus nOIse. 1954 B52: Interference from radar to communications equipment. 1958 B52: Problems with coupling between the 400 Hz power system and navigation and bomb aiming equipment. In this case the problems were so acute that production was stopped while cures were found. Eventually modifications to both cabling and equipment proved effective. A similar history exists for naval equipment [8]. 1939-45: Metric radar causes interference to HF communications. 1945-55: During this period there was a great increase in the use of servo equipment to control guns for example. New 10 f.1V sensitivity intercoms and tactical radio communications were now using voice channels with microphones distributed throughout the ship. New frequency
bands were used, sonar, radar and new navigational electronics were fitted. All these changes made for a more complex use of the electromagnetic spectrum and resulted in increased interference between systems. 1955-65: The change from DC commutator power generation to 440 V three-phase 60 Hz resulted in an improvement in the ship board EM environment, but at the same time the change from lead clad cables to plastic sheathed ones removed the natural screening offered by the older type. 1965 onward: The introduction of semiconductor technology greatly increased the number and sophistication of electronic eq uipments. Introduction of digital computers and automatic telegraphy all required greater vigilance in spectrum management and the control of spurious emissions from these devices.
1.3.3 The cost of EMC Development programmes on early ICBMs such as Atlas, Thor and Titan suffered delays owing to EMC problems. The cost of EMC remedies and programme delays ran into many millions of dollars. A subsequent, more complex missile development included an EMC control programme which cost an estimated $3.5M but resulted in minimal problems and no delays to the project. This shows, on a grand scale, that when a sensible EMC policy is adopted and implemented
10
A HANDBOOK FOR EMC'TESTING AND MEASUREMENT
early in product development considerable cost benefit can result. It is clear that failure to address EMC design issues can result in major budget overspends and programme delays. The cost of implementing an EMC control programme into product developme.nt is generally estimated at about 5% of the development cost. With careful design and choice of components and materials, the additional cost element on each equipment sold can often be small, and sometimes almost negligible in the case of large production runs.
persuaded programme managers over the preceding ten years that the costs associated with EMC engineering were affordable in order to produce equipment which not only met formal specifications, but also worked well in the field. In the late 1980s and early 1990s engineers and designers working in the civil electronics field faced a similar situation [5, 11, 12] and the commercial electronics business learned that good EMC design is important if their products are to compete successfully in national, regional and global markets.
1.3.4 Serious EMI problems
1.4 Technical discil?lines and knowledge areas wIthin EMC
As most EMC activity until the early 1980s was associated with military hardware, it is to be expected that problems would be associated with this type of technology. One of the most publicised incidents to military equipment in which EMI was implicated was that which occurred on the USS Forestall off Vietnam in July, 1967. It was reported that RF energy from a high powered ship's radar coupled into the firing circuits of a aircraft-mounted missile rocket motor, which ignited and fired the weapon into a number of other armed aircraft on the carrier flight deck. The resulting explosions and fire killed 134 people and caused $72M of damage not counting the 27 lost aircraft. More recently (around 1980), interference to avionics from a ground based transmitter was implicated in a mili tary aircraft crash in Germany. EMC activity in the nonmilitary world has concentrated on the need to keep the electromagnetic spectrum free from interference to enable comlTIunications systems to operate efficiently and to minimise the upset caused to radio and television broadcast reception. The last 40 years have seen a dramatic increase in the number of licensed radio services. In the USA it is reported [9J that since 1950 the number of broadcast stations has quadrupled to 11,000 radio and 1,400 TV stations. The FCC (Federal Communications Commission) has also licensed 2.7 million mobile and fixed systems using 12 million transmitters [10]. With the widespread introduction of digital computing technology and microprocessor controlled products into the commercial, industrial and domestic environments in the early 1980s the need for government agencies to act to control the possible explosion in EMI was overwhelming. By this time, EMC was firmly established in the industries designing military electronic equipment as an accepted part of the design and manufacturing/quality process. Many early EMC engineers working on military products had
This section considers briefly the wide range of technical skills that have a part to play in the field of EMC. Each skill or knowledge area discussed below is involved in solving EMC problems. 1'he task of the product manager involved with EMC is to blend these various skills together to engineer robust EMC solutions into the product at an affordable cost.
1.4.1 Electrical engineering Electrical and electronic engineers are expected to be knowledgeable in the following areas relevant to EMC design and installation practice: Analogue and digital circuit design Semicond uctor device technology Transien t suppression devices and circuits Circui t board design Component selection: operating limits and reliability /cos t EM aspects of mechanical design (propagation through slots, holes, joints etc), grounding and bonding impedances Power generation, distribution and switching systems Electrical safety and lightning protection filters and surge arrestors Grounding techniques, single/multipoint Differential and common mode cable coupling Transmission line theory Screening theory a~d shielding design Interface circuit design Data bus and interface circuit design Optoisolation techniques Radiation from cables and slots in screens Fourier transforms between freq uency and time domains Use of sophisticated RF test equipment Principles of RF receivers and transmitters Basic antenna theory Radiowave propagation theory (near-field ,effects being of particular interest)
NATURE AND ORIGINS OF ELECTROMAGNETIC COMPATIBILITY
11
1.4.2 Physics
1.4.4 Limited chemical knowledge
The physics of electromagnetic energy exchange between RF currents and waves is very important in understanding the complex processes that occur in a real EMI situation. The manner in which RF currents flow in and around the surfaces of conducting and nonconducting complex structures of the victim equipment determines to a large extent the nature of an EMI problem. Developing and implementing the means to contain, absorb or divert such currents in a harmless way is the core of good EMC design practice. The physical equations derived by Maxwell governing EM waves and their interaction with matter form the basis for a real understanding of EMI problems and their solutions. Such equations can be solved generally using large 3D finite element, finite difference, or boundary element computer codes which can predict the current flow patterns in a complex structure when illuminated by an external electromagnetic wavefield. The physics of EM wave propagation in both the near and far fields must be considered by the EMC engineer, along with standing-wave phenomena and the performance of radio absorbent materials in large shielded testing chambers if meaningful EMI measurements are to be made and explained. Understanding how EMC test antennas behave in the near field, possibly amid multiple test chamber reflections, is a difficult topic which represents a significant challenge to the physicist involved in EMC measurement.
Occasionally, good, apparently simple and costeffective solutions to an EMI problem are ruled out because of concern about the chemistry of the proposed design. Corrosion of RF gaskets due to contact of dissimilar metals in damp, salt-laden or corrosive atmospheres can be a real problem which not only renders some EMI solutions void, but can also result in serious damage to the equipment cases or containers.
1.4.3 Mathematical modelling Large projects that require EMC to be considered at all levels of development often make use of extensive computer models and the EMC designer or manager should be familiar with the different types used. These can include (i)
(ii)
(iii)
Models of physical processes, such as RF current distributions on structures due to imposed EM field, wavefield to transmission line coupling, lumped and distributed filter performance in circuits with arbitrary source and load impedances. Models of intersystem and intrasystem compatibility matrices to identify potential EMI problems because of unwanted frequency matches, noisy culprit equipments, oversensitive victims, or a high level of coupling owing to the close proximity of subsystems. Programme management software that can be used to monitor and control an extensive EMC activity.
1.4.5 Systems engirleering To be successful, EMC must be considered early, when a contract to develop and supply electronic equipment is being negotiated. Failure to do so can result in incorrect bid pricing and an eventual failure to comply with the contractual EMC requirements. The customer requirements must be interpreted and reflected in a system EMC specification which is then apportioned to each subsystem or element of the design. If these tasks are carried ou tit is then possible to consider a balanced hardening design of the system which helps to ensure the most cost-effective route to electromagnetic compatibility.
1.4.6 Legal aspects of EMC EMC requirements on products stem in part from legal req uiremen ts by government agencies for manufacturers not to produce electronic equipment that will pollute the electromagnetic spectrum. Failure to comply with such laws can result in possible litigation by customers, other suppliers, and of course, the official agencies. Under the new EC harmonisation directive ECI 89/336 within Europe it becomes a criminal offence to sell equipment which does not meet the community EMC requirements as set out in the harmonised or national (technically equivalent) standards. Corporate legal advice on EMC regulation is beginning to be sought in the USA [10J and it is inadvisable for EMC design or test engineers to venture a direct opinion on the legal implications of EMC technical matters. They should, however, be in a position to advise management and lawyers about the multitude of regulations and standards which relate to EMC throughout the world , particularly in those countries or trading blocks into which their products are being sold.
1.4.7 Test laboratories Often the formal proof of meeting an EMC requirement is obtained from a test In an accredited laboratory. This can be made within
12
A HANDBOOK FOR EMC TESTING AND MEASUREMENT
the company that is developing the product or by an outside specialist EMC test house. Clearly, the test engineers must comprehend the EMC standards and specifications, the philosophy behind the testing approach, the detailed technical methods used in testing and the quality control requirements which govern test procedures and test house management. I t follows that it is also in the in teres ts of designers and project managers to have a good understanding of all these aspects of EM C testing to be able to interact meaningfully with the test engineers and understand the results which are obtained. Providing such information is one of the principIe aims of this book.
1.4.8 Quality assurance: total quality management EMC test engineers and managers of test houses must be acutely aware of the need for discretion, impartiality and q uali ty assurance in the conduct of their business. The EMC facility manager is required to hold delegated ,quality authority from a senior company manager to ensure that the laboratory is run in a manner which satisfies the national laboratory accreditation scheme (NAMAS in the UK).
1.4.9 Practical skills Effective EMC designers often have first-hand practical engineering experience of equi pmen t development and solving tricky RF problems with relatively meagre resources. Experienced radio amateurs tend to have a good understanding of basic RF engineering, which incidentally, is being taught less each year in university courses. Radio hams often display a flair for EMC engineering, offering cheap effective design solutions perhaps as a result of using novel techniques. The skills mentioned above are req uired by individuals and teams in pursuing the goal of electromagnetic compatibility for electrical and electronic products being developed in today's commercial world of competitive costs and tight timescales. Many of the issues and technical topics touched on so far are amplified in the rest of this book with the aim of giving the reader an appreciation of design practices and a good awareness of EMC testing techniques.
1.5 Philosophy of EMC Engineering of any kind can be viewed as the creative process of defining, organising and distilling out of a range of possible outcomes the desired system performance. I t IS by the
application of consistent design constraints and accurate manufacturing processes that electronic equipmen ts with specific properties are produced. EMC engineering is concerned with identifying, understanding and managing the normally uncontrolled and often unexpected transfer of electromagnetic energy from device to device such that the desired product performance goals are not impaired. EMC engineering can therefore be viewed as the application of an extra set of controls or constraints to the design, manufacture, installation, operation and maintenance of the system in question, which ensures that the equipment performs only those functions for which it was designed, and does not respond to any spurious signals resulting from EM interference. This means that EMC engineers attempt to design-out all spurious system responses, leaving the product design engineers to design-in the wanted performance. I t is the role of those concerned with EMC to ensure that the second broad element of system design, the 'not engineering' is carried out cost-effectively in conjunction with the usual design process if the product is to avoid being compromised by EM!. Perhaps the essential difference between the two types of engineering is that the conventional designer is concerned in great detail with only a relatively !1arrow range of specific product related issues whereas the EMC engineer, or designer with responsibility for EMC, is concerned with all possible external electromagnetic influences on the proposed system. Not all good electronics designers take to EM C engineering: the subject seems to suit individuals who enjoy lateral thinking and have a wide interest in electronics, RF engineering and a broad knowledge of the many skills outlined earlier. However, most electronics designers can become competent EMC practitioners with appropriate training. The next chapter looks in detail at the range of EMC requirements and specifications used around the world with which EMC managers, designers and test engineers must be familiar. This is the essential starting point from which to develop an understanding of the technical factors that are involved in EMC design and testing aimed at meeting these specifications.
1.6 References JACKSON, G.A.: 'The achievement of electromagnetic compatibility'. ERA report 90-0106, ERA Technology, Leatherhead, Surrey, UK 2 DUFF, W.G.: 'Fundamentals of electromagnetic compatibility'. Interference Control' Technologies, Inc.
NATURE AND ORIGINS OF ELECT'ROMAGNETIC COMPATIBILITY 3 KEISER, B.: 'Principles of electromagnetic compatibility' (Artech House, 1987, 3rd edn.) 4 JOLLY, W.P.: 'Marconi' (Constable, London, 1972) 5 BRAXTON, T.E.: 'Selling EMC in a large organisation'. Proceedings of IEEE symposium on EMC, 1988, pp. 447-451 6 'Nature and characteristics of EM C'. USAF general information document AFSC DH 1~4, section IB, chap. 1 7 MORGAN, D.: 'An introduction to EMC'. Presented at lEE fourth vacation school on RF electrical measurernents, University of Lancaster, july 1979
13
8 FIELD, j.G.C.: 'Electromagnetic compatibility in warship design'. Proceedings of IERE symposium on EMC, April 1978 9 FCC Public Notice 2519, April 1990 10 FCC 54th annual report, fiscal year 1988 11 STAGGS, D.M.: 'Ethics within the corporate structure'. Proceedings of IEEE symposium on EMC, 1990, pp. 526-528 12 HILLIARD, D.E., DESOTO, K. E. and FELKERT, A.D.: 'Social and economic implications of EMC: A broadened perspective'. Proceedings of IEEE symposium on EMC, 1990, pp. 520-525
Chapter 2
EMC standards and specifications 2.1 The need for standards and specifications
TC62 and industrial process and con trol equipment was the responsibility of TC65.
2. 1. 1 Background
2.1.2 Contents of standards
As the use of electronic equipment grew and the need to allocate and protect the electromagnetic spectrum for communications became more importan t, there arose the req uiremen t to develop EMC regulations to ensure that an uncontrolled situation did not develop. Governments sought legislation through appropriate administrative departments, giving force to sets of standards which ensured that electromagnetic compatibili ty was managed properly in the design and use of certain categories of electronic equipment. The style and content of standards were usually characteristic of the nation which introduced them, although in many cases they were based on the, work of international bodies such as CISPR (International Special Committee on Radio Interference). This loosely co-ordinated national approach led to problems in the commercial trade of electronic equipment across national boundaries both within Europe and with the USA. The task of developing standards for the control of EMC can be said to have begun in 1934 with the formation of CISPR [1 J. The name CISPR is derived directly from the French, Comite International Special des Perturbations Radioelectriq ues, and was formed by several in ternational organisations coming together to institute a joint comnlittee to specify measurement methods and limits of radio frequency interference. Since 1950 CISPR has been a special committee under the sponsorship of the IEC (International Electrotechnical Committee) whose role is to issue international standards. CISPR has made considerable progress in developing methods of measurement and limits to deal with interference to communications equipment. More recently, the appearance of nonradio receiver interference phenomena led to the involvement of the IEC. In 1982 it was reported [2] that some 65 of the 200 committees were concerned in part with problems of EMC. For example, technical committee TC77 was concerned with power distribution networks, while TC18 produced the first EMC standard for electrical installations in ships (IEC pub. 533). Medical electronic equipment was covered by
The standards produced strive to ensure electromagnetic compatibility by requiring equipment designers to consider the subject as part of the design process from the earliest possible stage in the development of the product concept. They usually contain a section devoted to the definition of relevant technical terms used in the document and often specify the req uiremen ts for planning and project management of EMC test methods and specific test equipment specified limits which must be met specification for acceptable EMI measurement receivers are also referred to in these standards. As a good example of a well constructed standard one may refer to UK defence standard DEF STAN 59-41 [3J which is issued in five parts: 1 2 3 4 5
General req uiremen ts Management and planning procedure Technical requirements, test methods and limits Open-site testing Requirements for special EMC test equipment (draft) .
EMC standards usually undergo a process of evolution and updating to meet the developing needs of industry and society. They are produced after extended consultation between the regulatory authority and the supplying industry and other interested parties such as national standards institutes. They contain a list of definitions of words, phrases and technical terms related to EMC and present carefully ordered information with the aim of providing general guidance [4J and sometimes specific instruction in order to demonstrate compliance with the associated specification limits. An EMC specification usually contains numerical details and graphical representations of limits for measurable parameters such as radiated field strength or conducted interference current.
2.1.3 The need to meet EMC standards With the development of the EC as a trading entity, directives have been issued to harmonise 14
EMC STANDARDS AND SPECIFICATIONS
product standards in many fields. In the UK demonstrating compliance with EMC standards and related specifications for new and imported electronic products is a legal req uiremen t from 1996 onward. . EMC specifications are also invoked within commercial can tracts for the purchase of large equipment and military systems. They can go beyond. demonstrating compliance with the basic legal requirements for EMC and contain additional requirements that are specific to the equipment and the environment in which it will be operated. In this case, demonstration of compliance is normally required by the purchaser. Whether a manufacturer is satisfying the basic legal requirement or a more involved contractual one, the demonstration of compliance may be in the form of a technical dossier and/or an EMC test report. Such reports are normally produced by an independent test house, or certain specifications permit the supplier to self certify the equipment. In either case the measurements must be made in strict accordance with the test methods described in the specification and the test laboratory will be operated within strict quality guidelines and accredited by external q uali ty authorities such as NAMAS [5].
2.2 Civil and tnilitary standards 2.2.1 Range of EMC standards in use T'here are a large number of EMC standards and associated specifications in use in the world today covering an enormous variety of electrical, electromechanical and electronic equipment in various industrialised nations. Table 2.1 [1] contains a list of standards covering commercial equipment which are, or have been, in force in a selection of ind us trial coun tries. I t is inappropriate to discuss them all in detail in a book on EMC testing but some insights maybe gained into the nature of these EMC standards and the relationships between them by grouping them under the headings military standards and civil standards, as used in Europe, the USA, and other industrial nations. This section explores the nature of military and civil standards and specifications. Subsequent sections examine specific illustrative examples from the three groups.
2.2.2 Derivation of military standards The substantial differences between military and civil or commercial EMC standards and specifications are due to both equipment requirements and the environments in which they operate [6]. Generally, mili tary req uiremen ts are more wide
15
ranging and stringent than commercial ones, and thus more difficult and expensive to meet. As military standards cover more aspects of electromagnetic compatibility over wider frequency ranges than do commercial ones, many of the examples of measurement techniques discussed are taken from military standards. The aim of one set of military EMC standards is to ensure that mission success is not compromised by poor intrasystem control of spurious electromagnetic energy. Additional standards control intersystem EMC / ensure that individual systems do not compromise each other's performance in operation. _ Standards are imposed by military procurement authorities in the form of contractual conditions which companies tendering to supply equipments must meet. T'hey may insist on a contractor demonstrating his EMC project management, design, development and measurement capabilities prior to awarding a can tract. In the fu ture, can tracts for large commercial systems may also adopt _similar req uiremen ts as the need to achieve good management of EMC becomes more important. The concept that underlies intrasystem EMC standards is that if each identifiable electronics box and each electronic subsystem meets the standard then full system level EM C will be achieved. Writing specifications and defining test methods for measuring spurious and unintended emissions and unwanted responses is difficult and documents are written not in terms of wanted performance (as with most product design specifications) but by stating what may not be allowed up to a given level. A concept that is helpful in achieving system level EMC by specifying box-level limits is that of the source-victim margin. Each electronics box is considered both as a source of electromagnetic interference, either conducted or radiated and also as a potential victim of such interference. See Figure 2.1. The frequency, amplitude and modulation characteristics of interference sources and victims within each box are usually different. A matrix may be drawn up showing each box in a rank or file as a source and a victim. Any match in terms of frequencies, levels and modulation characteristics identifies the possibility of a system level EMC failure. See Figure 2.2. The key concept with these box-level military EMC specifications is to set the limits of emissions and susceptibilities such that there is a margin of safety between them. If this margin is achieved it is impossible for matches to be made in the cells of the matrix in Figure 2.2 and system level compatibility is ensured. Safety margins of 6dB
16
A HANDBOOK FOR EMC TESTING AND MEASUREMENT
Table 2.1 Examples oj electromagnetic compatibility regulations Istandards oj various countries Country
Ignition systems
CISPR
PUB 12
European norm
(72/245/EEC)
Australia
AS2557
Austria Belgium
Household electric applicances
Radio and TV/video
PUB II
PUB 14
PUB 13
PUB 15
PUB 22
EN55011
EN55014 EN60555-213
EN55013 EN55020
EN55015
EN55022
AS2064/2279
AS I044/2279
ASI053
AS2643
OVE-F65
OVE-F61/62
OVE-F61/62
OVE-F64
OVE-F61/62
o V E- F61/62/55022
RCI960/74/76
RCj6.6.1966
RC/1978/1983
Royal Dec. 1960
RC1978f1983
EN55022
Canada
SOR 75-629 CSAI08-4
SOR/75629 CSA22-1/3/5 CSAC235
CSAC I08-5-4 CSACI08-5-2
SOR/75-629 SOR/83-352
CSACI08-5
Brazil
RF equipment including ISM
Relevant CISPR PUBS
Fluorescent lamps and luminaires
Solid state controls
ASI054
IT & EDP equipment
EN55022
also IEC/TCn standards CSA22-4 VDI054
CSACI08-8
Czech.
CSN34-2875
CSN34-2865
CSN34-2860
CSN34-2870
CSN34-2850
Denmark
M04-j78 M03/83
M05689
M0416/NAHR3/2
M0396/NAHR4
M0416/ NAHR 5
NAHRI
M0416/568/396
Finland
PUB '1'35-65
PUB '1'33-86
PUB '1'33-86
T33-86/NAHR4
PUB '1'33-86
PUB '1'33
PUB '1'33-86
France
C91-103
C91-102 EN55011
NFC70-100 82/499/EEC EN55014 EN50006 EN6055.1
C91-104 C91-110 EN60555 82/499/EEC
NFC91-100 82/500/EEC EN55015
NFC91-100
NFC91-022 EN55022
Germany
72/245/EEC VDE0879
VDEOl60 VDE0750 VDE0871
82/499/EEC EN55014
EN55020 VDE0871 VDE0872 VDE0875 VDE0838
82/500/EEC EN55015 VDE0875
VDE0875
VDE0871
VDE087.1/6/7 VDE0838 EN60555
Italy
72/245/EEC
CISPRII
EN55014 82/499/EC CEIl 10-1 CEIn-1 EN50006 EN60555
CEIII0-3/4 EN60555
82/500/EEC EN55015 CEIII0-2
EN55022
Japan
CISPRI2 JR'rC/MP'r
RERART65 JRTC73/74
EA&MCLAW JRTC73/74/75 M P'f1970/71
EA&MCLAW J R'!'C 71 /74/75 &82 CISPRI3
EA&MCLA\V JR'!'C73/ 74/75 MPT1970/71
VCCI CISPR22
Netherlands
72/24·5/EEC NENI00I2
RCIOPREP NENIOOII EN55011 VDENI975
82/499/EEC RC402/1984 EN55014 NEN-EN50006 EN60555
NENJOOl3 EN60555
82/500/EEC RC401/1984 EN55015
RC/N.PNEP CISPR22 EN55022
New Zealand
CISPR 12 NFCCOM
RFS4·9-1
RFS49-1 AS2279 BS5406
RFS49-1 CISPR13
CISPR15
ClSPR22
Norway
Regs for motor vehicles 1969 43/6317 CISPRI2
NEMK0662/82 PUB. NO. 80POOS CISPRII
NEMK031/83 EN55014 NEN67-76 EN50006 EN60555
NAHR4 NEMK0661/77 LET-NO.32/83 EN60555
NEMK031/83 EN55015
South Africa
R2862-1979 (CISPRI6)
R2862-1979 CISPRII (CISPRI6)
R2862-1979 (CISPRI6)
R2862-1979 (CISPRI6)
SABS (CISPRI6)
Spain
UNE20505 UNE20503 CISPR12 (CISPR16)
UNE20506 CISPRII
UNE20507 EN55014
RC2704/l982 UNE20511 EN60555
UNE20510 82/500/EEC EN55015
United Kingdom
BS833 (CISPRI6)
BS4809 BS6662 EN75-31 BS4941 BS4999 ERP (SERIES)
82/499/EEC
82/449/EEC BS905 EN60555
82/500/EEC BS5394 EN55015 BS800/1983 BS6345
USA
SAEJ551C
FCC Ptl8 FCC MP-5 MDS2010004 NEMA ICS-2 I EEE518-1982 MIL STD461/2
FCC Ptl5 ANSIC63-2 ANSIC63-4 NEMA WD2-1970
FCC Pt2 FCC Ptl5/C
FCC Pt15.J FCC MD-4
EN55014 BS800 BS727 BS5406 EN60555
NEt\.1K031/83 (CISPRI4) NEMK0665-168
NEMK0662/82 NEMK0661/n NR33/83
NR32/83 EN55022 R2862-1979
SABS (CISPR22)
EN55022
BS800 Pt3
BS6527 EN5S022
FCC Ptl5J FCC MP-4
EMC STANDARDS AND SPECIFICATIONS MULTIPLE INTERACTIONS TAKE PLACE BETWEEN ALL UNITS
UNIT3 SOURCE 3 (f, I, m) VICTIM (f, I, m)
tI SUB-SYSTEMS or UNITS (1 • n)
SYSTEM
Each unit is a source and a victim with particular frequency level and modulation characteristics
Figure 2.1
Intrasystem-source-victim VICTIMS OF EMI V1
S1
~ w u.
S2
o
(f)
w
S3
V2
V3
V4
Vs
Vn
X X X
()
0:
::::>
o(f)
S4
X X
85
X
Sn
Figure 2.2
Intrasystem interaction matrix
are used in the derivation of MIL STD 1541 (USAF) [7] for the limits for power and signal cable interference levels, with a higher 20 dB margin for ordnance circuits. Established military specifications such as MIL STD 461 C (USA) [8] are often tailored to the specific requirements demanded by the procurement of a particular item of equipment, such as a naval aircraft. Without the ability to tailor general military EMC requirements it would be impossible to achieve electromagnetic compatibility for a unique system performing a specialist role in a particular environment at an affordable cost. In the case of commercial electronic equipment, specifications cover particular categories of related products. This approach does not involve incurring the unjustifiable costs imposed by meeting a general wide ranging specification. VDE 0871 [9] is an example of a commercial specification concerned only with ISM (industrial, scientific and medical equipment).
17
2.2.3 Derivation of commercial standards Regulations, standards and specifications relating to commercial electronic equipment are aimed at controlling pollution of the electromagnetic spectrum and protecting radio communications. The key intention is to limit the intersystem radia ted or conducted interference emissions from equipment to a level which does not cause problems for radio and television reception. An increasing number of commercial standards address the susceptibility or immunity of electronic equipment to electromagnetic threats. However, the majority are concerned with the control of unwanted emissions from the equipment. Commercial specifications are often limited to conducted interference limits below 30 MHz and radiated limits only above 30 MHz [6], since conduction along power or signal cables is more likely at the lower frequencies and radia tion is the more significant energy transport mechanism at the higher frequencies. The underlying assumption which determines specification levels for commercial eq uipment is that allowable emissions will be kept below the strength which will degrade radio and television reception based on practical receiving signal levels. Assumptions are made as to the probable distance separating a TV receiver and a radiating equipment such as a personal computer in a residential situation. This is taken to be between 3 and 30 m. I t is then possible to calculate the level of an interfering signal at test distances of 3, 10 and 30 m for an acceptable received signal-tonoise ratio. See Figure 2.3. Interference limits at the specified test distances are then issued together with careful test methods designed to determine whether an equipment meets them. The characteristics of appropriate measurement equipment are also specified, often based on CISPR standards. These documents may be referenced in the EMC regulations such as those E
/
= 300 - 1000 ~ V1m
Building attenuation
/ (
---d----3, 10, 30 meters
E dB ~ Vim
p'(gure 2.3
-
Required SIN ( Signal to noise) 20 - 40dB
+
Building attenuation 0- 10 dB
= Emission level limit at distance 3 -30 meters
Regulatory model for radiated emissions from electrical equipment
18
A HANDBOOK FOR EMC TESTING AND MEASUREMENT
required by the FCC in the USA e.g. FCC part 15J [10], the FTZ (Fernmeldetechnische Zentralamt or Central Telecommunications Office) in Germany e.g. VDE 0871/0875 [11] and in the UK by the DTI Radi.o Investigation Service [12,13] with specifications such as BS4809, BS800, BS6527, etc.
2.2.4 Generation of CENELEC EMC standards This section briefly considers the process by which the harmonised European Norm EMC standards (discussed in Section 2.3) are produced. These EN standards are now the most important for EC manufacturers and importers of electrical and electronic equipment. The European electrical standards committee is known as CENELEC and has been given a mandate by the EC to oversee the harmonisation of national standards and to prepare new ones when necessary. Technical committee TC 11 0 has the responsibility for EMC standards. EN standards are usually based on existing CISPR documents which may be familiar as they are similar to some British standards and German VDE emissions regulations. A simplified process of generating EN standards by CENELEC is shown in Figure 2.4. Government departments, trade associations and other interested bodies can stimulate and influence the appropriate British Standards committee to represent their interests via the national committee to CENELEC. Existing standards may be used or modified to generate a new draft EN standard. Once published for comment, the interested parties will reflect their views through the committee structure. A number of revisions may occur before the technical details and an agreed balance between the need for
EMC control and its economic impact on industry and commerce is achieved. Standards are continually developing and evolving and EMC designers and test engineers should keep themselves well informed of proposed introductions of and changes to EN standards.
2.3 UK/European com.m.ercial standards
2.3.1 UK standards relating to commercial equipment In the UK, EMC standards have been issued for many years by BSI. Examples of widely used EMC emission standards are listed in Table 2.2a together with equivalent EC harmonised European Norm (EN) standards where a ppropriate.
Table 2.2a EMC emission standards British Standard
EC equivalent
BS3GI00 pt 4 sec 2 BSG229 BS800 BS6527
EN55014 EN55022
BS905 pt 1 EN55013 EN55011 BS4809 BS5394
EN55015
BS5406
EN60555
BS833 BS1597
EN Documents
CISPR Standards IEC, ECMA, CEPT
Applicabili ty Equipment for use in aircraft Environmen t for aircraft equipment Household appliances Information technology equipment Radio and TV receivers Industrial, scientific and medical Fluorescent lamps and lighting Electrical supply networks Vehicles ignition systems Interference suppression in marine systems
There are also two widely used British Standards which relate to the susceptibility or immunity of commercial equipment to interfering signals. These are given in Table 2.2b. Other interested boards
Figure 2.4
Process oj generating EN standards by CENELEC
Table 2.2b EMC immunity standards British Standard
EC equivalent
A pplicability
BS905 pt 2 BS6667
EN55020 HD481
Broadcast receivers Industrial process control
EMC STANDARDS AND SPECIFICATIONS There are two other widely used UK standards which contain immunity limits:
19
surface of a solid metal ground plane as in Figure 2.5 1 m from the antennas. The technical details of antenna types, peak detectors, narrow/broadbandwid ths and screened rooms are all addressed and explained in the appropriate chapters. As an example, the radiated emission field-strength limits for this standard are shown in Figure 2.6. (For an explanation of the terms narrowband and broadband/referred to in this Figure see Appendix 1.1.) This BS3G 100 test may be contrasted with the arrangemen ts for measuring radiated emissions from data processing and electronic office equipment as specified in BS6527/EN55022 where the E UT is placed at 2. distance of 30 or 10m (depending on whether the equipment is for commercial or domestic use) from a measuring antenna on an open field test site. See Figure 2.7 for a plan of the test site. No screening of ambient signals is available on an open test site other than that afforded perhaps by nearby hills. Such a facility will preferably have been located in a radio quiet environment but background/ambient RF signals inevitably intrude on the measurement which can slow down the testing while they are identified and marked. The specified measurement antenna to be used in open range testing is a resonan t length balanced dipole for frequencies above 80 MHz and fixed at 80 MHz resonant length for frequencies below 80 MHz. The need to manually adjust the dipole at each test frequency is time consuming and slows down the test. Broadband an tennas such as the bow tie and log conical spiral used in BS3G 100 are sometimes permitted to speed up the measurement.
BS3G 100 pt. 4 sec. 2 'Equipment for use in aircraft' also contains limits for equipment susceptibility. NW0320 (National vVeights and Measures Laboratory) is concerned with 'Weighing and measuring equipment immunity to electrical disturbances' and sets limits for electromagnetic radiation, magnetic induction fields, electrostatic discharge, power line transients and radiated interference. The characteristics of special test receivers which are used to make interference measurements as required by the emission standards are themselves subject to another British Standard: BS 727 'Radio interference measurement apparatus'. This standard is in line with the requirements of CISPR publication 16.
2.3.2 Comparing tests E.ach emission or immunity standard specifies particular test methods and limits and in general it is difficult to read across from one standard to another. For example, the radiated emission measurements in BS3G 100 are conducted with a monopole, a bow tie (broadband dipole) and a logarithmic conical spiral antenna connected to a peak measuring receiver covering a frequency range from 150 kHz to 1 GHz. All measurements are made inside an RF damped screened chamber to eliminate ambient noise, and the EUT (eq uipment under test) carefully set out on the
Secondary power lines to load NORM~LOR SIMULATED
Power inputs
INPUT & OUTPUT CIRCUITS
Equipment under test
~
Monopole antenna 14 kHz - 30 MHz
90 em
Figure 2.5
Log conical spiral antenna 200 MHz - 1 GHz
\
Metal plate not less than 30 cm square
Typical radiated emission test corifiguration
Reproduced by permission of BSI
20
A HANDBOOK FOR EMC TESTING AND MEASUREMENT -g
1000: 61
;- 60
o
.0
~g 50 =.c 40
40
~
30
30
i
20
~
10
20 .:~ 10 .8 o
"8
~
50
N
~
E :>
::l.
co
co
'"C
'"C
1.0
10 FREQUENCY MHz
Figure 2.6
BS3G100 radiated emission limits
Reproduced by permission ofBSI
J o o e - - - - - - - major diameter = 2 F - - - - - - - - l ,.,....-
------T-----... I
1
/
I
,,
...........
--.--
~
\
F=10-30m.---7E:
RECEIVING AERIAL
~,
2.3.3 European commercial standards
........ ,"'-
1
j(
I \
-- ..........
minordiameter=V3F
///
-/-
)
EUT (equipment / under test ) / / //
--- ----,...,
Boundary of area defined by an ellipse
Volume above earth to be free of reflecting objects
Figure 2.7
Plan view of open range radiation test site
Reproduced by permission of BSI
Other differences between BS6527 and BS3G 100 relate to the movement of the measurement antenna to obtain maximum received interference signal strength and the use of a quasipeak detector in the receiver. The Class A limits for the field strength of radiated spurious signals in the frequency range 30 MHz to 1000 MHz Test distance ( F )
Frequency range
m
MHz
Quasi-peak limits dB
(~V/m)
30
30 to 230
30
30
230 to 1000
37
Class B limits for the field strength of radiated spurious signals in the frequency range 30 MHz to 1000 MHz Test distance ( F )
Frequency range
m
MHz
10
30 to 230
10
230 to 1000
Figure 2.8
Quasi-peak limits dB
(~V/m)
30 37
BS6527 radiated emission limits for open site testing
Reprod uced by permission of BS I
radiated emission field strength limits for BS6527 are shown in Figure 2.8. A quick comparison of these two standards clearly shows the difficulties that will be encountered in trying to read across between measurements made under different standards, even within the same country. Great care should be exercised in comparing standards, particularly if the comparison is being made between UK and other national standards for the purpose of certifying compliance when exporting equipment abroad. It is partly to overcome such problems that the EC has pressed forward with a programme of harmonisation of standards including those relating to EMC.
In the early 1970s there was a wide diversity of national regulations in Europe, some of which were related to CISPR standards but many were particular to a given country. This situation led to problems with free trage in electronic goods across national boundaries and gave rise to various public purchasing and subsidy policies which distorted fair competition [12]. With the signing of the Single European Act in 1985 governments were committed to a single European market which was to be achieved progressively by the end of December 1992. One of the main tasks was to remove technical barriers to free trade and this was to be achieved by harmonising technical standards which products must meet within all member countries. E:rvIC standards came to be harmonised under Directive 89j336jEEC of the 3rd May 1989 [14] (amended by 91j263jEEC and 92j31jEEC). These are complex and legalistic documents containing many articles dealing with issues such as definitions, applicability, release for sale, recognition of special measures in member states, relevant national EMC standards and declaration of conformity. The scope of the directive includes almost all electrical and electronic appliances, equipment, systems and installations which are brought into service within the community. The directive has been implemented in UK law by the Electromagnetic Compatibility Regulations 1992 (SI2372). From October 1992 manufacturers of electrical and electronic products governed by the regulations have the choice of either following the European Community regime, or continuing to comply with the existing national 'legislation in force' on the 30th June 1992 in the member states in which the product is to be marketed [15]. Existing UK legislation on EMC is listed in Appendix 1.2.
EMC STANDARDS AND SPECIFICATIONS
From 1st January 1996 most electrical and electronic products made or sold in the UK (including imports) must meet the requirements of the EC directive on EMC. Failure to comply will become a criminal offence and the product will be prohibited from sale in the EC. Most electrotechnical products are subject to these regulations; there are however a number of general and specific exclusions which are listed in Appendix 1.2. I t is envisaged that demonstration by measurement that the equipment conforms to a relevant harmonised EMC standard will be the normal means of complying with the directive. For some equipments it may be appropriate to submit a 'technical file' or for radio communication transmitting apparatus by EC-type examination. There will be cases where a manufacturer considers that it is inappropriate for equipment to be assessed against such a standard. I t is possible that no suitable standard exists. In such circumstances the assessment of compliance with the directive shall be by means of the production of a technical file and the involvement of a competent technical body designated by the Department of Trade and Industry, possibly a NAMAS accredited EMC design/test house. The directive permits the manufacturer to demonstrate compliance by self certification in its own test laboratories. Such facilities would usually have EMC accreditation by a body recognised for the certification of such laboratories. In the UK this would be a NAMAS approval. Listed in Table A1.3.1 in Appendix 1.3 are some of the European EMC standards, their applicability and equivalent national or other standards. Reference is also made to the closest equivalent US standard. I t is, however, dangerous to assume an exact read across to the US FCC standards as many detailed differences exist in test methods frequency ranges and limits. I t will be evident from comparing the two parts of the Table that there are no current equivalent FCC EMC immunity standards in the USA, though these migh t be introd uced for information technology equipment. Tables Al.3.2 and Al.3.3 of Appendix 1.3 contain lists in number order of EC EN standards [16, 17] which have been referenced in the official journal of the European Communities and are therefore notified for use in the self certification route to compliance with the EC EMC directive. Table A 1.3.4 [16] lists a number of proposed product specific EMC standards on which CENELEC is working (in 1993). The committee
21
aims to introduce these standards before the EC EMC directive becomes mandatory in 1996. The EMI limits and test methods recommended by CISPR have been adopted as the basis for many EN standards, European countries national standards and EMC standards throughout the world. Table A1.3.5 [1] in Appendix 1.3 lists the relevant CISPR documents. CISPR standards call for a quasipeak (QP) measurement of interference as this yields a result which is proportional to the su bjective annoyance effect experienced by radio broadcast listeners. The characteristics of the quasipeak detector as specified by CISPR publication 16 are given in Table 2.3.
Table 2.3 Characteristics of CfSPR EMf meters Frequency range 10 kHz150 kHz Electrical charge time constan t, ms Electrical discharge time constan t, ms 6 dB detector bandwidth, kHz Meter time constant, ms Predetection overload factor, dB Postdetection overload factor, dB
150 kHz- 30 MHz30 MHz 1 GHz
45 500
160
550
200
9
120
160
160
100
24
30
43.5
12
12
6
Similar specifications for EMI measurIng receivers exist in the USA but they are not identical to those of CISPR. The equivalent US ANSI (American National Standards Institute) C63.2 for measurement equipment contains a value for electrical discharge time constant of the q uasipeak detector in the freq uency range 150 kHz to 30 MHz of 600 ms rather than the CISPR 160 ms.
2.3.4 German standards Prior to the introduction of the EC EMC directive, many designers and exporters of electrical equipment paid particular attention to the mandatory national requirements which existed in West Germany. The VDE (German Institute of Electrical Engineers) standards called up in the national laws on EMC, promulgated by Vfg (Decree Verfugung) and issued by BPM (Deutsche
22
A HANDBOOK FOR EMC TESTING AND MEASUREMENT
Bundespost) are amongst the most stringent in the world. Therefore demonstrating compliance with the VDE limits would almost always assure technical compliance in most other countries. Table Al.4.1 [18] in Appendix 1.4 lists the important Vfg decrees issued by the BPM and the related VDE standards. In Germany the FTZ (Central Telecommunications Office) have the role of EMC adluinistration. This is comparable with that of the FCC in the USA. Because EMC standards are not yet fully harmonised at an interna tional level there have been many cases where a product which meets the FCC limits for a particular category of equipment has subsequently failed to meet the appropriate VDE limits. This can be due to the equipment designer failing to understand fully the differences which exist, both in the- general approach and in specific test methods between the FCC and VDE standards. It is therefore interesting to examine some of the similarities and differences between these two important EMC standards as the FCC rules apply throughout the USA and the VDE specifications have contributed to the basis for the harmonised European standards. There is one major feature which is common to both the FCC and VDE standards. Equipment is divided into different classes depending partly on operational use. For the US FCC part 15j regulations, classes are defined as:
Class A: equipment which is used solely in commercial environments where separations between equipment will normally be tens of metres and the professional eq uipmen t operators will have some incentives to position their equipments so as to cause the minimum interference to neighbours. Class B: equipment which can be used in either a commercial or a residential environment. The applicable limits are for emission only and are divided into conducted emissions below 30 MHz and radiated emissions above 30 MHz. These limits are given in Figure 2.9. For VDE 0871/0875 regulations:
Class A: Normally for commercial equipment requiring a test by VDE and individual permit from FTZ/ZZF (RFI registration office). Class B: General equipment for unrestricted distribution after self certification. Class C: A special on site test provision for large 'one of a kind' installations. Since the class A limits are less stringent than the
80 70~
I Radiated emissions at 3 meters
-
FCC Class A
E
>
•
50~
FCC Class B -
::1.
to
'0
-
301I-
I
10 10
100
1000
FREQUENCY MHz
100 Narrowband conducted emissions
90
FCC Class A
----.r--
> 70 ::1.
to '0
FCC Class B
50
30
.01
0.1
1
10
100
FREQUENCY MHz
Figure 2.9
FCC limits for radiated and conducted emzsszons
class B limits (see Figure 2. 10), the VD E is more particular about the testing and accompanying documentation for equipment tested to class A standards. This is the opposite of the FCC approach where class B products require FCC authorisation and class A products are self certified for compliance [18]. In Germany, equipment which is in the less stringent class A had to be tested by the VDE itself or at a VDE approved laboratory and additional paperwork was submitted. Products which fall within the more stringent class B can be self certified. Manufacturers may therefore choose to put their commercial eq uipment into class Band suppress their emissions to meet the lower levels to save test time and money by self certification and bring products to the market in the shortest possible time. Manufacturers who sell both to the USA and Europe, including Germany may opt to meet the class A limits of the FCC (easier to test) for the USA market and VDE 0871 class B limits which are easier to test and market in Europe. Lohbeck [18] gives an excellent precis of the main differences between FCC part 15j and the FTZ (VDE 0871 and 0875) standards. See Table Al.4.2 in Appendix 1.4.
EMC STANDARDS AND SPECIFICATIONS 100
r__---__
---~r__---......__---_
Narrowband conducted emissions
80
""
> ::l
CD "0
"",
._----
---,
60
VDE Class A VDE Class B
40 0.01
0.1
1
100
10
FREQUENCY MHz
80
_--------r--------_ Electric field VDE Class A
60 E
>
et
40
"0
VDE Class B
20 10
100
1000
FREQUENCY MHz wI"--
(l')LO
00
, ' g~
00
120-....~---
103.5 100 91.5 80
~
0'~140625 Z~A~25 109375 06375
63.5 60 ::l 51.5
CD
"0
40
41
± 001 .
078125 0625
20
.01
0.1
1.0
10
30
FREQUENCY MHz
Figure 2.10
VDE 0871 conducted and radiated limits
2.4 US commercial standards
2.4.1 US organisations involved with EMC FCC, the Federal Communications Commission, is responsible for spectrum allocations outside the government sector. NTIA, the National Telecommunications and Information Agency, has a committee named IRAC (Interdepartmental Radio Advisory Committee) which is responsible for spectrum allocations within the federal government sector. NASA is concerned with the EMC aspects of
23
equipment and systems for use in aerospace vehicles. NCMDRH, the National Center for Medical Devices and Radiological Health, is concerned with the safety aspects of both ionIsIng and nonionising radiation produced by electrical and electronic products. NBS is renamed the National Institute of Science and Technology (NIS1-'), concerned with metrology in general, of which EMC measurement is a part. NVLAP, the National Voluntary Laboratory Accreditation Programme, is administered by NIST and provides confidence that EMC testing carried out by laboratories in the USA within the progralume are to the desired q uali ty standard. Industrial organisations concerned with EMC standards and measurement within the US include: EIA, the Electronics Industries Association, IS concerned with equipment EMI. IEEE is concerned with EMC standards. SAE, the Society of Automotive Engineers, plays a special role in respect of both air and land vehicle EMC. For example, SAE Practice ARP-937 relates to jet engine EMI and applies to electrical systems and accessories, ignition systems, actuators, fuel con troIs, solenoids, servo con trol, electrical alternators, etc. This SAE document covers conducted and radiated emission and conducted susceptibility requirements. R TCA, the Radio Technical Commission for Aeronautics, issues documents on the airborne equipment environment, which includes sections on EMC and lightning. ANSI, the American National Standards Institute, is concerned with measurement techniques including those for EMC.
2.4.2 FCC requirements The involvement of the FCC in the realm of EMC began in 1934 when the Communications Act of that year gave the FCC the authority to impose rules and regulations on industrial, commercial and consumer devices which could radiate electromagnetic energy. The significant parts of the rules and regulations with regard to EMC are set out in title 47, parts 15, 18 and 68 of the US Code of Federal Regulation (also known as FCC docket 20780). A list of the parts relevant to EMC is given in Table A1.5.1 of Appendix 1.5. FCC part 15j covers any device that intentionally generates and uses electrical energy in excess of 9 kHz or 9,000 pulses per second for
24
A HANDBOOK FOR EMC TESTING AND MEASUREMENT
computation, control, operations, transformations, recording, filing, sorting, storage, retrieval or transfer of data. The rules encompass computing device peripherals but exclude transmitters and receivers and devices covered by other FCC regulations, computing devices used in transportation vehicles [19J, and control or power systems used in public utilities and a range of other ISM and household equipment that are covered by other regulations. FCC part 15 regulations address only emissions; however, the laws provide for susceptibility testing as well. The FCC has chosen not to mandate susceptibility or immunity limits for commercial equipment as it prefers to leave this responsibility to the manufacturer whom it supposes to have a self interest in the compatibility of its equipment with the EM environment in which it is to be operated. The FCC has five administrative arrangements for dealing with equipment that emits nonionising radiation including both intended and unintended emissions: (i)
(ii)
(iii) (iv)
(v)
Type acceptance is based on information and test data supplied by the manufacturer or licensee to the FCC, which may then choose to test the item. Type approval is granted when an equipment has passed the specified FCC test. Tests are carried out by the FCC. Certification is similar to type acceptance, however, no licence is held by the user. Verification, when the FCC carries out spot checks to ensure that the manufacturer's self testing has been suitably carried out. Notification. In this procedure the FCC may not require detailed data to accompany an application.
The applicable arrangement will depend on the nature of the equipment for which EMC clearance is being sought.
2.4.3 Other US commercial standards A list of some examples of the standards prepared by bodies other than the FCC is given in Table 2.5.2 of Appendix 2.5.
2.5 Cornm.ercial EMC standards in Japan and Canada These two countries are examples of important producers and markets for electronic products and both have laws or voluntary regulations governing EMC.
2.5.1 Japanese EMC standards Japan has had the Electrical Appliance and Material Control Law for many years which is concerned with regulating EMI from household electrical appliances, electrical tools, fluorescent lamps, radio and TV receivers. The radio equipment regulations, article 65, is concerned with radio frequency equipment including ISM. The measurement equipment and techniques are defined by the JRTC (Japanese Radio Technical Council) of the Ministry of Posts and Telecommunications (MPT). In 1985 the Japanese Electronic Industry Development Association, the Japan Business Machine Makers Association, the Electronic Industries Association of Japan and the Communications Industry Association of Japan came together at the request of the MPT to form the Voluntary Control Council for Interference by Data Processing Machines and Electronic Office Machines, known as the VeeI [24]. The goal of the VCCI is 'To take voluntary control measures against electronic interference from data processing eq uipment and electronic office machines, and thereby contribute to the development of a socially beneficial and responsible state- of affairs in the realm of electronic data processing equipment inJapan'. Membership is not limited to Japanese companies and is open to foreign organisations. Some overseas manufacturers of electronic equipment have found membership of, and compliance with, the VCCI beneficial in competing in the Japanese market place. Members must test the product for conformance and submit a report in Japanese to VCCI before placing the equipment on sale. When the report is registered a certificate of acceptance is awarded to the company for that product. There are two classes of equipment specified in the VCCI standards: Class 1: information technology equipment (ITE) used in industrial and commercial applications Class 2: ITE used in residential situations. The limits applied to the two classes for equipment manufactured after December 1989 are equal to those specified in CISPR 22 [25 J. The VCCI limits for radiated emissions are given in Figure 2.11 and those for conducted emissions in Figure 2.12. These VCCI limits are similar to the FCC and VDE limits but there are differences in frequency coverage, test methods and the arrangement of the equipment under test. A limi ted comparison of the test methods set out in
EMC STANDARDS AND SPECIFICATIONS
80
I
Radiated emissions at 3 meters ~
E
60
> ::t
CD 'U
-
-
VCCI Class 1 _
-
VCCI Class 2. _
40
-
20
:~
-
-
I
1000
100
10
FREQUENCY MHz
VCC! radiated emission limits
Figure 2.11 100
Narrowband conducted emissions
80
VCCI Class 1 Quasi-peak limits
> VCCI Class 1 Mean limits
::t
CD
'U
60
40 1
0.1
10
100
FREQUENCY MHz
1 0 0 - - - - - - - - - - - - . . - - - - - -.......... Narrowband conducted emissions
80
>
'-
::t
CD
'U
60
VCCI Class 2 Quasi-peak limits
"'-. ,
I
......-----V-C~C..I Class 2 Mean limits
40 0.1
1
10
100
FREQUENCY MHz
Figure 2.12
VCC! conducted emission limits
the three standards has been made by Hoolihan et al. [24J, see Table A1.6.1 of Appendix 1.6.
25
Communications announced that they intended to regulate radio frequency electromagnetic noise emissions from digital electronic devices. All such devices manufactured or imported after 31st January 1989 must have been tested and shown to comply with the new regulations [26J and carry an identifying mark to that effect. Under these new regulations products are divided into two groups: class A, industrial and class B, residential. The class B limits are set at a lower level of tolerable emissions than class A and in this respect are the same as the FCC and VDE standards. As in the USA there are some products which contain digital devices which are exempt from the regulations. For Canada these include Transportation vehicles Public utility or industrial plant Test equipment in industrial, medical or commercial environments Certain motor driven domestic appliances Some medical equipment and monitors Some central office telephone equipment Systems using radio transmitters and receivers. The Canadian EMC regulations are referred to by the publication number SOR/88-475 [27]. The associated testing procedures are given in CSA standard C 108.8-M 1983 [28J. The testing procedure is similar to that defined by the FCC, but it is not identical [29J. The Canadian Department of Communications (DOC) makes the important stipulation that a product tested and shown to be compliant with the FCC regulations need not be retested. The FCC report will be accepted as proof of compliance provided that a note is attached indicating that the results are considered to be satisfactory proof of compliance with the Canadian regulations. Compliance with the DOC regulations will be verified by the Canadian authorities if a complaint of interference caused by the product is received, investigated and subsequently confirmed. The applicable limits for radiated emissions 30 MHz-1 GHz and the conducted interference limits from 450 kHz-30 MHz are given in Table A1.6.2 of Appendix 1.6.
2.6 Product safety
2.5.2 Canadian EMC standards Canada has legal requirements for EMC covering vehicle ignition systems, radio frequency equipment including ISM and TV receivers under regulation SOR/75-629 1975. Voluntary compliance to CSAC108.5 standard has been sought for products such as household electrical appliances and fluorescent lamps. In January 1987 the Canadian Department of
2.6.1 Safety of electrical devices Electrical product safety and the safety of humans and ordnance devices when exposed to high power electromagnetic radiation is not the primary subject of this book. However, these topics are related to EMC, and often both safety and EMC specifications must be met simultaneously in a single equipment design.
26
A HANDBOOK FOR EMC TESTING AND MEASUREMENT
Occasionally, the design that gives the best EMC resul ts might be in contraven tion of the electrical safety rules. This can occur, for example, when considering the optimum grounding or earthing policy for an equipment where a safety earth can act as an effective radiating antenna.
2.6.2 Product safety From the manufacturer's point of view there are a number of legal and commercial advantages to be gained by designing and testing equipment to meet the widest range of electrical and electromagnetic safety regulations which exist world wide. He can thereby ensure that his equipment is operable and reliable within the boundaries of recognised minimum safety standards [30J. Safety agency periodic audits of approved products will also encourage manufacturers to maintain their product safety quality control during production. Probably the most important benefit to be gained by meeting recognised safety standards is the almost certainly reduced liability judgements that may be made against a manufacturer in the event of safety litigation. For example, in Germany when equipment carries a separate VDE or TUV [31J laboratory safety mark (see Figure 2.13), then liability of the manufacturer in the German courts is minimal. When the approving agency mark is applied to the product, this places the burden of proof on the end user not the manufacturer.
is human. There are diverse opinions with regard to safe levels of exposure. In the West the generally accepted criteria for exposure to CW irradiation are based on the thermal effects genera ted wi thin the body by the absorption of RF energy. The approach in the former USSR to human exposure seems to be based on the effects of RF energy on the central nervous system, which are stated to occur at between one to two orders of magnitude lower power density than for thermal effects [32]. There are little data available in any country on the effects of intense short pulses or on chronic exposure at low levels. Concerns are beginning to be expressed about safe levels of low frequency magnetic fields at power line frequencies [33]. l'he biological effects of exposure to electromagnetic fields are likely to become a significant technical, legal and social issue in the 1990s. In a modern ind ustrial society many millions of people may rou tinely be exposed to RF radiation of some kind during their occupation. Typical categories of workers are listed in Table 2.4 [34]. Table A1.7.2 of
Table 2.4 Industries involving use oj electromagnetic energy Sector
Application
Industrial
Food preparation by mIcrowaves India rubber processing Adhesive processing Heating or fusion of metal and crystals
Medical
Diathermy Magnetic resonance imaging Computer imaging of x-rays Ultrasound procedures H ypothermy-sensi tising malignant tumours
Military
In the USA, the Underwriters Laboratories and in Canada the Canadian Standards Association are the recognised agencies for safety testing of electrical products. Typical national and international electrical product safety standards are given in 1'able A1.7.1 of Appendix 1.7 and EMC engineers should be familiar with these technical requirements as design measures which ensure product safety will always override those for electromagnetic compatibility if a choice has to be made.
Communications Radio navigation Aviation Preparation of precooked foods Weapons guidance and triggering devices Radar systems Radio communications
Research and scientific applications
Food processing Plastics Chemical applications Metal stress tests
2.6.3 Radiation hazards to humans
Communications
Radio stations Portable transmitters Satellite hookups
Figure 2.13
Product safety mark
A special case of electromagnetic compatibility exists when the potential receptor or victim system
EMC STANDARDS AND SPECIFICATIONS 1Q3------IUK MoD(1982)
r-'-..- -..\ l:fSARMV"1L-------.., I STANAG 2345
:
I STANDARDI (WORKERS AND I POPULATION)
NEW(1982~ANSI
1()2 US AIR
:
FOR~~~.~.~.~~~.~.~. ~.~11
E ~
E
~
T -\
NIOSH 101 STANDARDS (WORKERS)
~
~\.
"'~
ffi
0.
" 10-1
/
.... /
-I , r -
\\..... _ ..... _ ..... _ ..... /'.... \ NRPB(1989) / /
~
~
/'7
ART. 5
\.
1 INIRC (1988>"
I US NAVY
llj--:r;;EvIOUS ANSI STANDARD :' .....tl'J:.::lbln.:r!:._._._i::n:.lI:'l:'l:l'\I.lII'I:Ill'I:_ _ .::nt. EEC
\.
~
HUMAN BODY RESONANCE REGION
'-- __ -0/ ~~--'
SOVI ET WORKER STANDARD
UK NRPB (1986) PUBLIC ACCESS
5 HOURS/DAY 10-2
10·3 1--_ _--'0.1
L..---'-_........_ L - - - - L_ _----'
10
100
1GHz
100Hz
FREQUENCY MHz
Figure 2.14
Comparison of electromagnetic wave exposure standards
Appendix 1.7.7 contains examples of standards regulating human exposure to RF energy. A comparison of the limits contained wi thin some of these standards is given in Figure 2.14 including those proposed by INIRC (1988). Safe levels of exposure were recommended by the NRPB (1986) in a consultative document which was revised in 1988 and issued as NRPB-GS 11. The recommended reference levels are gIven In Table A1.7.3 of Appendix 1.7.
2.6.4 Hazards of electromagnetic radiation to ordnance The history ofRF ignition hazards goes back to the First World War when experiments showed that cotton bales could be ignited by RF power induced into the restraining metal bands [41]. The high-power RF hazard has long been recognised in the USA and studied by a variety of agencies [42-45]. Work in the UK by Excell [46] assisted in updating BS4992 (1974) 'Guide to protection against ignition and detonation initiated by radio frequency radiation'. BS4992 has now been superseded by BS6657 'Prevention of inadvertent initiation of EEDs (electro explosive devices) by RF radiation', and BS6656 'Flammable atmospheres and RF radiation'. In BS6657 a comprehensive list is given of minimum safe distance for EEDs from transmitters employing various powers, polarisations and modulation
27
types. In Germany, the standard for RF ignition hazards is VDE 0848 (DIN57848 issued by the German standards institute). There is also an international standard issued as IEC 79-2, 'Electrical apparatus for explosive gas atmospheres'. The RF ignition of ordnance (HERO) is a safety-cri tical feature of military eq ui pmen t which con tains electrically triggered ini tia tors (EEDs). They may be used to activate switches, separation fastenings and set off explosives or rocket motors. Accidental ignition is clearly a serious issue bu t if the explosive mixture is heated and not detonated by currents induced into the firing fuse in the EED this can render the explosive chemicals unresponsive to a subseq uent real firing pulse. This is called d udding and is also a concern for the safe operation of the system. In the UK, the pertinent regulations posted as Ordnance Board (OB) Proc.41273 [47], are now superseded by OB Proc.42413 March 1986. EMC engineers are often called on to design and test EED circuits as part of the wider system and they should be familiar with the appropriate regulations.
2.7 ESD and transients 2.7.1 ESD (electrostatic discharge) With the conjunction of the spread of digital instrumentation, process control equipment and general information technology in to office buildings and industrial plants fitted with synthetic materials and h umidi ty controlled air conditioning, a particular type of transient EMI became a problem [48]. Digital equipment is inherently sensitive to short high-level widebandwidth voltage transients caused by the discharge of electrostatic energy which can easily build up on people, chairs and tables etc. in environments with abundant insulators (plastics and manmade fibres). These transients can cause digital data disruption, microprocessors to change function and even to lock up. ESD and fast transients are becoming a more important part of EMC as the need increases to understand and con trol this phenomenon across a wide range of equipment. The measurement procedure defined in BS6667 Part 2 (and in IEC80 1 Part 2) recommends test levels of between 2 and 15 kV using a specially designed spark discharge probe. See Figure 2.15. The in ten tion is to simula te the discharge of static electricity built up on an equipment operator into sensitive circuits via conducting metalwork.
28
A HANDBOOK FOR EMC TESTING AND MEASUREMENT ESD GENERATOR PROBE
I I
MAINS OUTLET WITH EARTH TERMINAL ~
PROTECTIVE CONDUCTOR
/ EARTH REFERENCE PLANE PLANE
CLAMPING DEVICE
Figure 2.15
Typical ESD test
Reproduced by permission of' BSI
2.7.2 Transients and power line disturbances Immunity to fast transients due to switching on mains supplies is also increasingly important with the widespread introd uction of semiconductors and digital microprocessors into domestic, commercial and industrial equipment. It is only relatively recently that international agreement has been obtained with regard to suitable test methods and limits. The methods call for transients to be induced into the equipment mains leads directly or via a distributed capacitance. The characteristics of the transients which must be injected are given in IEC 801 part 4 and have a risetime of only 5 ns. The regulation specifies Risetime Pulse length Burst duration Burst period Rep. frequency (within burst)
5 ns ± 30% 50ns 15 ms 300ms 5kHz (for amplitude up to 1 kV) 2.5 kHz (above 2 kV)
for military electronic equipment have been developed around them. Because of this, many of the examples of the wide range of EMC measurement techniques discussed in this book are described with reference to these standards. Some of the EMC tests included in these standards are not yet required for commercial electronic equipment. With the inevitable proliferation of equipment in the future, and the consequent increase in the importance of EMC, adaptations of some of these tests which currently apply only to military systems may become relevant to commercial equipment. MIL STD 461/2/3 documents and their successors are issued by the US Department of Defence and are now used around the world. MIL STD 461 A entitled 'Electromagnetic interference characteristics requirements for equipment' and first issued in August 1968 aims to ensure that interference control is considered and incorporated into the design of equipment and subsystems. I t also provides a basis for evaluating the electromagnetic compatibility of these equipments and subsystems when operated in a complex electromagnetic environment. The standard is concerned with both radiated and conducted emissions and susceptibility of an equipment as illustrated in Figure 2.16. The associated standards are MIL STD 462, 'Measurement of electromagnetic interference characteristics', first issued in July 1967, and MIL STD 463, 'Definitions and systems of units for electromagnetic interference technology'. These can be applied to single or multiservice procurements and are called up in the equipment specification and contract. Compliance with the requirements of these standards does not guarantee that the equipment will not suffer from any EMC problems, but it should ensure that they are minimised without incurring excessive costs for over protection.
There are also standards which specify the limitations of disturbances caused in electricity supply networks by domestic and other equipment; BS 5406 (1988) or IEC 555 are examples. The practising EMC engineer must be aware of these power line transients and ESD specifications and may be called on to design and test systems to which they are applicable. emIssIon
2.8 US tnilitary EMC standards
I. ¥ h
INTERCONNECTING CABLE
POWER CABLE
2.8.1 MIL STD 461/462/463 These standards are among the most formal, technically comprehensive and widely used of all EMC standards. Many other tests, particularly
"7
Conducted susceptibilIty
f-'igure 2.16
Unit- or box-level en7ission and susceptibility testing (MIL SYD 461)
Reprod uced by permission of' ICT Inc.
EMC STANDARDS AND SPECIFICATIONS MIL-STD 461A 1st August 1968
NOTICE 1 CORRECTIONS 2nd February 1969
NOTICE 2 (AIR FORCE) 20th March 1969
NOTICE 3 (AIR FORCE) 1st May 1970
NOTICE 4 ( ARMY) 9th February 1971
NOTICE 5 ( MOBILE ELECTRIC POWER) 6th March 1973
NOTICE 6 ( CORRECTIONS TO NOTICES) 3rd July 1973
Figure 2.17
MIL SYD 461A and revision notices
Reproduced by permission or ICI' Inc.
MIL STD 461A has been the subject of many revisions and amendments over the years and this is illustrated in Figure 2.17. Similarly, revisions have taken place to the testing methods and limits in MIL STD 462. See Figure 2.18. Many of the amendments have been concerned with procuremen ts for a single service (army, navy or airforce). The test req uirements are organised logically and can be represented as shown in Figure 2.19 in the form of a flow chart or tree showing the successive subdivisions into emissions and susceptibility, then conducted and radiated mechanisms for each, and so on. Not all of these tests need be performed on all equipment which falls within the MILSTD462 31st July 1967
NOTICE 1 CORRECTIONS 1st August 1968
NOTICE 2 ( AIRFORCE ) 1st May 1970
NOTICE 3 (ARMY) 9th February 1971
Figure 2.18
MIL SYD 462 and notices
Reproduced by permission or rCT Inc.
29
general regulations. Equipments are grouped into classes and further divided into subclasses. See Table A1.8.1 of Appendix 1.8. Only certain tests are carried out within a given class as illustrated in Table A1.8.2 of Appendix 1.8. In 1980 a major revision to MIL STD 461A was issued as MIL STD 461B. It was the outcome ofa significant change in the philosophy which underpinned the intentions of the regulations. All equipment purchased for the US military was required to meet EMC specifications derived from the equipment type or class and the intended installation and criticality of the equipment to 'mission success'. Thus MIL STD 46IB is sectioned into parts 1 to 10 which correspond to the types of installations in which the equipment will be used. See Figure 2.20. The relationship between the equipment classes and subclasses to the numbered sections or parts of 46IB is shown in Table A1.8.3 of Appendix 1.8. The inclusion of the impact of installation and mission aspects in addition to the equipment classes is a further attempt to ensure enhanced operational electromagnetic compatibility, but again at a reasonable cost, by tailoring the requirement for only certain tests as appropriate. In some cases, further tailoring will be necessary for specific missions such as navy carrier aircraft [49] which have to operate in close proximity to powerful radar and satcom antennas. In such situations field strengths for the RS03 test may need to be increased to 200 V 1m or more to ensure operational EMC. There are many differences between MIL STD 461A and 461B which are too numerous to be described in detail in this book. However, some of the major changes to tests are listed in short form in Table A1.8.4 of Appendix 1.8. Some of the tests introduced into MIL STD 46IB are only applicable to procurements for particular services and readers should consult the Standard for further detail. A second upgrade and revision of this series of military standards arose in 1986 with the introduction of MIL STD 461C. The new standard retains the same format as 46IB and introduces a number of modifications [50]. The most important addition is that concerned with tests designed to ensure survival of military equipments against EMP (electromagnetic pulse). The following tests have been added and are applicable to installations listed in parts 2, 4, 5 and 6 of the document: CSIO: pulse injection onto equipment pins CS 11: pulse injection onto interconnecting cables The characteristics of the damped sinusoid to be injected are given in Figure 2.21.
30
A HANDBOOK FOR EMC TESTING AND MEASUREMENT MIL STO 461 Specification test requirements
RS 01 30 Hz - 30 kHz RS 02 Magnetic induction
Electric and electromagnetic
Electric and electromagnetic
RE 02 14 kHz - 10 GHz RE 03 10 kHz - 40 GHz spurious & harmonic RE 05 150 kHz - 1 GHz
vehicles
~
CE 02 30 Hz - 20 kHz CE 04 20 kHz - 50 MHz CE 05 30 Hz - 50 MHz
RS 03 10 kHz - 10 GHz RS 04 10 kHz - 30 MHz (parallel plate)
engines
RE0614kHz-1 GHz
overhead power lines Transmitter: key up Transmitter: key down Receiver: LO leakage
Figure 2.19
MIL STD 461 test specifications
Figure 2.20 MIL STD 461B and 462 (notices 4 and 5)
MIL STD 461 B 1st April 1980
MIL STD 462 31 st July 1967
PART 1 - GENERAL
NOTICE 1 - CORRECTIONS 1st August 1968
PART 3 - SPACECRAFT & GSE
or ICT Inc.
NOTICE 2 - ( AIR FORCE) 1st May 1970
NOTICE 3 - ( ARMY) 9th February 1971
PART 4 - GROUND FACILITIES
NOTICE 4 - INTERIM NEMP (CS 09 TEST) 1sf April 1980 (Navy)
PART 5 - SURFACE SHIPS
NOTICE 5 - INTERIM NEMP (CS 10 & RS 05) 5th January 1983 (Navy)
PART 6 - SUBMARINES
PART 7 - NON-CRITICAL GROUND AREA
PART 8 - TACTICAL & SPACE VEHICLES
PART 9 - ELECTRIC POWER
or ICT Inc.
CS 03 30 Hz - 10 GHz Intermodulation CS 04 30 Hz - 10 GHz Signal rejection CS 05 30 Hz - 10 GHz Cross modulation CS 07 Squelch circuit CS 0830 Hz -100Hz (signal rejection)
Reprod uced by permission
PART 2 - AIRCRAFT & GSE
Reprod uced by permission
CE 01 30 Hz - 20 kHz CE 03 20 kHz - 50 MHz CE 05 30 Hz - 50 MHz
PART 10 - COMMERCIAL
EMC STANDARDS AND SPECIFICATIONS 100 I mtlX
nnnII n
1\
A
_
!~nME
~ 10
E
31
electrical and electromagnetic in terference phenomena. The aim of the standard is to ensure a compatible total system under operational condi tions and therefore is concerned with
II Q)
j5
The system electromagnetic environluent Lightning protection Static electricity Grounding and bonding.
1
8 II
c::
0.1
'0..
0.01 ""--_-"'-_ _...&.-_--'-_ _.... 0.01 0.1 100 FREQUENCY MHz
Figure 2.21
Limits for CS10 and CS11 NEMP current injection
50 45 40
a:
35
~ 30 LU
-
:E 25
~ 20 15
10
I
I
I I I I
I
I
I
I
5 --~-----~-------------------I I
0-----1---""'-----------------'--1-t r - I - t d - i - - - - - - t f - - - - - - - i
~5 X 10-9
~ 38 X 10-
9
Figure 2.22
550 x 10-9 LIMIT FOR RS 05
US MIL Syn 461C NEMP susceptibility
RS05: pulsed E-field (only for mission-critical equipments located outside of an intentionally hardened area). The E-field pulse shape to be used is shown in Figure 2.22.
A summary of the test identifiers, short titles, applicability to parts of 461 C and detailed notes is given in the form of a requirements tree in Figure 2.23 [51]. The process of continuing to modify and update this very important set of military standards is continuing. A triservice committee has been set up to further revise MIL STD 461/462/463 with one of the aims being to eliminate the controversial area of defining and measuring narrow (CW) and broad band (impulse) signals [51 J, as referred to in Appendix 1.1.
It suggests guidelines for generating procedures to define system compatibility demonstration tests by using the normal system performance indicators to reveal EMC failures. The system is tested under real operational load conditions and interference is injected into it at critical points and at a level which is 6 dB (or greater) than the worstcase levels created by the system or experienced in the environment. The aim is to prove that a margin of compatibility exists over and above the level required for fault-free operation during a simulated mission. The imposition of MIL-E-6051 D on a system is not contingent on the individual equipments within it having been designed or tested to MIL STD 461/2/3. I t requires a contractor to prepare detailed proced ures and test plans to demonstrate compatibility at system level. Clearly, such a contractor could make considerable use of the equipment test results from MIL STD 461/2/3 in understanding the electromagnetic characteristics of these equipments. One may initially choose to model EMC at system level using computer codes such as IEMCAP [53J to predict ranked systemlevel incompatibilities (starting with safety critical circuits [54J) and thereby to better define a costeffective detailed test plan for satisfying MIL-E6051D. Without such computation and subsystem test work, meeting this requirement to demonstrate a final check on complex system level compatibility under operational conditions (simulated) cart be lengthy and expensive in its preparation and execution.
2.8.3 Other US military standards A list of relevant EMC related US standards can be found in Appendix 1.9.
2.9 UK tnilitary standards
2.8.2 MIL-E-6051D
2.9.1 Service and establishment-specific standards
This standard is entitled 'Systems electromagnetic compatibility requirements'. It is applicable to all items of equipment within a definable system, the performance of which may be influenced by
Military interest in EMC has been growing significantly in the UK since the mid 1960s following the introduction of compact, complex, semiconductor based equipment into platforms for all three
spikes 10~~ ~ OV,151..1.s 20OV, 10 I..l.S 40o V, 51..1.s
UM 03 E field 2=-"7"M-=-=-H-=-z-·--=1-=-0--:0::7H"7"z-
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. . . CS 11 Damped sine trans. Cables
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CS 09 Structure current 60 Hz -100 kHz
CS 07 Squelch circuits
CS 06 Spike power leads 100 V, 10 Jl s 200 V, 1511 s 200 V, 101..1. s 400 V, 5 Jls
CS 05 Cross modulation 30 Hz -200Hz
CS 04 Rej. of Undes. signal 30 Hz· 200Hz
CS 03 Intermodulation 15 kHz -100Hz
CS 02 Power-inter. leads 50 kHz - 400 MHz
H field 30 Hz - 50 kHz
l
UM 05 E field 150 kHZ - 100Hz
UM 04 E field 14 kHz -1000 MHz
UM 03 E field 150 kHz -100Hz
RE 03 Spur & harmonics Radiated technique
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.
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UM 05 Power lines 50 kHz - 50 MHz
UM 04 Power lines 15 kHz - 50 MHz
CE 07 Transients Time domain (power)
CE 06 Antenna terminals 10 kHz - 260Hz receivers TRS (key up) TRS (key down)
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EMC STANDARDS AND SPECIFICATIONS
services. Early specifications such as FVRDE 2051 section 4 [55J, which was concerned with radio interference from military vehicles, have been progressively superseded by more modern standards MVEE 595 and specific amendments [56J in the same way as the US MIL STD 461/2/ 3. Where possible these standards now form part of the central (nonproject specific) UK standard DEF STAN 59-41. Each service tends to generate standards which are designed to produce cost-effective EMC for particular types of platform, weapons systems and equipment which must function together in the particular EM environment in which that service operates. For example, MVEE 595 divides military vehicles into two classes: FFR, vehicles fitted for radio, and nonFFR, vehicles without radio. In this way, the interference limits can be more relaxed for nonFFR vehicles so that acceptable electromagnetic compatibility can be achieved economically without compromising the performance of a particular system. Where appropriate, military standards refer to British Standards such as BS 727 (RFI measuring equipment) and BS833 (vehicle ignition) for example. The navy has produced a set of standards for equipment on board ships at sea which takes into accoun t issues such as screening offered by metal hulls below deck and the need to use electrochemically compatible metals for equipment boxes, access doors and RFI gaskets where these are exposed to a corrosive salt atmosphere. Examples of navy standards are NWS 3 (amendments 1-4) 1981: Electromagnetic compatibility of naval electrical equipments NWS 1000 part 1 Chap. 5 1986. 'Electromagnetic compatibility-design guide' NES 1006 1988 [5 7J : 'RF environment and acceptance criteria for naval stores containing EEDs' (supersedes NWS 6) In 1974 the Royal Radar Establishment produced a comprehensive EMC standard RRE 6405 [58J to cover the EMC performance of RRE (later RSRE, now Defence Research Agency) controlled projects. I t comprised three parts: 1: Guide Terminology and definitions Con trol and test plans Related documents 2: Requirements/limits 3: Measurement techniques This standard specified equipment classes (Table A1.8.5 of Appendix 1.8) and contained a comprehensive set of limits and test methods (Table A1.8.6 of Appendix 1.8) which could be applied to
33
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FREQUENCY MHz LIMITS FOR HIGH FREQUENCY BROADBAND CONDUCTED VOLTAGE N
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FREQUENCY MHz LIMITS FOR HIGH FREQUENCY NARROWBAND CONDUCTED VOLTAGE
Figure 2.24
RRE 6405 conducted emission limits
Reproduced by permission
or HMSO
all three services. This was achieved by the use of different limits in relation to land, shipboard and air service equipments. See Table A1.8.7 of Appendix 1.8. For example, the limits for RCE03 where a line impedance stabilisation network (LISN) is used in the measurement of RF conducted voltages show the progressive relaxation of the limits from grade L. See Figure 2.24. Many aspects of this comprehensive and clear document have been carried over to DEF STAN 59--41 which is the current central UK military E~1C standard. It too covers equipment for land, shipboard and air use with various limits based on equipment classes.
2.9.2 Project-specific standards With the aim of economically achieving the best possible EMC performance, many large military projects have generated comprehensive EMC specifications specifically for that project. They make reference to the general EMC standards which exist but tailor the limits and tests to help ensure good E~IC performance of the system with respect to its particular operational mission and environment. The generation of project specific EMC requirements can also be of value in harmonising the views of sections of industry which commonly operate with national EMC defence
34
A HANDBOOK FOR EMC TESTING AND MEASUREMENT
standards, when they form multinational collaborative ventures for large aerospace or defence projects. Examples of project-specific specifications are SPP-90003 SPP-90203 EA98 QO 10J SPE-J-000- E 1000
Tornado IDS Tornado ADV EH 101 helicopter European fighter aircraft
National project-specific EMC requirements are also produced, e.g.: rrS1527 TS 1727
Ptarmigan} communications systems Wavel
2.9.3 DEF STAN 59--41 (1988) This standard was first issued in March 1971 and was revised as Issue 2 in August 1979. Until Issue 3 in 1986, individual establishment EMC specifications were invoked for land, shipboard and air applications. Issue 3 of DEF STAN 59-41 was intended to stand alone and contained many test methods and limits which were differentiated by equipment class, service use and grade. The June 1988 issue of DEF STAN 59--41 is in five parts: 1: General req uiremen ts 2: EMC management and planning procedures 3: Technical requirements, test methods and limits 4: Open site testing 5: Technical requirements for special EMC test equipment The standard ou tIines typical electromagnetic environmental requirements for equipment used in the services but the limits may be tailored to meet the needs of specific projects. I t covers all equipments for military use that may give rise to or may suffer from electromagnetic interference. I t is invoked, wherever practicable, in all designs, contracts etc. for equipment for military use. Specific requirements for projects may be generated based on DEF STAN 59-41 by considering Selection of appropriate types of test Modification of the scope of certain tests as a ppropriate Modification of test limits to match the environ mental requirements of the project Consideration of transients or other infrequent emISSIons. The standard does not directly cover issues connected with explosive, nuclear effects (other than EMP), hazards to personnel or ignition of fuels by electromagnetic energy. Suitable references to other documents are made. rrhe
standard also does not cover immunity or protection from lightning strikes and is not concerned with spurious signal outpu ts, spectral purity, IF rejection or intermodulation products in connection with communications or other sensitive receiver equipment. Part 2 of the standard sets out the requirements for management and planning procedures, which are mandatory in order to ensure that potential interference problems are properly addressed as early as possible in the life of a project. Specific actions are required: An EMC co-ordinator should be appointed for the project. An EMC working group should be formed for a complex project. A suitable control plan must be developed. EMC technical policies for screening, filtering, grounding and bonding should be established. Appropriate test plans must be prepared. Suitable, test facilities should be available. Configuration control must establish the build standard of all items for test. Part 3 of the standard defines the equipment types that are subject to certain tests, see Table 2.5.
Table 2.5 DEF STAN 59-41 equipment types Type 1 Type 2
Type 3
All equipments fitted with electronic components Motors, genera tors and electromechanical selectors (wi thou t electronic control) Relays, solenoids and transformers
The tests applicable to each equipment type are indicated in Table 2.6. The application of DEF STAN 59-41 tests are further tailored by introducing the grading of limits in relation to service environment. See Table 2.7.
Table 2.6 Equipment type and tests Tests All tests All emission and imported spike tests Imported and exported spike tests only
Type 1 Type 2
Type 3
Y
N
N
y
N N*
N
N
Y
*DRE02 test at power frequency is required if EDT is fed with AC power. A list of DEF STAN 59-41 test methods and their applicability to a service is given in Table Al.8.8 of Appendix 1.8.
EMC STANDARDS AND SPECIFICATIONS
The tests specified in DEF STAN 59-41 are comprehensive, but great care has been taken to make them as straightforward as possible to implement in an effort to eliminate some of the inconsistency that may be associated with EMC measuremen ts. T ~sts are to be carried out in large shielded chambers equipped with radio absorbent material (RAM) if possible. See Figure 2.25.
SCREENED ROOM WALL
FILTERING ........
EUT
Detail
Air Ship
One grade Above decks Below decks Class A
All test methods
Class E Class F
........
........
, I
EUT
I I
CASE BOND
Grade
Class D
........
I I
Use
Class B Class C
INPUT / OUTPUT TEST GEAR
MEASURING SET
Table 2.7 Grading oj limits
Land
35
CAS'E BOND I
GROUND PLANE
Equipment within 2 m of comms. antenna 2-15 m of comms. antenna 15-100 m of comms. antenna > 100 m from comms. antenna Shielded equipment Civil standards are adequate
}'igure 2.26
Test lnethod DGYE02: signal/control line current 20 Hz-l00 MHz DEI? STAN 59-41 (Part 3) /2
Reprod ueed by permission or H M SO
LIMITS FOR AIRCRAFT USE Z Q_130 (J)<S;.
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DEF STAN 59-41 has many test method and equipment layout features in common with MIL STD 462. It could be considered somewhat in advance of the current US MIL SPEC in respect of its employment of wideband bulk current injection tests for conducted susceptibility DCS02, DCS04 & DCS05.
55 o
FEEDTHROUGH CONNECTORS OR FEEDTHROUGH FILTERS TEST ROOM AREA cleared of any unnecessary equipment and personnel
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FREQUENCY
LIMITS FOR SHIP USE
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8° FEEDTHROUGH CONNECTOR
GROUND PLANES
Suggested layout .for screened room complex DEF STAN 59-41 (Part 3) /2
Reproduced by permission or HMSO
40
-20'--_..a.-._--'-_ _L . . . - _ . . . . I - _ - - I ._ _....I 100 Hz 1kHz 10 kHz 100 kHz 1 MHz 10 MHz 100 MHz
FREQUENCY
}-'igure 2.27
DE}' S T A,N 59-41 (Part 3) /2 ( DGYE02)
Reproduced by permission or HMSO
36
A HANDBOOK FOR EMC TESTING AND MEASUREMENT
The adoption of fixed receiver bandwidths for emission measurements means that the problems normally associated with the interpretation of broadband and narrowband signals have been eliminated. A future revision of MIL STD 461/2/ 3 may also adopt this approach. An example of the test layout for DCE02 can be seen in Figure 2.26 with the associa ted limits for conducted emissions appropriate to air, shipboard and land use in Figure 2.27. A list of examples of some widely used UK EMC civil and military standards can be found in Appendix 1.9.
2.10 Following chapters This book has been prepared with the aim of guiding the reader from a simple introduction to EMC in Chapter 1, through issues relating to regulations and standards in Chapter 2, to a detailed consideration of the sensors, antennas and measuring apparatus used in EMC testing which are discussed in subsequent chapters. In this way, readers with some familiarity of the basics of EMC can progress to the main body of information on EMC testing in the most direct way. An introduction to design techniques for EMC has been provided in Chapter 12 to complement' the emphasis on EMC testing. Much has been written in detail elsewhere on EMC design and the reader is referred to appropriate references in the text of Chapter 12. Readers new to EMC and who wish to cover the basic elements of design before continuing with the details of EMC testing in Chapter 3, may read Chapter 12 now and then return to Chapter 3.
2.11 References
2
3 4 5 6 7
MERTEL, H.K.: 'International and European RFI regulations'. EMACO Inc. 7562 Trade St., San Diego, CA 92121, USA, also section 10/2 IEEE conference on EMC) 1982 SHO\VERS, R.M.: 'Wide scope of IEC work on electromagnetic compatibility problems', Trans. South Afr. Inst. Electr. Eng.) Jan. 1982 DEF STAN 59-41 (parts 1-4). Ministry of Defence Directorate of Standardisation, Glasgow, UK KEISER, B.: 'Principles of electromagnetic compatibility' (Artech House, 1987, 3rd edn.) NAMAS Executive, National Physical Laboratory, Teddington, Middlesex, TWll 01 W, UK GERKE, D.: 'A fundamental review of EMI regulations', R.p'. Design, April 1989, pp. 57-62 MIL STD 1541 'Electromagnetic compatibility requirements for space systems'. US Department of the Air Force, Washington DC, October 1973
8 MIL STD 461 C 'Electromagnetic emISSIon and susceptibility requirements for the control of electromagnetic interference'. US Department of Defense, Washington DC, August 1986 9 VDE 0871 Equivalent to EN55011, CISPRll, BS4809. Similar to FCC Title 47 pt. 18. Obtainable in UK from BSI, Milton Keynes, MK14 6LE 10 FCC Title 47 parts 15,18 and 68 of the US code of Federal Regulations, Federal Communications Commission, Washington DC 11 VDE 0875 'Regulation for the radio interference suppression of electrical appliances and systems'. English translation available from BSI, Milton Keynes, MK 14 6LE 12 WHITEHOUSE, A.C.D.: 'EMC Regulations within Europe', Electron. Commun. Eng. J.) March/ April 1989, pp. 57-60 13 Radiocommunications Division of the Department of Trade and Industry, Waterloo Bridge House, Waterloo Road, London SE1 8UA, UK 14 89/336/EMC 'EC council directive on the approximation of the laws of the member states relating to electromagnetic compatibility', OJ!. J. Eur. Communities) L 139/19, 23.5.89 15 'Electromagnetic compatibility product standards, UK Regulations' Department of Trade and Industry and Central Office of Information, INDY J1896NE.40M, April 1993 16 'EMC directive, technical report part 2', New Electronics) May 1993 17 'EMC and the European directive', one day seminar papers. Assessment Services and Hewlett Packard, Nov. 1991 (Assessment Services, Tichfield, Hants, UK, Hewlett Packard, Stoke Gifford, Bristol, UK) 18 LOHBECK, D.: 'West German RFI laws and regulations'. Interference Technology Engineers Master 1988, pp. 302-342 19 'U riders tanding the FCC regulations concerning computing devices'. OST Bulletin 62, FCC Office of Science & Technology, Washington DC, May 1984 20 Standard J -551 C, 'Measurement of electromagnetic radiation from motor vehicles (20-1000 MHz)'. Society of Automotive Engineers, Warrendale, PA, USA 21 Practice APR-93 7, 'Jet engine electromagnetic interference test requirements and test methods'. Society of Automotive Engineers, Warrendale, PA, USA 22 Report AIR-114 7, 'EMI on aircraft from jet engine charging'. Society of Automotive Engineers, Warrendale, PA, USA 23 ANSI C63: 1980 'Specifications for electromagnetic interference and field strength instrumentation, 10kHz-10GHz'. ANSI, 1430 Broadway, New York, NY 10018, USA 24 HOOLIHAN, D.D., JOHNSON, J.W. and WEBBER, C.L.: 'Compliance with Japanese Standards creates opportunities'. Interference Technology Engineers Master 1988, pp. 291-344 25 'Limits and methods of measurement of radio interference characteristics of information technology equipment'. CISPR publication 22, 1985, first edn.
EMC STANDARDS AND SPECIFICATIONS 26 'New radio interference regulations amendment 28/ 9/89'. Canada Gazette part 2, vol. 122 no. 20 27 'Radio interference regulations amendment'. SOR/ 88-475 Canadian government publishing centre, Department of Supply and Services, Ottawa, Ontario KIA 059, Canada 28 C108.8-M1983, Canadian Standards Association, 178 Rexdale Boulevard, Rexdale, Ontario M9W lR3, Canada 29 MA.Y, C.R.: 'EMI and amended Canadian radio interference regulations'. Interference Technology Engineers Master 1988, pp. 84-90 30. LESCHAK, D.: 'Safety and regulatory compliance'. Interference 1'echnology Engineers Master 1989, pp. 325-354 31 TUV, TUV Rheinland of North Anlerica Inc., 108 Mill Plain Road, Danbury, CT 06811, USA 32 DUFF, W.G.: 'Fundamentals of electromagnetic compatibility'. Interference Control Technologies Inc., Gainsville, Virginia, USA 33 SMITH, C.W. and BEST, S.: 'Health hazards in electrical environment (Dent, London) 34 CIPOLLONE, E., BEVACQUA, F. and AMICI, S.: 'Electromagnetic field measurements for radiation hazards evaluation'. Interference Technology Engineers Master 1989, pp. 190-288 35 ANSI standard C95.1, 'Safety levels with respect to human exposure to radiofrequency electromagnetic fields, 300 kHz-100 GHz'. American National Standards Institute, 1982, New York, USA 36 'Advice on the protection of workers and members of the public from the possible hazards of electric and magnetic fields with freq uencies below 300 GHz'. National Radiological Protection Board, (IIMSO, 1986) 37 'Guidance as to restrictions on exposures to timevarying electromagnetic fields and the 1988 recommendations of the international nonionising radiation committee', NRPB-GSII (HMSO, 1988) 38 DEF STAN 05-74/1: 1989 'Guide to the practical safety aspects of the use of RF energy'. Directorate of Standardisation, 65 Brown St., Glasgow G2 8EX 39 INIRIC 'Interim guidelines on the limits of exposure to radiofreq uency electromagnetic fields in the frequency range from 100 kHz to 300 G Hz' , Health Phys.) 1984, 46, pp.975-984. Updated in Health Phys.) 1988, 54, p. 115 40 EEC articles 4 and 5 41 LE ROY, G.A.: 'Sur les incendies provoques par les ondes hertziennes', Comptes Rendus 1991, 168, pp. 244-247 42 'Control of high-frequency radar equipment'.
43
44
45 46 47
48
49
50
51 52
53
54
55
56
57
58
37
National Fire Codes vol. 10 Transportation, Boston. National Fire Protection Association, 1972 'Electromagnetic radiation hazards'. Ground Electronics Engineering Installation Agency standard T.0.31z-10-4, 1970 'Naval shore electronics criteria: EMC & EM radiation hazards'. Naval Electronic Command, doc. Navelex 0101, 106, 1971 Hazards of EM radiation to fuel (HERF). N avsea OP3565/Navair 16-1-529, vol. 1, Chap. 7 EXCELL, P.S.: 'Radio frequency ignition hazards', Hazard Prevention, May /J une 1984 OB procs. 41273 and 42413. Ordnance Board, Empress State Building, Lillie Road, London SW6 ITR 'The achievement of electromagnetic compatibility'. ERA report 90-0106, Feb. 1990, ERA Technology, Cleeve Rd., Leatherhead, Surrey, UK SIEFKER, R.G.: 'Tailoring MIL STD 461B for naval avionics applications'. Proceedings of IEEE conference on EMC, pp. 387-395 SIKORA, P.A.: 'A comparison of MIL STD 461C to the previously issued MIL STD 461B'. Interference Technology Engineers Master 1988, pp. 72-111 Electro-Metrics Company, 100 Church St., Amsterdam, New York 12010, USA GOLDBLUM, R.D.: MIL STD 461/2/3 revision process. Interference Technology Engineers Master, 1990, pp. 208-357 'Intra-system electromagnetic compatibility analysis program (IEMCAP)'. Rome Air I)evelopment Center, USAF LEE. G. and ELLERSICK, S.D.: 'Methodology for determination of circuit safety margins for MIL-E6051 EM C system test'. Proceedings of IEEE symposium on EMC) 1985, pp. 502-510 'Radio interference from vehicles'. Fighting Vehicle Research and Development Establishment, FVRDE 2051 section 4. (FVRDE is now Military Vehicles and Engineering Establishment, Chobham, Surrey) 'Electromagnetic interference and susceptibility requirements for electrical equipments and systems in military vehicles'. MVEE 595, 1975 'Radio frequency environment and acceptance criteria for naval stores containing EEDs'. NES 1006, 1988, Procurement Executive, MoD, Deputy Controller Warships, Foxhill, Bath, UK (restricted document) 'Requirements for electromagnetic compatibility of electronic equipments'. RRE 6405, 1974, Royal Radar Establishment, DRA, Malvern, Worcs., UK
Chapter 3
Outline of EMC testing 3.1 Types of EMC testing
relate the measurements made to field, current or voltage standards to within a few dB. In many cases it is sufficient to make comparisons between two alternative items for test and check the amplitude and spectral characteristics of emissions or immunity to resolve a design question. I t is an ad van tage for the electrical and mechanical designers of the product to be involved with these simple bench tests, perhaps with assistance from an EMC engineer, as they gain first-hand knowledge of the reasons why EMC issues must be taken into account and sometimes take precedence over straightforward design preferences. On large projects where there may be an EMC control board and specialist EMC design engineers in both the prime contractor's organisations and in many of the subcontractor firms, these simple tests should be carried out as a routine matter, having been specified in the EMC control plans which call for tests at all the various levels of development. These control plans may require exploratory tests to be carried out that are neither simple nor quick to perform. For example, it may be that sophisticated measurements need to be made to gather information about the RF induced currents on an existing system or installation, as a guide to the EMC design features of a new type. Whatever the scope and scale of these preliminary tests, they will help to build up a picture of the electromagnetic behaviour of the equipment and give an insight into the optimum design solu tions.
3.1.1 Development testing A key element in cost-effective EMC design and certification of a civil or military equipment or product is careful programme planning. The EMC aspects of the project must be considered at all stages and may require some design and cost compromises to be made. The particular solutions arrived at will of course be determined by specific project conditions, but it is vital that the existence and timing of decisions involving electromagnetic compatibili ty are known and as far as possible planned for. To this end, it is advantageous to be able to submit key components, circuits, boards, cable forms and hardware such as cases or racks, to quick and simple tests during the design process. In this way it is often possible to differentiate more easily between the EMC attributes of competing designs than by calculation or computer modelling (which may be inappropriate for small projects). Simple tests carried out on the bench, with perhaps a signal generator, oscilloscope and spectrum analyser, can help to build a sound design based on a number of small tests devised to resolve EMC queries as they arise. This activity also allows the designer or EM C engineer to build up a broad picture of the 'electromagnetic landscape' of the system being developed. This will help to put into context particular EMC issues which rnay arise during the design task. For example, it will be possible to balance the effort and programme funding devoted to case screening, compartment filtering and screened cable design, to ensure that no one feature is over designed and too costly for the EMC performance which will be achieved as a whole. These simple bench tests can be carried out in minutes rather than hours and many will not require the use of a screened room, although setting up such tests on an RF ground plane in an electrically quiet corner of the laboratory can be an advantage. Measurements are made at close range with inexpensive simple E- and H-field sensors, small current probes or high-impedance RF voltage probes. They need not be accurately calibrated to test house standards but it should be possible to
3.1.2 Measurement to verify modelling results On large projects, or in cases where it is inappropriate or prohibitively expensive to test a critical EMC design feature, it may be necessary to model the situation in a computer to gain enough information about options and possible solutions to 'enable the design to progress. The models used may be relatively simple and inexpensive to acquire and run. The results and limitations of simple models are usually well understood and data are used only as a guide. This is the case for exampIc when calculating the capacitative crosstalk between parallel wires [1, 2J or the transfer impedance of shielded cables [3]. 38
OUTLINE OF EMC TESrrING
In some situations, however, large models (such as numerical electromagnetic code, NEC [4]) may be complex and the limitations on the validity of the results may be difficult to determine. In these cases it is necessary to construct careful experiments based on specific and simple situations where the results can be calculated analytically and the measured, calculated and modelled results can be compared and any modelling errors estimated. In this way it is possible to gain confidence that the complex computer model being used is well behaved and its predictions are meaningful. An example of such a situation is where a system in a complex metal case is being tested in an RF anechoic chamber and it is required to assess the surface currents induced in the casing by planewave free-space illumination [5]. In such a situation it is possible to create a patch model of the system casing for input to NEC and model its behaviour. One might also model a simple short fa t dipole antenna of similar proportions to the metal case, calculate the induced current pattern using an analytical approach [6J and make careful measurements in a high performance RF anechoic chamber. The aim is to assess the agreement between the analytical solution and the measurements for the simple case and then to compare these with the computer model prediction and estimate the size and nature of any model errors. The model may then be run with the adopted patch configuration of the system casing with increased confidence. A great deal of reliance can be placed on computer models that are used to support the EMC design of large projects and it is vital that the predictions are questioned and tested whenever possible by comparison with measurement.
3.1.3 Preconformance test measurements It is best practice to build on the development testing which takes place during a project, by checking the EMC performance of complete subsystems or prototype equipments against the specification by testing them in a semiformal way using the test methods associated with the specified standards. This practice requires access to an appropriate test house or suitable company facility. Consequently, the test can sometimes be expensive and is reserved for confirming EMC design progress at key stages of development where screening, grounding, filtering, cable and structure policies are to be fixed. Preconformance test checks need not attempt to cover all the specified tests listed in the standard. They may concentrate only on emiSSions or immunity or a wide range of tests
39
may be carried out for example on safety or performance critical items. If these tests are performed by an external test house then it is vi tally important to select a laboratory which is not only competent to carry out the testing using the specified methods, but one which can also analyse the resulting data and interpret them to give clear guidance to the customer for improving the design should this be required. These semiformal tests are best carried out in the company's own facilities if they are available, as there is no substitute for hands-on experience by designers and EMC engineers in assessing equipment performance against the specified standards which the final product must meet. Simple development and preconformance test facilities can be constructed for a few tens of thousands of pounds. Well appointed EMC test facili ties wi th large semianechoic chambers can cost around a million pounds to build but they can usually be hired for a few thousand pounds per day. Both large and medium-sized projects make use of development models which are dedicated EMC test beds [7J on which a variety of assessments can be made during the design phase and a good reference database can be constructed. These EMC physical models can then be subject to the semiformal tests towards the end of development with increased confidence that the preprod uction and production items submitted for test will meet the EMC requirements.
3.1.4 Conformance testing The final stage of testing formally demonstrates whether the equipment or system will meet the EMC limits set out in the standard against which it was designed. The contractor should have confidence that the equipment will meet these limi ts, as preconformance tests on critical subsystems and iterative design testing will have contributed to a full EMC database on the equipment in line with the EMC control plan for the project. Conformance testing should be for confirmation of compliance; there should be no surprises at this late stage. The exact circumstances surrounding confor mance testing will depend on a number of factors: (i)
The constraints on the conformance test station imposed by the appropriate national law governing the registration, sale and operation of the equipment being submitted for test. In the past, some countries have specified the use of central national test laboratories for certain conformance tests.
40
A HANDBOOK FOR EMC TESTING AND MEASUREMENT' The conditions con tained in the con tract to supply the equipment may have a section devoted to the EMC. Conditions with regard to conformance or acceptance tests may be laid down in such a section. I t is possible that the procuring authority will specify the conformance test house to be used. I'his can sometimes be his own facility. The type of conformance testing undertaken will depend on the specification to be met, e.g. system level or equipment level, and on the size and complexity of the equipment under test. Not all certified test houses will be able to cover every specification for both civil and military use and may not be able to accomrnodate large, distributed or complex E-UT (equipment under test). In general, test houses have facilities that are ei ther directed to testing civil/commercial equipment or to military equipment.
with the rapid expansion of the need for as a result of harmonisedEC regulations applied to almost all electronic military and aerospace test facilities have more active in the commercial field. Even for tests on military systems, few industrial would have all the facilities to carry out acceptance of for example a new aircraft without the support of the procurement or authorities to provide a flight test range environment for mission electroYrl'lcrr\""r'lI' environment simulation. to the specialisations and limitations which most test houses will have, the selection of a will depend on the civil or nature the product and its size. Test facilities m.ay only be able to support limited '-'.....,"'~~,'J-'--'-'-'IJ of for conformance testing: "r>.'n-lr'.':lYllP."
f>r'\p.r·--:\t-l,n.Yl<:l1
Commercial equipment (small < 1 m cube) Commercial equipment (large or distributed) Commercial equipment (as installed) Military equipment (small < 1 m cube) Military equipment or distributed) Military safe service worthiness) acceptance trials) 1'he technical details of EMC test equipment and conformance rnethods applicable to these of are discussed in detail in later 1-L>r.. r."V",.o.0
3.1.5 Conformance test plan Whatever the equipment, specification, test method or test it is important to work to a preprepared where necessary,
customer approved) test plan. 1'he plan will have been called for by the project EM C control plan. The size, content and style of the test plan will vary depending on the type of conformance test. For example test plans for equipment being tested to MIL STD 461 must be prepared in accordance with DI-EMCS-80201 [8J and it is difficult to generalise with regard to the contents of possible conformance test plans. They can be written by the equipment design authority contractor or can be subcontracted to an EMC consultant, who as an independent third party, will liaise with the test house on the contractor's behalf. Or test plans can be written by the selected test house as part of the testing contract. In any event, the plan will contain information from both the equipment contractor and the test house in order that planned, accurate and trouble-free testing can be conducted quickly for the rninimum cost. Test plans should include as a minimum the following information to be supplied by the equipment design authorityj.contractor or the test house: Functional and physical description of the item to be tested detailing its purpose, method of operation, modes of operation, interfaces to external objects Performance and modes of operation should be identified as critical mission or performance critical reversionary or backup modes self test/check/calibration modes Statement of the objectives of the conformance test Statement of the standard or specification against which the item shall be tested and a list of tests to be performed Statement of any tests not to be performed, or limitations of tests to be performed List of applicable documents with regard to the EUT, e.g. operating manuals etc. reference to addi tional information in the con trol plan rrime schedule for the tests with daily test goals including EUT position, 1-'est layouts orientation, details of cable layouts and bonding/grounding arrangements Criteria for determining the locations of EUT monitoring points and methods of monitoring (immunity tests) Applicable limits of performance degradation (for immunity tests) Description, methods of connection, position and opera tion of any associa ted test equipment needed to function the EUI' In a
OU1'LINE OF EMC TESrfING
representative manner to include all E UT stimuli, instrumentation, electrical services, cooling and hydraulic supplies Electromagnetic and physical req uiremen ts for a conformance test site (open range or chamber testing) with special reference to EUT size, weight, disposition and special requirements (e.g. Dynamometer/rolling road for vehicle testing). In particular, the selected test house contribute statements with regard to
must
Any modifications or limitations applied to the test requirements set out by the EUT manufacturer/contractor for the test to be carried out The necessary approvals from a national quality organisation to carry out the specified conformance tests 1'he state of calibration of all equipment to be used (including screened rooms) and the source of such calibration Test equipment, including sensors, receivers and recorders (type and serial numbers) to be used in the tes t Test methods to be employed and relevant procedures Standing orders and safety proced ures Experience of testing staff who carry out the work An agreed test programme plan The nature and content of test reports (e.g. for reports on MIL STD 461 tests these should be in conformance with MIL STD 831) Delivery date for the test report, and An estimate for EUT setup and stripdown times and consequent facility occupation (usually charged at a lower daily rate than for a test day). Test houses are usually booked well in advance and it is prudent to fix a date and duration for the testing and then stick to it. Setting the test date can be difficult as the manufacturer will have to balance a desire to set an early date to get the product to the market quickly, against the possibility of delays through development over runs which could cause him to miss his slot and perhaps not get another for some months. If slots are missed and the selected test house is full, manufacturers may have to change test house and modify the test plan. The manufacturer has to make the test date judgment in the light of dates available at suitable test houses. If the equipment is large and complex and is to be thoroughly tested to an involved specification (usually for a military project), there may be very few test facilities which can accommodate the equipment and there
41
will be limited scope in setting the test date. This explains why many of the larger and more established test houses are within military equipment or aerospace manufacturers, as they need the assurance of being able to test according to the dictates of their project programmes rather than by the availability of external test facilities. As EM C testing becomes routine in the commercial sector, larger electronics corporations are setting up similar extensive in-house test facilities. The number of test facilities which can carry out testing of commercial electronic equipment to the European EN standards or the US FCC regulations is increasing all the time, but there is predicted to b~ a shortfall in the mid 1990s in the UK when the full impact of EEC harmonisation Directive EEC/89/336 is felt across the whole of the electronics industry. If the manufacturer can justify the return on investing in an in-house EMC test capability then most regulations and specifications permit self certification of an equipment. A small in-house facility may start with development testing and grow to meet the company's developing requirement for EMC conformance testing.
3.2 Repeatability in EMC testing 3.2.1 Need for repeatability and accuracy The basis of the scientific method is founded on the theoretical postulation and experimental verification of propositions. Thus the need for experiments and measurements to be conducted in a well ordered, accurate and repeatable manner is fundamental if hypotheses are to be confirmed or denied. These experiments must be able to be repeated by others, and only when this has been done is a consensus established with regard to the validity of the theorem or notion. In the case of practical engineering, the need for careful test rnethods which can be carried out to yield accurate and repeatable results is just as important as in basic science. Indeed, in the practical world of near market design, developmen t and sale of electronic products which are covered by national and interna tional laws (including those relating to EMC and electrical safety), it is vital that accurate wellfounded test data can be generated to support the release of a product. Failure to certify the electromagnetic compatibility of a product to a recognised standard is not only an impediment to successful sales but can be an infringement of the law, for which individuals within a company can be held responsible. EMC conformance measurements can influence the success or failure of a product and thereby affect
42
A HANDBOOK FOR EMC TESTING AND MEASUREMENT
the prosperity of the company which is developing them. For electronic engineers engaged on projects both large and small, understanding EMC test methods and the limits of accuracy which may be expected is becoming ever more important. Accurate EMC measurements are essential for Engaging in disputes over product liability Ensuring system or product functionality in its intended EM environment, and Cost-effective EMC in product development. It is an unfortunate fact that some EMC measurements are difficult to undertake simply and repeatably. Increasingly in situations where legislation is involved and litigation may turn on EMC test data, every effort must be made by members of the EMC community to define, specify, understand and work to agreed test methods which yield the most reliable results. Many years of con tin uous effort by engineers and regula tory committees go into developing and ilnproving test methods in the light of realworld measurement experience to arrive at test methods which are technically meaningful, economic to conduct and are as reliable, repeatable and accurate as possible. Such test methods should be regarded with respect and followed carefully as they embody the collective experience of many people. They should not, however, be followed blindly, and an effort should be made to understand how the tests were originated and developed and where they are still difficult to perform, or may be inadequate or inaccurate in some way. EMC symposia held throughout the world have sections devoted to new or improved test techniq ues with a view to improving accuracy and repeatability. Such technical development accoun ts, in part, for the con tin uous changes to EMC standards and specifications.
3.2.2 Accuracy of EMC measurements Measurements of fundamental quantities of mass, length and time have reached levels of extreme accuracy with the advent of lasers and atomic clocks. For example, time and frequency can be measured accurately to about 1 part in ten to the power 13. In everyday engineering some quantities can be measured in the work place to a fraction of a percent regularly and without effort. Unfortunately, EMC measurements of quantities such as electric and magnetic field strengths, power density and wave impedance, surface and cable currents over a very wide frequency range from Hz to GHz, are not particularly easy to conduct or to repeat. When wavefield measurements are made in relation to equipments
being tested which do not have known electromagnetic characteristics, e.g. radiation pattern, farfield distance, etc. and are conducted in imperfect surroundings such as screened enclosures which give rise to multiple signal reflections, then considerable errors can arise. A full discussion of EMC measurement errors is given in Chapter 11 and follows the detailed description of the various probes, sensors and antennas used in testing. In this section a preliminary indication is given as to the source and Inagnitude of some of these errors such that EMC measurement accuracy may be compared with more familiar measurements. Consider a typical EMC measurement system as shown in Figure 3.1. The sources of EMC measurement error will depend on •
•
•
•
The accuracy of the measurement meter, usually an EMI receiver, spectrum analyser, power meter or other sensitive calibrated measuring device. The nature of the sensor or probe connected to the meter. It is important to know what quantities it actually measures, whether it loads or disturbs the quantity being measured and whether its output is an average over space Qr time. Such questions apply particularly to wavefield measuring sensors or antennas. This area of measurement uncertainty has prompted considerable research by national standards bodies, academics and practising EMC engineers into defining exactly the characteristics of specific antennas, what they actually measure in a given test situation, and how to calibrate them. The connection of the sensor Ian tenna to the receiver or measuring meter can play a significant part in introducing errors into the measurement. Anomalies of up to 10 dB have been observed with regard to the movement of antenna cables on open test sites [9]. The surroundings in which the test is being made have a major impact on wavefield measurements. EMC tests are usually carried out in a laboratory with a ground plane on a bench for conducted EMI tests a screened chamber, possibly lined with radio absorbent material (RAM) for military radiated emISSIon and susceptibility tests. Errors have been reported of 17 dB in measurements between standard antennas in RAMlined chambers [10 J an open site, sometimes referred to as an OATS (open-air test site) for commercial radiated and conducted emission tests. Errors of around 10 dB have been reported concerning just the calibration of open sites [11, 12J
OUTLINE OF EMC TESTING SCREENED ROOM TEST CELL EUT AND CABLE PLACEMENT
~SUREMENTSENSOR
UNKNOWN RADIATION FIELD
43
MEASURING METER - EMI RECEIVER OR SPECTRUM ANALYSER
(A BICONIC ANTENNA IN THIS EXAMPLE)
CALI BRATION ERRORS DRIFT ERRORS EQUIPMENT FAULTS
EXTERNAL SIGNALS REACH ANTENNA AND CONFUSE MEASUREMENT
+
C.tt8(
;
12 0 ;:'-1.1::' !V0r: I-t(
o
000
UNBALANCED CURRENTS FLOWING ON MEASUREMENT CABLE INTERACTION BETWEEN ANTENNA AND CABLE
UNKNOWN NEAR FIELD I FAR FIELD DISTANCES FOR RADIATED EMISSION COMPONENTS
~
RADIO ABSORBENT MATERIAL HELPS TO MINIMISE REFLECTIONS
METALLIC FLOOR - GROUND PLANE
~ ,
MUTUAL COUPLING OF ANTENNA ~ TO ITS IMAGE CHANGES ITS CALIBRATION
#i\
~: ~
ANTENNA IS PHYSICALLY LARGE AND AVERAGES THE FIELD STRENGTH
\ ~, "
tI,'
ELECTRICAL IMAGE OF ANTENNA IN GROUND PLANE
'1\
I
I I \ I I \ I \
',I ; ~4I~
Figure 3.1
Possible sources of measurement uncertainty in a measurement oj'radiated emitted jield strength
the actual installation where the equipment is to be used. This is usually confined to large systems which may occupy a whole building, such as a central computing facility or a telephone exchange. Problems encountered in carrying out such testing have been reported and discussed [13 J. In each of these locations the surroundings will affect and sometimes dominate the accuracy of EMC measurements which are made, particularly radiated emission and susceptibility measurements. Multiple reflections from the inside of a screened room have been reported as leading to errors at some frequencies of up to 50 dB in field strengths [14 J. The radiated-field measurement errors are further complicated by the interaction of the measuring meter with the connecting cable and the antenna, which manifests itself as a VSWR mismatch. The cable also acts as a parasitic element to the receiving antenna and disturbs the field being measured [12]. I t can also act as an active element if the antenna or sensor is unbalanced and a significant common-mode curren t flows on the outer of the connecting coaxial cable [9J. Thus the construction of dipole antenna baluns with low leakage and good balanced transformer properties is important in minimising interactions between the antenna, cable and receiver [15-1 7]. Significant errors can be in trod uced into radiated EMI measurements by the interaction of
the antenna and the ground plane at an open site or the walls of a shielded chamber. Such effects can be understood in terms of the mutual im pedance between the test antenna and the electrical images set up by the conducting planes. Monopole antenna input impedances can vary wildly with frequency by an order of magnitude or more inside shielded chambers [18-20J. Such variations will not be allowed for in the antenna calibration factor and will appear as a measurement error. Because the frequency dependent antenna impedance variation will interact with the mismatched antenna cable, the measured field strength against frequency will be a function of the particular antenna at a particular location in the screened room and with a particular connecting cable which is laid out in a particular way. •
One of the largest sources of error is that introduced by the configuration of the EUT itself. The precise layout of cables which cond uct RF currents from the energising source (the E UT) will significantly affect both the amplitude spectrum and the spatial distribution of current along the cables. This in turn will affect the field radiated from it. Changes in cable length or shape will affect the cable resonance and the nature of the conducted and radiated signals particularly at frequencies around cable resonances where changes of 10 dB have been reported for various ways of bundling the same cable [21 J.
44
A HANDBOOK FOR EMC TESTING AND MEASUREMENT
3.2.3 Implications of repeatability of EMC measurements From this overview of some of the issues which determine the accuracy of EMC measurements it is clear that radiated measurements in particular are generally (i)
(ii)
(iii)
(iv)
more complicated than some more common measurements of physical dimensions such as mass, time, pressure, or direct voltage or current the measurements cannot be easily isolated from the surroundings in which they are being made the full nature of the object being measured (in the case of an EDT) is not known and it is far too time consuming and uneconomic to investigate it fully such that the most appropriate measurements of maximum radiated field strength, for example, may be made over a wide range of freq uencies the measurements are consequently far less accurate than those for more common quantities. In general it may be supposed that conducted current measurements can be made with a repeatable accuracy of around 6-10 dB, but radiated measurements can contain errors in excess of 20 dB. White [22J indicates that under some circumstances total measurement uncertainty at some frequencies can be as high as ±40 dB for radiated EMC tests.
With this level of uncertainty in the repeatability of these costly EMC measurements and the legal! financial importance of being able to prove that an equipment meets the national and international EMC standards before it may be sold or imported into a country, it is clear that pressure from government standards and regulatory bodies together with industrialists and engineers will gradually result in developing higher accuracy EMC test methods. There are those, like T.]. Dvorak in Switzerland [12J, who believe that: 'A real improvement would mean adopting entirely new concepts based on recent advances in inverse modelling and modern computer technology or even to completely abandon the measurement of radiated fields in favour of conducted measurements which still have a large developing potential'. I t is conceivable that electronic products of the future will be released against specification limits which are computer derived from standard calibrated source current distributions which simulate the EDT. The EDT conducted current
patterns would be measured and a computer would calculate the full 3D wavefield (including radial componen ts in the near field) as a function of distance which would be compared with a new type of specification. As the need for more accurate results intensifies in the coming years and the cost of manpowerintensive testing increases while the cost of computer modelling decreases, there may be a powerful incentive to move towards Dvorak's technically superior computer simulation methods for EMC clearance rather than relying on measurement. For the present, EMC testing with all its difficulties and uncertain ties will con tin ue to be carried out worldwide on an increasingly wide range of electrical and electronic products as the importance of EMC grows.
3.3 Introduction to EMC test sensors, couplers and antennas
3.3.1 EMC sensor groups Sensors and coupling devices for EMC testing can be grouped basically according to whether the RF coupling is by conduction and induction or by radiation. In some cases the same coupling devices can be used for signal injection and emission measurement. I t is convenient to group probes, sensors, couplers and antennas based on coupIing mechanism and their use in emissions or susceptibility measurement as shown:
Conduction and induction Conducted susceptibility (CS) signal injection Conducted emission (CE) signal measurement Radiative coupling Radiated susceptibility (RS) antennas Radiated emission measurement (RE) antennas
3.3.2 Conduction and induction couplers As an example, an artificial composite conducted EMC test arrangement (CE and CS) is shown in Figure 3.2 where the conduction and induction couplers used in a wide range of emission and susceptibility tests have been illustrated. The direct conduction couplers include 10/lF low-inductance RF feedthrough capacitors Line-impedance stabilisation networks (LISN) Wideband RF capacitor, used in conjunction with a cable break-out box High-impedance wideband voltage probes (oscilloscope probes) Direct ESD injection.
OUTLINE OF EMC TESTING
10uF FEEDTHROUGH CAPACITOR _ TO STRIP °RFo FROM MAINS SUP?LY
45
~ ~~AL WINDING AR.OUND EUT BOX ~ MAGNETIC INDUCTION TEST
r".-----......
~
DISTRIBUTED CAPACITIVE COUPUNG WIRE
CURRENT PROBE MEASUREMENT OR CURRENT INJECTION ON ON POWER UNES
/'or
INDUCTIVE
Figure 3.2
CAPACITIVE CABLE CLAMP
RF GROUND PLANE
RF coupling techniques usedJor conducted emitted (CE) and conducted susceptibility (CS) testing
Figure 3.3 Examples oj coupler frequen(Y coverage for typical conducted EMC testing
DEVICE FREQUENCY COVERAGE
COUPLING DEVICE
(Power line frequencies) SPIRAL WINDING ( BOX) SPIRAL WINDING (CABLE) 10 uF CAPACITOR USNs AUDIO TRANSFORMERS TORROIDAL CURRENT PROBES
(SJ: 'ke Injection) (Sp kes)
(Pm er frequ.)
INDUCTIVE CLAMP CAPACITIVE CLAMP DIRECT CAPACITOR INJECTION ESD PROBE HIGH IMPEDANCE VOLTAGE PROBE 10
100
1k
10k
100k FREQUENCY Hz
Induction couplers include Cable and surface current probes Spiral induction windings for boxes Spiral induction windings for cables Distributed capacitance parallel wires Capacitive and inductive cable clamps Indirect ESD to nearby conducting plane. There are a large number of sensors and probes which are commonly used to make EMC measuremen ts by using direct connection or close spaced induction. The approximate frequency coverage of each type of coupler Isensor is shown in Figure 3.3. The technical details of these devices is discussed in subsequent chapters where their use in carrying out typical EMC tests is explored.
3.3.3 Radiative coupling - EMC antennas Figure 3.4 shows a collection of typical standard antennas used for radiated emission and susceptibility testing. The approximate frequency
1M
10M
100M
1G
coverage of these antennas is given In Figure 3.5. They include the following types: H-field loop antennas for measuring the magnetic field component of an EM wave Small shielded loop antenna (155 mm dia.) Large shielded loop antenna (0.5 m dia.) E-field monopole antennas for measuring the electric component of the wavefield (with respect to an RF ground plane) 1 m effective height rod antenna Small battery powered ~ensor with fibre optic readout for susceptibility field-strength levelling Free-field antennas such as: Long wire an tenna Tuned dipole sets Broadband biconic antenna Logarithmic conical spiral antenna' (circularly polarised) and log-periodic antenna (linearly polarised) Horn antennas of various sizes Dish reflector type an tennas for microwaves
46
A HANDBOOK FOR EMC TESTING AND MEASUREMENT
~IGH GAIN REFLECTOR MICROWAVE ANTENNA
~ ~ORNANT~
0/
SMALL LOOP "H FIELD" ANTENNA
_
.----:-;;:;C --;to\S\ ~NCS
_
~m-~;30fn
SPECTRUM ANALYSER OR EMI RECEIVER
~m,3m,
.~RO\SS
LOG. CONICAL
LjSPIRALANTENNA
SIGNAL IN
o
000
10Hz - 40GHz
MONOPOLE ANTENNA TUNED DIPOLES
PARALLEL PLATE LINES TEM CELLS ETC (not to scale)
I
I
RF LEVELLING AMPLIFIER CONNECTED TO FIELD SENSING PROBE via FIBRE OPTIC CABLE
I
BATTERY POWERED "E FIELD" SENSOR WITH FIBRE OPTIC CONNECTION TO FIELD LEVELLING AMPLIFIER TO CONTROL FIELD LEVEL DURING A SUSCEPTIBILITY TEST
LOOP ANTENNA
(H FIELD)
O
t--------
RF POWER OUTPUT TO
SUSCEPTIBIUTY ANTENNAS ....-
p"'igure 3.4
...
Antennas and sensors used in radiated emission (RE) and radiated susceptibility (RS) testing
Figure 3.5 Examples of antenna frequency coverage .for typical EMC antennas
ANTENNA or SENSOR
DEVICE FREQUENCY COVERAGE
SMALL LOOPS LARGE DIAMETER LOOPS 1m MONOPOLE LONG WIRE TUNED DIPOLES
-- ---- ------
--
-
BICONIC DIPOLE LARGE LOG. CONICAL SPIRAL LARGE LOG. PERIODIC LARGE HORN SMALL HORNS
-
-
--- -
SMALL LOG. CONICAL SPIRAL DISH REFLECTOR + FEEDS
-
--
-
,.....
-
.......
FIBRE OPTICLY COUPLED E FIELD SENSORS PARALLEL PLATE LINES T.E.M. CELLS G.T.E.M. CELLS
10
Bounded-wave antennas usually used for susceptibility testing: Parallel-plate lines
"rEM (transverse electromagnetic) cells erawford cells
GTEM cells (gigahertz TEM).
3.4 References WHITE, D.R.J.: 'A handbook series on EMI', vol. 3, section 6.3, pp. 6.20-6.21 (Formulas for
100
1k
10k
100k 1M 10M FREQUENCY Hz
100M
1G
10G
1000
capacitive crosstalk between parallel wires) DWC Inc., PO Box D,Gainesville, Virginia 22065, USA 2 PITT, A.D.: 'EMC guidelines'. BAe ST23357, Oct 1979, pp. 10.2-10.6. British Aerospace Dynamics, Six Hills Way, Stevenage, Herts, UK 3 RICKETTS, L.W., BRIDGES, J.E. and MILETTA, J.: 'EMP radiation and protective techniques' (Wiley) p. 147 4 BURKE, G.J. and POGGIO, A.J.: 'Numerical electromagnetic code (NEC) - method of moments'. Technical document 116, Naval Ocean Systems Center, 1981
OUTLINE OF EMC TESTING 5 PRICE, W.O. and CANAGA, K.W.: 'Accuracy estimation in anechoic chamber testing'. Proceedings of IEEE conference on EMC, 1985, pp. 161-169 6 KING, R.W.P.: 'The theory of linear antennas' (Harvard University Press, 1956) 7 'ESA GEOS spacecraft development control plan'. BAe Dynamics, Filton, Bristol, UK, 1974 8 DUFF, W.G.: 'Fundamentals of electromagnetic compatibility'. Interference Control Technologies Inc., Gainesville, Virginia, USA, p. 9.22 9 DEMARINIS, J.: 'The antenna cable as a source of error in EMI measurements'. Proceedings of IEEE symposium on EMC, 1988, pp. 9-14 10 AG.ARWAL, N.K.,JOSEPH, P.C. and KESAVAN NAIR, P.N.: 'Evaluation of shielded anechoic chamber for EMC measurement'. Proceedings of IEEE symposium on EMC, 1985, pp. 134-138 11 TSALIOVICH, A.: 'Absorber-lined open area test site - a new type of EMC test facility'. Proceedings of IEEE symposium on EMC, 1988, pp. 106-111 12 DVORAK, TJ.: 'Fields at a radiation measuring site'. Proceedings of IEEE symposium on EMC, 1988, pp. 87-93 13 COONCE, H.E.: 'On premises emission testing of digital switching systems'. Proceedings of IEEE symposium on EMC, 1988, pp. 15-18 14 JOFFE, E.B.: 'Are RS03 limits in the HF band realistic?'. Proceedings of IEEE symposium on EMC, 1990, pp. 196-201 15 DASH, G.: 'A reference antenna method for
16
17
18
19
20
21
22
47
performing site attenuation tests'. Proceedings of IEEE symposium on EMC, 1985, pp. 607-611 BERRY, J., PATE, B. and KNIGHT, A.: 'Variations in mutual coupling correction factors for resonant dipoles used in site attenuation measurements'. Proceedings of IEEE symposium on EMC, 1990, pp. 444-450 BRENCH, C.E.: 'Antenna differences and their influence on radiated emission measurement'. Proceedings of IEEE symposium on EMC, 1990, pp. 440-443 MISHRA, S.R., KASHYAP, S. and BALABERDA, R.: 'Input impedance of antennas inside enclosures'. Proceedings of IEEE symposium onEMC, 1985, pp. 534-538 McCONNELL, R.A.: 'An impedance network model for openfield range site attenuation'. Proceedings of IEEE symposium on EMC, 1990, pp. 435-439 MISHRA, S.R.: 'Effect of ground plane and chamber walls on antenna input impedance'. Proceedings of IEEE symposium on EMC, 1988, pp. 395-399 HUB lNG, T.H.: 'Bundled cable parameters and their impact on EMI measurement repeatability'. Proceedings of IEEE symposium on EMC, 1990, pp. 576-580 'A two-part five-day comprehensive training course in EMC, Part 2'. VG A15393, Don White Consultants;' PO Box D, Gainesville, Virginia 22065, USA
Chapter 4
Measuretnent devices for conducted EMI 4.1 Introduction
Ridged horns Horn-fed dishes LF magnetic loops Ferri te-cored loops Leakage probes
The approach taken in this book with regard to ordering and presenting information about the dozens of sensors, couplers, probes and antennas used in EMC measurement is to discuss them in groups that are defined by the manner in which they couple to the signals being measured. The different physical processes related to each type of coupling can thereby be appreciated and a practical insight gained in to their operation which is reflected in the structure of the various test methods employing these devices. Although this particular chapter concentrates on coupling devices used in conducted EMC testing, all the groups of sensors discussed in this book, including radiated emission and susceptibility antennas, are listed here to show the number of groups and to illustrate the range of sensors considered. Sensors are grouped as follows:
Devices for radiated susceptibility (RS) testing •
•
•
Conducted emission and susceptibility tests (CE, CS) •
•
•
Direct connection devices Line impedance stabilisation network (LISN) 10 flF feedthrough coupling capacitor Injection transformers High-impedance voltage probes Inductively coupled devices Cable current probes Cable current clamps Surface current probes Straight and spiral wire inductive coupling Inductive loops around boxes Ferri te wands or probes Direct electrostatic discharge Handheld spark generators
•
4.2 MeasureD1ent by direct connection Devices that are directly connected to the E UT are of two types: (i)
Devices for radiated emission (RE) tests •
Free-field antennas Short transmission lines Long wire antenna Biconic dipoles I...Jog. spirals and periodics Standard and ridged horns Dish-type reflectors Bounded-wave antennas Parallel-plate strip lines Crawford cells TEM and GTEM cells E-field sensors Battery powered monopole sensors Electrically short dipoles Diode detection dipoles Fibre-optic coupled sensors Monopoles Monocones Radhaz monitors Indirect ESD Various metal plates onto which the spark is discharged
(ii)
Radia ted emission antennas Long wire antenna Monopoles (passive and active) Tuned dipoles Biconic dipoles Log. conical spiral Log. periodic Standard horns
48
Networks that produce a defined load impedance as a function of frequency to the circuit being tested. The most common device of this type is the line impedance stabilisation network (LISN). Probes that can be connected to the circuit to be measured wi thou t significantly altering the RF voltages, currents and circuit impedance. High-impedance RF voltage probes commonly used with oscilloscopes, transient recorders or spectrum analysers are an example of this second type of direct connection device.
MEASUREMENT DEVICES FOR CONDUCTED EMI TO EQUIPMENT UNDER TEST
4.2.1 Line impedance stabilisation network The development of the LISN goes back to the 1950s when early EMC engineers needed to develop a means of providing a standard power supply impedance network for aircraft, in an attempt to correlate equipment EMC bench test results with those carried out using actual aircraft supplies. In the civil sector, the parameters of the mains network were determined by analysing many measurements of the RF impedance of domestic, industrial and other supply systems [1]. The mean values were found to be well represented by an equivalent circuit with 50 ohrns in parallel with an inductance of 50 pH. Good agreement was possible between several countries and so this network was adopted by CISPR in publication 16 [2J and has since been incorporated into a number of standards including BS800jEN55014. When an LISN jartificial mains network with a defined impedance is used, it is possible to simply and quickly measure the RF voltage as a function of frequency appearing on the conductor under test. The specified levels for the test are written in terms of a measured RF voltage as a function of frequency. Thus the voltage measurements made using an LISN can be compared directly with specified limits and many commercial standards, such as BS800jEN55014, BS6527jEN55022, VDE0877 and VDE0871, etc., specify conducted EMI only in terms of RF voltage across a standard LISN. However, knowing the RF impedance of the network also permits the calculation of the RF curren t flowing along the cond uctor into the LISN terminal. This can then be compared with measurements made using alternative direct techniques such as inductive current probes as used in MIL STD 462 tests CEOI to CE05. Practical LISNs fulfil three functions: Filter incoming RF noise from the maIns supply Provide a known impedance as a function of frequency to the equipment under test (EDT), and Provide a matched 50 ohm RF connection to the EMI meter. One of the most commonly used LISN designs is that required for meeting tests CE02, CE04, CS02 in notice 3 of MIL STD 462 [3]. The circuit of the LISN is shown in Figure 4.1a with its impedance profile from 14 kHz to 10 MHz in Figure 4.1b when the measurement port is loaded with 50 ohms. Note that the EDT line impedance is 50 ohm down to about 1 MHz. A second circuit
L (APPROX. 56uH)
A
8
49
TO POWER SOURCE 50A 15A
O.1uF
600V
Z1
C2 25 k ohm 20W
0.03uF 600V
R4
NC
Zs
50 OHM COAXIAL TERMINATION IN PLACE OF RFI METER
(aj (J)
:;E I
50
0
UJ
40
()
Z
UJ
a.. ~
30
LL
0
UJ :::::>
20
....J
>
UJ
r-
10
:::::>
....J
0
(J)
co
0 0.014
0.1
1.0
10
FREQUENCY MHz
(bj Figure 4.1
56p,H line impedance stabilisation network ( LISNj ( a) Circuit diagram (b) US MIL-STD 462
is given for use in MIL STD 462 N3 which is suitable for measurements over a higher frequency range up to 50 MHz, see Figures 4.2a and 4.2b. Figure 4.3 shows the basic circuits for three LISNs required to carry out testing to CISPR 16, FCC (MP4) and VDE 0876. Specifications BS3GI00, NWS3 and DEF STAN 59-41 require the use of a 5 pH LISN for tests up to 150 MHz which incorporates special design elements to suppress unwanted high-frequency impedance variations owing to component self resonance. See Figure 4.4a for the circuit details and Figure 4. 4b for the limits of acceptable impedance with frequency. DEF STAN 59-41 specifies the addition of a 10 pF feed through capacitor connected across the power input to the LISN for AC lines and a 30,000 pF capacitor for DC supplies to further stabilise the power supply impedance. Figure 4.5 shows the limits of acceptable impedance for the LISN with the power supply terminal connected to the case.
50
A HANDBOOK FOR EMC TESTING AND MEASUREMENT, TEST SAMPLE
L
COAXIAL
~g~~~~~~R
...__.....
MEASURING SET
(aj
50
Iii' E ..c
.2w ()
40
30
z
« 0
w
a..
~
20
z
(J)
::J
10
0 0.1M
10M
1·0M
100M
FREQUENCY Hz
(bj
Figure 4.2
US MIL-STD 462 HF 5 IlH LISN ( aj Circuit diagram (b j LISN impedance
TO EUT
TO MAINS SUPPLY
TO 50 ohm RECEIVER
CISPR PUBL. 16
L150uH TO EUT
TO MAINS SUPPLY
TO 50 ohm RECEIVER or 50 ohm TERMINATION
FCC MP-4
TO EUT
>
TO MAINS SUPPLY
TO 50 ohm RECEIVER
A 50 ohm 50llH design (9 kHz-30 MHz) is given in BS727 which allows the connection of the EMI meter to both the live and neutral terminals of the power lead of the EDT by operation of a switch. See Figure 4. 6a. Note the pi-section input filters formed from L 1 , C1/R 1, C2 /R 2 . Figure 4.6b shows the acceptable impedance limits. The impedance profile of the higher frequency 5 IlH 50 ohm LISN (150 kHz-l 00 MHz) specified in BS 727 is that given in BS3G 100. BS 727 also defines the componen t values for a 150 ohm resistive LISN, see Figure 4.6a. A more sophisticated 150 ohm LISN that allows measurement of both the symmetrical and asymmetrical interference voltages from an EDT is specified in BS905 pt I/EN550 13, see Figure 4.7 . LISNs are usually mounted on a specified metallic ground plane alongside the EDT as in Figure 4.8 and the EMI meter is connected via a high performance low-loss coaxial cable to the measurement port. LISN measurement ports on lines which are not being measured are terminated in 50 ohms. Typical narrowband voltage limits (BS3G 100) for conducted interference measured in this way can be seen in Figure 4.9. Limits applicable to interference from information technology equipment as specified by FCC part 15 and the now superseded VDE 0871 (measured using a quasipeak detector in the EMI meter) are given in Figure 4.10. LISNs can be usedjust to provide a stable known terminal impedance for EDT power lines and the actual measurement of conducted current is made using inductive current probes. This is the case for test DCEOI in DEF STAN 59-41, see Figure 4.11a. The conducted current limits in this case are specified in dB IlA not dB 11 V. See Figure 4.11 b. The LISN is a widely used coupling and standard impedance device which is easy to use and has been incorporated into most commercial and military EMC standards. It represents realworld power line impedances well and is an inexpensive device, but is limited in respect of its freq uency span and its upper frequency limit, which is usually determined by spurious component resonances. The LISN can be used to inject an RF signal directly onto a power line that feeds the equipment under test. Consider the circuit of the simple network shown in Figure 4.2a. The signal is injected on to the line under test via the high quality low-inductance/low-loss capacitor C 1 . The series inductor L 1 provides a high impedance to the injected RF being shorted out by the mains circuit outside the test area. The capacitor C2 dumps any RF signal which passes the inductor to ground and prevents it from contaminating the external mains system. Y
VDE 0876 Teil1
Figure 4.3
Various LISNs for use in testing commercial electronic equipment
MEASURElVIENT DEVICES FOR CONDUCTED EMI For details of inductor see table
I
"'TOSOohm MEASURING RECEIVER
}-'igure 4.4 LISN suitab Ie for UK BS3G100 Pt.4 Sec.2 ( a) Circuit details ( b) Limits of acceptable impedance
rl
C1 O.OS,UF
C2 TO.OSpF ,.....,................
51
,.......~~r~"r-,.
TO EQUIPMENT
TO MAINS SUPPLY
10 ohm
DETAILS OF INDUCTOR CURRENT RATING
INDUCTANCE
INSIDE DIA. LENGTH
A
~H
mm
mm
10 100
5 5 5
25.4 50 90
32 115 178
500
No. OF TURNS
CONDUCTOR CROSS SECTION
mm 20
1.6dia 6dla 12.5 x 12.5 sq.
18 11
(aJ 80----------r-----------r---------.,.------.
(j)
E
60
.c
~
----------"".,.-',""
UJ
~ 40
o
UJ
a.. ~
20
"--
O ~
0.1M
...L.__
1.0M
10M
___IL..._
...I___
100M
____'
200M
FREQUENCY Hz
(b)
Reproduced by permission of BSI
50 __---r---...,.---r------r----,----,------,
_
The use of an LISN in this way is required for some conducted susceptibility tests such as MIL STD 462 Notice 3 test CS02 (method B 150 kHz to 65 MHz). Signal voltages of less than 1 V RMS are normally injected but RF power of up to a few tens of watts can be injected by this means for a short time.
40
~ "§ 30 (II
:g. w
o
20
z
(§
w
10
c.. ~
0 -1 0 L------'......_ _..,.;._ _"--_---L._ _--I--_ _ 10k 100k 1M 10M
~_----J
10
FREQUENCY Hz
* ACTUAL L1SN IMPEDANCE MUST LIE WITHIN THESE LIMIT LINES Figure 4.5
LISN,for use with UK DEF STAN 59-41 limits for impedance against frequency with supply or load terminal connected to case
Reproduced by permission of HMSO
4.2.2 10 JlF feedthrough capacitor MIL STD 461/2 calls for the use of a good quality 10 flF RF feed through capacitor which can be used to provide a known low RF terminating impedance on power lines in tests such as CEO 1, CE03 and CS06. See Figure 4.12. In the spike immunity test CS06, the RF short circuit
52
A HANDBOOK FOR EMC TESTING AND MEASUREMENT
Figure 4.6 Dual line LISN suitable for use with BS727: Radio interference measuring apparatus ( a) -Circuit of 50 JlH design (b) Acceptable impedance limits
C3
L
R2 ~ -E
R3
I
: EQUIPMENT: ----1.._---;,-0 UNDER I I TEST I
MEASURING RECEIVER
0-_..--.....
R2~
~
N
COMPONENT VALUES FOR 50 ohm I 5O,..H NETWORK COMPONENT
VALUE
COMPONENT
R1 R2 R3 R4 R5 R6
10 ohms 5 ohms 1kohm 50 ohm (short) 50 ohms
C1 C2 C3 L1 l2
COMPONENT VALUES FOR 150 ohm NETWORK
VALUE
COMPONENT
VALUE
1.2pF 8JJF O.25J,1F 250,uH 50}lH
R1 R2 R3 R4 R5 R6
open short open 150 ohm 100 ohm 50 ohm
COMPONENT C1 C2 C3
l1 l2
VALUE open 0.251JF O.1I-'F short short
(a) 100
r-------.,..--------.------,..--..,
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50
en
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E ..c: ,£.
; ;;,;
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;
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"
TOLERANCE +/- 20%
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1'--10k
1--
...1.-
100k
1M
--1._--1
10M
FREQUENCY Hz Reprod uced by permission of BS I
provided by the two capacitors ensures that the full high-frequency spike voltage is applied to the EDT power input. Because these capacitors are intended for use with power lines they must operate up to 600 V DC and be designed to handle mains currents of more than 100 A without significant loss. Such capacitors are quite large, 85 X 85 X 70 mm, and are designed to be bolted to the RF ground-plane or to the screened room wall when being used as a power line filter. The RF attenuation characteristic of a commercial 10 JlF capaci tor (Solar 6512-1 06R) [4] is shown in Figure 4.13.
4.2.3 RF coupling capacitors High-quality low-inductance RF coupling capacitors are widely used in EMC testing to
(b) inject RF voltages onto the power lines connected to EUTs. The principle is to isolate the mains low-frequency AC, or DC line voltage from the signal injection RF power amplifier while efficiently coupling the injected RF energy into the power line circuit. Typical circuits may have an RF impedance from tens of ohms up to a few hundred ohms. The coupling capacitor must therefore have negligible series reactance at the lowest frequency of the test. The capacitor should not exhibit self resonance at high frequencies where the self inductance due to foil construction or lead lengths becomes significant. In MIL STD 461, a capacitor with a reactance Xc < 5 ohms is required for the CS02 test over a frequency range of 50 kHz to 400 MHz. This requires a capacitor with a value of at least 0.1 fiF. See Figure 4.14. Practical coupling
MEASUREMENT DEVICES FOR CONDUCTED EMI
r-----------------------------------
5 : ~
0::
S
---~
I
R12
I
L
::>
o u:: z o o
53
Figure 4.7 150 ohm LISN for measuring conducted interference ( EN55013 [BS905 pt.1])
~ ....J UJ o I-
::>
a.. ~
,r
(j)
I
~
I
z
::?E
I
2
OUTPUT TO EUT
l_~monaifiiter il;'~;;:-ed- ----- - -- -
I
- - - -- -- -.1:--- - J
SWITCH S - Position 1 - symmetrical interference Position 2 - Asymmetrical interference Resistor values for ISO ohm LISN
Measuring apparatus
Value ohms 120 150
390 270 22 110 50
capacitors are constructed from a number of different capacitors in a network designed to minimise self resonance in the VHF and UHF ranges. Coupling capacitors are also used for exan1ple in IEC 801 part 4 (BS6667) where high-frequency transient bursts are injected into industrial process measurement and control equipment. See Figure 4.15. The capacitors have a value of 33 nF which results in a negligible impedance to the frequency components of the 50 ns (50% width) voltage spikes. Note also the use of ferrite damped inductive and capacitive decoupling of the injectors from the mains input. The draft for part 5 of BS6667 of 1990 [5 J considers the immunity of industrial process control equipment to voltage surges. In this
Reproduced by permission of BSI
standard, capacitors with values of 9 or 18 JlF are specified for injecting surges onto power lines and smaller capacitors with a value of 0.5 JlF are used for injection onto input/output (I/O) or control lines. See Figure 4.16a. For line-to-ground testing a series resistor of 10 ohms is used with the 9 JlF capacitor for power lines and a 40 ohm resistor in series with the 0.5 JlF capacitor for I/O lines. See Figure 4.16b and 4.16c. Boresero et al. [6J describe a current injection method for evaluating the immunity of broadcast receivers to RF interference based on an injection capacitor. See Figure 4.17. The reactance of the capacitor must be small compared with the 150 ohm resistive component of the total generator and coupler impedance. Once more an inductive/capacitive filter forms an integral part
LINE IMPEDANCE STABILISATION NETWORKS (ONE FOR EACH POWER LINE)
.-------
POWER INPUTS
-----
AC or DC
50 ohm resistive termination on line not being tested
Figure 4.8
Conducted emission measurement LISNs mounted on ground plane bench
54
A HANDBOOK FOR EMC TESTING AND MEASUREMENT MEASURING SET
POWER SUPPLY
BROADBAND CONDUCTED INTERFERENCE LIMITS
SCREENED ROOM WALL
80 W C)
~
70
30,OOOJJF CAPACITOR FOR DC SUPPLIES
....l
0
>
60
N
WI
()~
z> W=t
L1SNs USED TO PROVIDE A DEFINED RF IMPEDANCE FOR POWER LINES
50
0::,...
w(1)
u.. >
0::0
40
~en
30
W.c .-0 0"0 Z
20
en 0
CURRENT PROBES ARE USED TO MAKE THE MEASUREMENTS
0
10
0::
en
0
10k
100k
1M
10M
EUT
100M
FREQUENCY Hz
GROUND PLANE
NARROWBAND CONDUCTED INTERFERENCE LIMITS NORMALRFMEASUREMENTPORTS TERMINATED IN 50 ohms
90
w
<:>
.
..J
80
0
TEST METHOD DCE01 (POWER LINE CONDUCTED INTERFERENCE 20Hz - 150MHz)
(a)
> w
()
z
w a:: w>
~=l
W,...
,-(1) Z _
70
(j) .0
o
~ 130
60
~
~ 110
0>
0"8
50
Zm
(i5-o
~
90
~
70
~
50
"0
~
40
0
a:: a::
:J
z
z
w
30 10k
100k
10M
1M
100M
ffiLl.. 0::
W I-
FREQUENCY Hz
Figure 4.9
~
30 10 L -_ _J..-.._ _J..-.. 10Hz
100Hz
Typical conducted emission limits using LISN (BS3G100 pt.4 sec.2)
'--
1k
10k
' - - _ - - - L ._ _---L._ _- - '
100k
1M
10M
150M
FREQUENCY Hz
(b)
Reproduced by permission of BSI
Figure 4.11 of the coupler and isolates the wan ted signal source from the injected interference. The use of coupIing capacitors for injecting RF interference into power and signal circuits is a common technique. It is simple and inexpensive. I t can be trouble free if good quality non self100 , . - - - - , . - - - - , - - - - , . . . . - - - - r - - - . , - - - . , - - - - - - , CI)
UK DEF STAN59-41 test DCE01 using LISNs (a) Use oj current probes ( b) Limits for aircraft use
Reprod uced by permission of HMSO
resonant components are used. The EMC test engineer must take care however to ensure that the connection of the coupling capacitor does not lead to the following problems.
...J
W
> W
...J
80
W
- - - CLASS A F C C -
()
r-------
Z
W
~$
0::"'-
I
60
~ ~
~~
CLASS A & C VDE
CLASS B VDE & FCC
OOJ
~"O
40
()
RECEIVER BANDWIDTH--I_---RECEIVER BANDWIDTH 200Hz -19kHz
:J
o
... !
Z
o
()
20
r.....-_--'"_ _--lo-_ _--I,..-_ _- i -_ _- ' - -_ _- ' - - _ - - - '
10k
30k
100k
300k
1M
3M
10M
30M
FREQUENCY Hz
Figure 4.10
FCC and VDE interference limits for conducted emission tests on IT equipment using LISN
(i)
For AC power circuits
High-power line voltage can be coupled back through the capacitor into the signal source and damage the source output circuit. The power line voltage fed back through the coupling capacitor also must not damage the input circuit of any RF voltage measuring meter which is connected to monitor the injected RF susceptibility signal. Some proprietary coupling capacitors (e.g. Solar 7415-1) have built in high-pass filters to the power line frequency in the monitor output to preven t this.
MEASUREMENT DEVICES FOR CONDUCTED EMI (ii)
50
TRANSIENT GENERATOR
~
::r:
TRANSIENT WAVESHAPE
10IJF CAPACITOR
10IJF CAPACITOR
-=-
SERIES INJECTION ON AN AC LINE
(a)
(b)
(c) Figure 4.12
MIL STD 461 test using 10 J-lF capacitors (a) Spike injection test (b) Test CEOI (c) 10 J-lF Jeedthrough capacitors
55
For signal I/O and control lines
'The connection of a coupling capacitor can load the signal circuit thus reducing the wanted signal level below that specified for correct circuit operation. When the interference voltage is applied via the capacitor the ratio of the normal functional signal to injected susceptibility signal will be artificially low and the EUT will tend to fail the test at a lower level of injected interference than it should. Direct capacitive coupling via lumped components is not particularly suitable for circuits operating at RF or carrying data with fast-rising and falling (nanosecond) edges, as the high frequencies will see the injection capacitor as a low-impedance short.
4.2.4 Distributed capacitance couplers The use of lumped injection coupling capacitors has a number of technical drawbacks which have been mentioned earlier. An additional problem is the time, trouble and cost involved in making the direct connection to the conductor under test by splitting the cables or using special break-out boxes. It may not be possible to make such a connection at all in the case of overall screened multiconductor cables or coaxial signal cables. For some circuits to be tested, many of these problems of direct connection can be overcome by the use of distributed capacitance clamps if the testing standards permit. Using such a device the cable under test can simply be laid in the clamp with no direct physical connection being made. A device specified in IEC80 1 part 4 is shown in Figure 4.18a. I t is used for testing as shown in Figure 4.18b. Such a clamp however only has a capacitance of between 50 and 200 pF for cables of 4 to 40 mm dia. This low coupling capacitance restricts efficient injection to high-frequency interference signals or pulses with fast nanosecond risetimes. In some cases it will not be possible to physically place the signal or I/O cable under test in the capacitance clamp owing to the inflexibility of the cable in question or the size of the clamp. In such circumstances IEC80 1 pt 4 permits the use of a simple wire or conductive metal tape to be used by placing it alongside the cable under test to form a distributed coupling capacitor. See Figure 4.19. 1'he capacitance between the cable under test and the injection wire should be adjusted to be in the same range as for the purpose designed clamp (50-200 pF). Distributed capacitance probes can also be used in conducted emission measurements. White [7] illustrates a spaced wire technique on a ground plane for measurifig common-mode cable
56
A HANDBOOK FOR EMC TESTING AND MEASUREMENT 70
r-------.,-------~-------r_------~------__,.------___.
TYPICAL PERFORMANCE [SOLAR 6512-106R] 60 50
co "0
SAE MINIMUM ARP-936 REQUIREMENT
40
CJ) (/)
9
30 20 10
10M
1M
100k
10k
100M
1G
FREQUENCY Hz INSERTION LOSS MEASURED IN a 50 ohm CIRCUIT as per MIL-STD-220A
Figure 4.13
Attenuation oj RF conducted current through 10 J-lF Jeedthrough capacitor
Reproduced by permission of Solar Electronics
~5cm~ BLOCKING INDUCTOR
..---
TEST SAMPLE
r-'lOWER SOURCE
Xc< 5 ohms WAVEFORM MONITOR
POWER AMPLIFIER
Figure 4.14
500hms
OSCILLOSCOPE OR TUNEABLE RF VOLTMETER
Conducted susceptibility test using RF coupling capacitor (MIL Syn 461 50 kHz-400 MHz)
Figure 4.15 Use of coupling capacitors in IEC801-4)· transient injection onto power lines
33nF COUPLING CAPACITORS TRANSIENT BURST
GENERATOR EUT
L N E
INTERNAL POWER SUPPLY
Mains filter section
Reproduced by permission of BSI
Insulating support
REFERENCE GROUND PLANE
emissions. AIm-long coupling wire is spaced 5 cm above the cable under test, which is itself 5 cm above the ground plane. See Figure 4.20.
4.2.5 High-impedance RF voltage probes It is often required to make a measurement of RF voltage on a conductor or component without
100,uH DECOUPLING INDUCTORS (With lossy ferrite damping)
loading the circuit to any appreciable extent so that a true representation of the signal characteristics can be observed. This implies that any direct connection device should have a high input resistance and a low shunt capacitance to present a high impedance across the frequency band of interest. For EMC measurements, such probes need to cover at least DC to 1 GHz if possible.
MEASUREMENT DEVICES FOR CONDUCTED EMI
57
Decoupllng network
-oI AC(DC)
POWER SUPPlY
I---~L....L-L-J-L---------_+_--
----__(
I
T -0-------------------------------~
EUT
(a)
Decoupling network
OI AC(DC)
R=10ohms
~--('(Ynl---------_+_---_+_
--___<
I
POWER
o
SUPPLY
I O - - - I I t - - - - - - - - - - - - - - - t l I - - - - - - - - - -___
EUT
~___i!_____,._-'
I± (b)
R = 400hms
C=O.5pF
Decoupllng network 1.5 tnH INDUCTORS
AUX. EQUIP.
EUT
(c) Figure 4.16
Various coupling capacitors used in IEC801-5 for transient burst injection ( a) Test set up for capacitive coupling on ACI DC lines)· line-to-line coupling) generator output floating (b) Test set up for capacitive coupling on ACIDC lines)· line-to-ground coupling) generator output floating or earthed (c) Test set up for unshielded 110 lines)· line to ground coupling
Reproduced by permission of BSI
58
A HANDBOOK FOR EMC TESTING AND MEASUREMENT Coaxial cable, twisted pair or multi-lead cable - screened or unscreened Coupling unit
I
other cables for mains, loud speakers etc each terminated with a coupling unit (150 ohms)
t
EQUIPMENT UNDER TEST
-- D
J
-----_--.. -----,,----....
~C1--
~
\
1
L
100 ohms
+ Wanted signal generator or auxilary equipment
.. /
//
/
/
/
/
/
/
/
/
L = Isolation inductance INTERFERENCE RF SIGNAL GENERATOR
C 1 & C2 capacitors with low RF impedance
I = INTERFERENCE CURRENT R int + R1 = 150 ohms
Figure 4.17
Coupling capacitor used for signal injection (EN55020 [BS905})
Probes are available in two types: passive and active. Passive probes are usually built as voltage dividers with ratios of 10: 1 or 100: 1 and can present input resistances of greater than 1 megohm. The physical construction of the probe and the self capacitance of the input resistor determine the shunt capacitance. Probes of this type are commonly used with oscilloscopes for voltage measurements within a limited bandwidth of around 100 MHz. If the signal to be measured is large enough to withstand the probe attenuation and the bandwidth of the probe is sufficiently high, these oscilloscope probes can be adequate for EMC
Reproduced by permission of BSI
measurements. When such probes are used to measure susceptibility signals impressed on power lines care must be taken to ensure that, although the amplitude of the RF signal to be measured is acceptable, the maximum probe input voltage is not exceeded by the power line voltage. Input parameters for typical passive probes can be found in Table 4.1. Care must be taken to establish the upper frequency limit of such probes when used for EMC testing as they will typically not perform satisfactorily much above 50 or 100 MHz. Special high-frequency probes (HP 85024A for example) can be obtained with
Table 4.1 Examples ofprobes suitable for use in EMC measurements Active! paSSIve
Probe
Division ratio Input R
P P P P P
10430A 10435A 10436A 10440A 10437A
10: 1 10: 1 10: 1 100: 1 1:1
A A A A A A A
85024A
1: 1 10: 1 1:1 10: 1 100: 1 10: 1 100: 1
41802A 1124A 1141A
(All equipment -
Hewlett Packard)
Shunt C
(ohms) 1M 1M 10M 10M 50
(pF) 6.5 7.5 11 2.5
1M
0.7
Compensates for input
Max. volt
1 M, 6-9pF 1 M, 10-16pF 1 M, 18-22 pF 1M,6-14pF
450 450 450 450
Freq. range 0.3 M-3 G
1M 10M
12 10
5 Hz-100 M DC-100M
1M
7
DC-200M Diff. mode
1.5 15 50 10 100 200
MEASUREMENT DEVICES FOR CONDUCTED EMI
59
. - . . - - - - - - - - - - 1000 mm ----------~I
I
o
o
o
o
o
o
o
70mm
COUPLING PLATES
INSULATING SUPPORTS
~UND ~=~metre f
.--
PLANE SHAll HAVE A MINIMUM AREA OF
EXTENDING BEYOND THE DEVICE ON ALL SIDES BY MORE THAN O.1m
1050 mm
- - - - - - _....
(a)
EUT(2)
EUT(1)
CAPACITIVE COUPLING CLAMP i1
i2 AC MAINS SUPPLY
AC MAINS SUPPLY
a.1m /
Insulating support GROUNDING CONNECTION TO MANUFACTURERS SPECIFICATION
Figure 4.18
TO INTERFERENCE SIGNAL GENERATOR
GROUNDING CONNECTION TO MANUFACTURERS SPECIFICATION
(b)
Distributed capacitance clamp for use in IEC80 1-4 susceptibility testing ( a) Construction of capacitance coupling clamp (b) Use of the clamp for injection into control and signal cables
Reprod uced by permission of BSI
Figure 4.19 Tape or wire distributed capacitor injection (for HF transients IEC801-4) THIS CONNECTION SHALL BE AS SHORT AS POSSIBLE AC MAINS SUPPLY
COMMUNICATIONS LINES 110 CRICUITS PROTECTIVE EARTH
/ The coupling device shall be a conductive tape or a metallic foil in parallel or wrapped around as closely as possible to the cables to be tested The coupling capacitance shall be equivalent to that of the CLAMP
1 Reproduced by permission oCBSI
60
A HANDBOOK FOR EMC TESTING AND MEASUREMENT
RF GROUND PLANE
10 x 15 em FOAM CHANNEL 1m long
//
CONNECTOR FOR SHORT / OPEN CIRCUIT
5mm FOAM SUPPORT FOR CABLE
Figure 4.20
Capacitive coupling wire used for cable emission measurements
an input resistance of 1 megohm and a shunt capacitance of only 0.7 pF. Active probes can be used to make highimpedance measurements of small m V -level RF signals over a wider frequency range. Care must be taken in the probe preamplifier design to minimise distortion products (important for EMC measurements) and to maintain a wide dynamic range. Probes such as HP 41800A cover the frequency range 5 Hz to 500 MHz with a unitary transfer function and have an input impedance of 100 kohm with 3 pF shunt capacitance. As a further example, the Anritsu MA44B active probe has a frequency range from 10kHz to 30 MHz, a unitary transfer function and an input impedance of 300 kohm with 20 pF. Its maximum RF input voltage is around 0.25 V. Active wideband probes are used mainly in low signal level development or diagnostic testing applications where circuit boards are being examined for sources of interference. Some passive direct connection probes can be used to measure the relatively large signals up to 300 V pk. from exported transients as in the ·DEF srf AN 59-41 DCE03 test for example, see Figure 4.21a and b. They can be used to monitor the impressed RF interference signal voltage of a few volts RMS on power lines as in MIL STD 462 (N3) CS02. See Figure 4.22.
4.2.6 Directly connected transformers Direct connection transformers are used in EMI testing for:
Reproduced by permission or leT Inc.
Series injection of audio- and low-frequency interference signals onto power lines Measurement of low-frequency conducted emissions on power lines Series injection of transients into power lines Generating several kV pulses for use with spark gaps in discharging transients into screened cables or structures. Audio-frequency signals may be injected into circuits for EMC tests such as MIL STD 462 CSO 1 using directly connected iron cored transformers. Such transformers have primary windings with less than a 5 ohm impedance, a turns ratio of 2: 1 and a heavy-duty secondary winding capable of carrying 50 A AC or DC. They are employed as shown in Figure 4.23 to series inject the audio interference signal of up to about 100 W into the mains circuit of the EDT. In some cases an additional secondary winding is provided which is used to connect a meter/oscilloscope to monitor the injected signal level. Audio-frequency transformers can also be used to couple low frequency conducted emissions from power cables to an isolated measurement meter as in Figure 4.24. For AC lines it may be necessary to in trod uce a mains-freq uency notch filter to prevent input overload of the EMI meter. Spikes or transients with voltages up to around 600 V pk., such as that shown in Figure 4.25a, can be series injected into power circuits as in Figure 4.25b using high-frequency ferrite cored' pulse transformers. Higher voltages of up to 1200 V can be injected into 50 ohm circuits by using transformers with a high turns ratio. Typical output winding
MEASUREMENT DEVICES FOR CONDUCTED EMI
61
AUDIO ISOLATION TRANSFORMER
I
AF. OSCILLATOR
W
A.F. AMPLIFIER
~ P
EQUIPMENT UNDER TEST
?'-~ ~TEST_
~ ~
DC orAC LINE PRIMARY WINDING P < 5 ohms FREQUENCY 30Hz 250kHz SECONDARY POWER LINE CURRENT 50A AC/DC BALANCED OSCILLOSCOPE PROBES
U
I II
Figure 4.23
SECONDARY WINDING S - 10hm POWER CAPACITY 100W TURNS RATIO 2:1
Current injection via heavy current iron-cored transformer
DIFFERENTIAL
~ OSCILLOSCOPE
NOTCH OR HIGH PASS TO SUPPRESS AC POWER LINE FREQUENCIES
PROBES CONNECTED 50mm from EUT CONNECTOR
(a) POWER SUPPLY
INSTRUMENTATION SUPPLY SCREENED ROOM WALL
30,OOOuF CAPACITOR FOR DC SUPPLIES
TRANSFORMEri - - - - - -
+ LINE
--..J
::c
-=-10MF
~
V
SWITCH
NOT USED TEST SAMPLE
10 MF I::UT
(b) Figure 4.21
Direct voltage probe measurement used in UK DEp-' STAN 59-41 DCE03 ( a) Test configuration for A C supply ( b) Test method DCE03 exported spikes on power lines
]?igure 4.24
Audio transformer used as a pick-up device for conducted interference at audio flow frequencies
Some EMC specifications such as MIL STD 1541 (for spacecraft hardware ) require the injection of a high voltage transient onto the structure of the system under test. An EHT pulse transformer such as Solar 7115-1 can be used to generate up to 15 kV which can then be discharged to the structure or equipment case being tested via a spark gap as in Figure 4.26.
Reproduced by permission of HMSO
4.3 Inductively coupled devices {MIL·STD.461 CS02} (N4-50kHz·400MHz)
}zgure 4.22
EMI METER MEASURES RF CURRENT
Passive and active probes used to measure amplitude of injected RF signal
inductance is around 300,uH. I t is possible for the transient wave shape to be slightly degraded by the transformer which has higher output impedance than the spike generator.
4.3.1 Cable current probes The most widely used inductively coupled EMC measurement device is the cable current probe or clamp. I t is a conveniently small device, about 5-10 cm across which is constructed in two halves hinged together. It can be clamped around a single conductor or cable bundle with ease. When connected to an EMI meter, sensitive measurements as low as a few microamps of RF current can be made. Figure 4.27 shows a typical probe with an aperture of about 4 cm in diameter. A circumferential magnetic field exists round any conductor carrying the RF current to be
62
A HANDBOOK FOR EMC TESTING AND MEASUREMENT
H FIELD AROUND THE CONDUCTOR H CONDUCTOR CARRYING A CURRENT
TIME (microseconds)
CURRENT FLOWING IN CONDUCTOR UNDER TEST
(a) BASIC RF CURRENT PROBE
SPLIT FERRITE TOROID CONCENTRATES H FIELD AROUND CABLE UNDER TEST THROUGH PICK-UP COIL PARALLEL INJECTION ON DC LINE
SERIES INJECTION ON AC LINE
(b) Figure 4.25
Transient injection from spike generators with transformer outputs (a) Transient shape with unterminated output as required by MIL STD 462 (b) Series injection on A Cline (left) and parallel injection on DC line (right)
:::::; 600 V OUTPUT
TRANSFORMED UP TO :;:::,-10kv
TRANSIENT GENERATOR
EUT
t FERRITE CORED HIGH FREQUENCY TRANSFORMER
Figure 4.26
SPIKE DISCHARGE TO EUTCASE
HF step-up transformer for spike injection
Figure 4.28
Principle of cable current probe field around conductor
measured as. ,shown in Figure 4.28. The toroidal ferrite core of the current probe concentrates this flux within itself. The toroidal core is split to allow the probe to open. The mating surfaces of the two halves must be carefully machined and positioned such that a low reluctance path is maintained around the magnetic circuit of the toroid. The permeability of the ferrite, and not that of the air gap, must be the limiting factor minimising the overall toroid reluctance. The concentrated flux in the toroid induces a vol tage in the ou tpu t winding proportional to the permeability, cross sectional area of the ferrite toroid and the number of turns in the output winding.
where
Vout
kflANf I cond .
Vout
output voltage constant core permeability CSA of core number of turns frequency current in conductor
k fl A N f I cond
\ CABLE UNDER TEST
Figure 4.27
PROBE OUTPUT CABLE TO EMI METER
Typical EMGY current probe for measuring conducted RF emissions
4.1
The RF current probe is therefore a wideband RF transformer which uses the conductor in which the current to be measured is flowing as the single-turn primary winding. See Figure 4.29. Note that an electrostatic shield is included In the construction to prevent capacitive
MEASUREMENT DEVICES FOR CONDUCTED EMI
63
OUTPUT TO COAXIAL CABLE, 50 OHMS IMPEDANCE
~ SECONDARY
V(:)
/J
POWER LINE
~~~~~~;ECAN FERRITE CORE
====
/
~
o
50 OHM LOAD
/
PRIMARY (TEST SAMPLE LEAD)
\ RF CURRENT TO BE MEASURED
1.0
~
ELECTROSTATIC
w
SHIELD (CASE)
z «
.5
()
o
/ O---~---~---:--i(f)--I: 0
TRANSFORMED DOV'
2.0
(J)
I
w
Cl.
~OWER
~
a:
LINE AC OR DC
~C~~iN~U~:~~~GH
CONDUCTOR UNDER TEST
I:l: (J)
.05
z
« a:
f-
.01
Figure 4.29
" - - _ - - A . _ " - - _ - - - & _ . a . - - _ - - - & _ " " ' - -_ _.L..--...I-o-....
.01
RF cable current probe is transformer
.05
.1
5
.5
10
50
100 200
FREQUENCY IN MEGAHERTZ
(a)
coupling between the windings and the cond uctor under test. The performance of the current probe may be expressed in terms of a sensor transfer impedance:
+10 ....----....---__- - - . , - - - - - . , . - - - . . . -__- -
r-----,
+5
4.2 -5
where Vout is the voltage developed across a 50 ohm termination on the output and I cond is the current flowing in the conductor being measured. Typical transfer impedances are about 1 ohm, giving 1 volt output per amp. Practical current probes are optimised with internal loading to yield flat transfer impedances over most of their operating frequency range. The transfer impedance of a typical general performance current probe (Carnel Labs. Corporation 91550-2) [8] can be seen in Figure 4.30a. The frequency dependence indicated in eqn. 4.1 can be seen below 100kHz. The upper frequency limit of a current probe design may be determined by factors such as selfresonance and ferrite losses. The probe transfer impedance is often expressed in terms of dB above or below 1 ohm as in Figure 4.30b. This convenient notation allows the EMC engineer to calculate the measured current from:
-10
-15
1 - -_ _..L.----'--_ _--L...----I"--_ _.L..--&-_ _- - I . . . - - - l L . - - - I
.01
.05
10
.5
50
100
200
FREQUENCY IN MEGAHERTZ
(b) Figure 4.30
Transfer Junctions oj typical current probe ( a) Transfer impedance in ohms ( b) Tran~fer impedance in dB ohms
Reproduced by permission of' Camel Labs Corp.
::2:
I
o W z o ~
9w
+30
co a:
~
+20
>
~ «
fg
+10
~
w
()
The
transfer function of a more sensitive (Z == 0 to 15 dBohm) and higher frequency (1 MHz-1 GHz) current probe (Carnel Labs. Corporation 94111-1) [8] can be seen in Figure 4.31. Current probes are sensitive well-behaved sensors which are easy to use and give repeatable results. RF currents from microamps up to 20 A or more can be measured. However, care must be taken with regard to the total current (RF and power line components) flowing in the conductor being measured. At high combined current levels it is possible to saturate the ferrite toroidal core and thus change its small-signal permeability. This results in incorrect measurement of the RF component of the total current. Power line
z « o
w
Cl.
~
a:
I:l: (J)
-10
L . -_ _--A._--'--
1
z « a:
10
-'-----&
50
100
-"
500
1000
FREQUENCY IN MEGAHERTZ
f-
Figure 4.31
Transfer Junctions oj HF current probe
Reproduced by permission or Camel Labs Corp.
currents of the order of 200-400 amps, depending on power line frequency, can be accommodated before saturation results. Because the current probe forms a transformer with the conductor under test as the primary winding, a small series resistance of a fraction of an ohm is introd uced in to the primary circuit owing to transformation of the 50 ohm (plus any
64
A HANDBOOK FOR EMC TESTING AND MEASUREMENT
internal probe resistance) secondary load by the effective turns ratio of the probe. In most measurement situations the additional transformed resistance has no effect on the operation of the circuit under test. Care should be exercised when current probes are used to monitor circuits which may be sensitive to additional resistive loading. Freerunning RF oscillators and circuits exhibi ting spurious oscillation are examples where frequency pulling may result from the additional loading. The spectrum observed with the probe in such cases may not be truly representative of that under unloaded conditions. Current probes are used extensively in EMC conducted emission testing to military specifications such as MIL STD 462 and DEF STAN 5941. Their use is less evident in EMC standards relating to testing of commercial electronic equipment where the LISN method currently predominates. A current probe will measure the sum of all
currents flowing in a cable bundle, or the screen curren t in the case of a screened cable, see Figure 4.32a. If the probe is placed around one conductor in a bundle with another conductor close by, then although the probe construction offers considerable rejection of unwanted signals, it may not measure just the current flowing in the cable under test, see Figure 4.32b. In such cases the cond uc tors should be separated a few centimetres if possible and any changes in the measured current noted. During development testing it may be advantageous to determine not only the magnitude but also the nature of the interference current flowing in a set of conductors to learn something about the probable source of the interference. The differential mode current can best be measured as shown in Figure 4.32c and the common-mode current as shown in Figure 4.32d. The RF current in a cable will usually exhibit standing wave patterns along its length. In conducted emission EMC tests this phenomenon
iSCREEN CURRENT DIFFERENTIAL
COMMON MODE
MODE
.. (a)
(b)
i DIFFERENTIAL MODE CURRENT
(c) F'igure 4.32
Making cable current measurements with a current probe ( a) Measuring shielded cable screen current ( b) Measuring of common Idifferential mode ( c) Maximising differential mode pickup ( d) Maximising common mode pickup
Reproduced by permission of' leT Inc.
i COMMON MODE CURRENT
( d)
MEASUREMENT DEVICES FOR CONDUCTED EMI TEST DCE01 POWER SUPPLY
MEASURING SET
SCREENED ROOM WALL
65
compared against the limits. The conducted emission limits for these two tests (for military land systems EUTs) are shown in Figure 4.34a as examples of the levels of conducted RF current which can be measured with toroidal current probes.
30,OOOjJF CAPACITOR FOR DC SUPPLIES
4.3.2 Current injection probes
EUT
GROUND PLANE
(a) TEST DCE02 POWER SUPPLY
MEASURING SET SCREENED ROOM WALL
EUT
EUT
50Q
GROUND PLANE
(b) Fzgure 4.33
Positioning of RF current probes on power and signal cables in UK DEF STAN 59-41 tests
Reproduced by permission of HMSO
IS only important when measuring VHF frequencies on cable longer than a few metres. The position and amplitude of RF current maxima and minima can be explored by moving the probe along the cable. In some EMC standards that call for the use of current probes, their position along the cable is specified with regard to the distance from the cable connectors on the EUT. In the DEF STAN 59-41 DCE01 test this is 5 cm from the LISN connection on the power lead. See Figure 4.33a. For the DCE02 test (see Figure 4.33b) with leads longer than 1 m and for frequencies above 30 MHz the probe is positioned 5 cm from the connectors at both ends of the cable and the greatest emission levels are
Specially designed low-loss current probes can be used to inject RF current into cables. Formal EMC tests using this technique are a recen t introd uction and have been pioneered in the UK primarily for testing military equipment. However, the technique can also be used for the developmen t testing of commercial electronics. The transformer action of a toroidal current probe is exploited to produce a current injection test for cables which does not require direct connection to the circuits being tested. This makes the tests quicker and potentially more representative of the functioning system with an undisturbed electromagnetic topology. An example of the use of injection probes is that called for in the clearance 'of military aircraft under RAE technical memorandum FS510 [9J. The current probe injection test is now included in DEF STAN 59-41 as DCS02 for power, con trol and signal leads over the frequency range 50 kHz-400 MHz. The precise test method is complex and the reader should refer to the appropriate reference for a full understanding. The construction and performance of the injection probe will be considered here as representative of this type of EMC test device. A low-loss high-power current injection probe design capable of handling 200 W input RF power was produced initially by ERA Technology in the UK and designated ERA36A and ERA37A. The Carnel Labs. Corporation company in the USA has produced an equivalent injection probe designated model 95242-1. The characteristics of this probe are given in Table 4.2. The probe has been designed to handle high input RF power and has more core material and smaller core gaps than the general emission measurement probes. The low insertion loss of about 5 dB (see Figure 4.35a) over most of the freq uency range also ~akes the device an extremely sensitive emission measurement probe with a transfer impedance as high as +40 dB ohm. See Figure 4.35b. In the injection mode the probe is used with a 200 W amplifier and a directional coupler to measure the forward power to the probe. This is necessary because when fixed around a bunch of conductors with unknown RF impedance, the probe can present a poor VSWR to the amplifier.
66
A HANDBOOK FOR EMC TESTING AND MEASUREMENT
Figure 4.34 Typical test limits for conducted current as measured with current probe
TEST METHOD DCE01/2
~
LIMITS FOR LAND SYSTEMS
y-----.,..-----...,.--------------,..-----...,.------
140
120
2-
co
~ z w a: gs
100
80
U
z
@
60
(J)
~
w o w
100 MHz, 40 40
~---------...... CLASS 0
I-
U :::>
o z
100 MHz, 25 ~--------...... CLASSC
20
100 MHz, 10
ou
~------... CLASS B
o -20 Reproduced by permission or HMSO
2 MHz, 0,..----10-0-M-H-z,-0-t CLASS A and E
""-100
1kHz
As a check on the RF current being induced into the cable a measurement current probe is placed 5 cm from the connector of the EDT and the injection probe as in Figure 4.36.
4.3.3 Close magnetic field probes RF pickup devices that could be considered as small magnetic field an tennas have been included in the section on inductively coupled
Table 4.2 High power current injection probe Specifications model 95242-1 Frequency range 2-400 MHz Window diameter 40 mm Outside dimensions 102 x 130 x 60 mm Weight 1.60 kg Input connector Type N Maximum input power 200 W Maximum core temperature 80°C Recommended maximum 35°C tempera ture rise Maximum time for 30 min. continuous rating at full power Turns ratio 1: 1 Inductance 0.8,uH, ±20 0/0 Self-resonan t frequency 350 MHz, ±10 0/0 Impedance at resonance: unloaded 76 + jO ohms 95241-1 calibrations jig 50 + jO ohms, ±5 % Frequency 2 5 10 20 50 100 200 300 350 400 (MHz) Insertion 17 9.5 6.5 6.5 5 5 5 5 5.5 6 loss (dB)
"""'10kHz
100kHz
1MHz
"""--
__
10MHz
100MHz
devices as they operate at frequencies up to 1 GHz and at a distance of only a few mm from the curren t carrying conductor. Thus they operate in the near field, and with the probe elemen ts being small loops, the predominan t coupling is inductive. Small probes or wands as they are sometimes referred to, are an important tool for diagnostic and development testing at board and box level. I t is possible to make a set of probes covering various frequency ranges and to calibrate them sufficient for nonconformance test purposes. Proprietory equipment employing dual pickup loops and a balun can be obtained such as HPl1940A/11941A. These probes have calibrated transfer functions to yi~ld the absolute magnetic field st~ength in dB,uA/m from the EMI meter readings in dB,uV. For example, from 9 kHz to 30 MHz with probe HPl1941A and 30 MHz to 1 GHz with HPl1940A [10]. Close-in probes can be used for tracking down sources of RF emission from components, circuit boards, apertures in equipment boxes and leakage from connectors and screened cables. When connected to a spectrum analyser that can rapidly sweep over wide bandwidths to reveal the amplitude of components of complex signals, the EMC engineer has a powerful diagnostic tool.
4.3.4 Surface current probes These small inductively coupled sensors are designed to measure the RF current flowing in a particular direction on a conducting surface. The unit is a proximity RF current transformer where the surface on which the insulated device sits forms the primary circuit. The probe is placed with its
MEASUREMENT DEVICES FOR CONDUCTED EMI
Figure 4.35 Performance characteristic of typical current injection probe
20
\
18
\ \ \ \ \ \ ~
16 CO
"0
14
~
en en
0-J Z
0 i= a: w
en
~
\
12 10
\
\
67
:\ :\ \
8
~
'" "....-.---, ----------
'-'-
6
...................
"
4
,~
"""'----
.",/
--------------_ ......
.,;
,
2 0
1
2
10
5
(a)
20
50
100
200 300 400500
FREQUENCY IN MHz
Injection Probe Model 95242-1
50
en ~ :r:
20
0
~
w z
10
w a. ~
5
()
« 0
a: w
LL
C/)
Z
« a:
.-
2
2
(b)
5
10
20
50
FREQUENCY IN MHz
insulated base against the conducting surface in which the RF current is flowing, as in Figure 4.37, and is orientated to obtain the maximum response on the EMI meter to which it is connected. The RF current flowing in the test surface beneath the effective area of the probe is obtained by dividing the measured EMI voltage by the transfer impedance of the probe in ohms, in the same manner as for a cable current probe.' See Figure 4.38 for the transfer impedance of typical devices such as Fischer probes [11] model numbers F-91 and F-92. The total RF current flowing in a surface can be
100
200
500 Reproduced by permission o[ Camel Labs Corp.
measured by dividing the surface into probe footprint segments along a line normal to the direction of current flow and summing the measurement made at each segment. These probes operate over a frequency range from around a hundred kHz up to a few hundred MHz and can be used to measure the skin currents which are set up on system structures such as cable trunking, equipment racks or mainframe computer cabinets when illuminated by electromagnetic radiation. They are also useful in determining the distribution of interference ground currents within systems such as military vehicles and spacecraft.
68
A HANDBOOK FOR EMC TESTING AND MEASUREMENT
Figure 4.36 Current injection and measurement probes used together to stimulate cable to a known level
TEST SAMPLE
AMPLIFIER
SIGNAL GENERATOR
DIRECTIONAL COUPLER FORWARD POWER /TO PROBE ___ INJECTION PROBE
Injection currents controlled to give known/wanted measured current levels in the cable under test
Reproduced by permission of HMSO
4.3.5 Cable RF current clamps In his survey of EMC measurement techniques Jackson [1] gives a brief history of the development of the cable current clamp from initial experiments in Sweden. Experiments were carried out to enable radiated field measurements to be made in a laboratory by using a movable coaxial filter on the mains lead of an EDT to maximise the radiation from the equipment. Further work by the Swiss PTT laboratories led to the use of ferrite rings, and this evolved into the ferrite cable clamp. For a full history of the development of this probe consult the reference given in CISPR publication 16, Appendix S. The probe specified in CISPR 16 was conceived in 1966 for use in radiated emission testing in an attempt to obtain better agreement between measurement results and operational experience. The use of the clamp requires the position of the Surface current probe is orientated for maximum pick-up
Produce records of power meter readings for given measured cable current values
Cable into which current is being injected presents variable impedance to injection probe leading to VSWR variations
EMI METER
MEASUREMENT CURRENT PROBE
RF POWER METER
absorbing element to be adjusted along the length of the mains input cable to an EDT. The im pedance discon tin ui ty represented by the absorbing clamp on the single-wire transmission FISCHER PROBE model no. F·91 -10 - - - - - - - - - . - - - - - - - - . . - - - - - - - - , . - - - - . ~
I
0 T""
UJ
>
0
CD
«
-20
.0 "0
UJ
0 Z
/
« 0
UJ
a.. ~
-30
0: UJ lL (f)
Z
«a:
I-
-40
1MHz
10MHz
100MHz
FREQUENCY
FISCHER PROBE
B FIELD ~
model no. F-92
+10
I
0 T""
UJ
> 0
CD
«
.c
"0
UJ
0 Z
«
0
UJ
a.. ~
0: UJ
-10
lL
(f)
Z
« a:
I-
1MHz
10MHz
100MHz
1000MHz
FREQUENCY
Figure 4.38 Figure 4.37
Using surface current probe
Reprod uced by permission of' ICT I ne.
Transfer impedance oj surface current probes
Reproduced by permission of Fischer Custom Coms.Inc.
MEASUREMENT DEVICES FOR CONDUCTED EMI Typical exanlple
Figure 4.39 Use oj absorbing clamp Jor measurement of interference power 30 MHz-300 MHz
MOVEABLE CLAMP
d A
~
\..--f=E::::::::t--_H_---l
69
F
~1-B---t-C-+--F=D=f----EtG-J-----B-II-_~~~~~~-E,t=:==:::1...;;.;;.;;;..;\~-~Q: 1----\--When
L~4.2m,
F min ~ 40MHz FIXED ABSORBER
CABLE UNDER TEST
DETAILS OF ABSORBING CLAMP SINGLE TURN SHIELDED CURRENT PROBE
MEASUREMENT CABLE ABSORBER RINGS
/
---,..........,.-------~-----E~~~~~~~}~-~i-+-H--,.~1~-:
d
EUT
60rl~~~_____
METER
~~5.f8~
l=1~~r=:H:~t:=+~~I:::::::l------===RRRR_=Jf
--~12-18==m~ 0 EfEjEj~ TO
::---t=:rF=:rt==:t-,.---r:==n:==n:==n==nF=rl::=:r1F=r_==--:.=-=-:.=
3
r~gs
U
56 r?ngs
CABLE UNDER TEST
\
B
ABSORBING FERRITE CLAMP
line, causes the line impedance presented to the in terference source to change in such a way that the maximum RF power is transferred to the associated measurement probe. In this way, a measurement can be made at each interference frequency which represents the maximum possible power which can be radiated by the equipment and its cable. The construction of the clamp is shown in Figure 4.39. The key measurement component of the clamp is the Faraday loop of coaxial cable which is wound around three ferrite rings, labelled C in the diagram. This acts as a toroidal current probe and produces an output voltage proportional to the RF current flowing on the cable under test. Further along the cable from the EUT which generates the interference emission, are anum ber of lossy ferrite rings which act to absorb the interference signal once it is past the current probe. The assembly is moved up and down the cable under test to obtain maximum readings at each frequency of interest. Other ferrite rings are placed around the measurement coaxial cable from the current probe to absorb any spurious signal pickup on the shield. The measurement of RF power available to be radiated from the cable under test at each frequency of interest is made by a substitution method where the signal reading from the EMI
MAINS
Reproduced by permission of BSI
meter is compared with that obtained during a calibration of the clamp in a special jig and recorded on a calibration chart. The chart is a record of the clamp insertion loss as a function of frequency from 30 to 300 MHz. Details of the current clamp and the calibration method can be found in EN55014/BS800 and VDE0875. The probe- may ,give readings which correlate well with everyday experience of radiated interference from typical commercial equipment, but it is time consuming to have to adjust the position of the probe along the line to obtain a luaximum signal reading at every frequency of interest in an emission spectrum. The need to physically move the probe means that it is very difficult to automate the process in order to speed up the measurement. The absorbing clamp method, although widely used for testing commercial electronic equipment, is not without its critics. Kwan [12] presents a keen analysis of the drawbacks of the method and suggests how the device may be improved by separating the current probe and the mismatch/absorption unit so that they can be moved a part as necessary, to ensure that the absolute maximum energy transfer to the probe is obtained. An idea of the interference RF power levels that can be measured with the absorbing clamp
70
A HANDBOOK FOR EMC TESTING AND MEASUREMENT
Table 4.3 Typical interference levels on cables that can be measured using absorbing clamp device (EN55014) Frequency range
Household and similar appliances
Portable tools: rated power of motor ~700W
700-1000 W
1000-2000W
MHz
dB (pW) dB (pW) quasipeak average*
dB (pW) dB (pW) quasipeak average*
dB (pW) dB (pW) quasipeak average*
dB (pW) dB (pW) quasipeak average*
30 to 300
Increasing linearly with the frequency from 45 to 55 35 to 45
Increasing linearly with the frequency from 45 to 55 35 to 45
Increasing linearly with the frequency from 49 to 59 39 to 49
Increasing linearly with the frequency from 55 to 65 45 to 55
* If the average limit is met when using a quasipeak detector receiver, the test unit shall be deemed to meet both limits and measurements with the average detector receiver need not be carried out.
CABLE UNDER TEST (not power cables)
MIL STD 462CS 06
\
BOX INDUCTION WINDING
EUT or LOAD
CABLE INDUCTION WINDING
TRANSIENT SOURCE
POWER LINE FREQUENCY SOURCE
> o o
AC POWER MAINS VARIAC
Figure 4. 40
Magnetic induction tests using simple wire coils on boxes and cables
method can be seen in Table 4.3. This shows the limits set out in EN55014 for the interference which is tolerable from household appliances and portable power tools. Levels of around 35 to 50 dB PWare specified when measured using an average and quasi peak detector.
4.3.6 Magnetic induction tests The susceptibility of boxes of electronics and attached cables to magnetic induction fields at very low frequencies can be determined by generating fields from nearby wires or coils carrying heavy current at the power line frequencies. Such tests are specified in MIL STD 462 (RS02) and call for heavy-duty wires to be wound around the interface signal or control cable under test in a spiral with a pitch of two turns per metre. See Figure 4.40 for the layout of
the induction cable. A current of 20 amps is passed through the wire at the power line frequency appropriate to the unit under test (usually 50, 60 or 400 Hz). The field produced around the induction cable is closely coupled to the cable under test and the coupled power line interference signal enters the EDT via the cable connectors. EDT power line cables are exempt from this test. Signal cables are to be tested in bundles or groups. However, White [13] questions the validity of spiralling the test wire around the cable to be tested. He suggests laying the wire along side and parallel to, the cable under test to maximise the induced voltage in any cable pair in the harness being subjected to the induction field. A very similar power line ind uction field test is carried out on the EDT itself by wrapping the wire around the eq uipmen t case at a pitch separa tion of 30 cm over the heigh t of the box and applying the 20 A current. See Figure 4.40. Spikes or transients which are produced from the generator detailed in test CS06 of MIL STD 462 are also fed into the same induction cable wrapped around both the box and interface cables, to perform transient pulse tests. The correct applied stimulus level is obtained by setting the spike genera tor ou tpu t to 100 V measured across a 5 ohm load.
4.4 References JACKSON, G.A.: 'Survey of EMC measurement techniques', Electr. Commun. Eng. ]., March/April 1989 2 'Radio interference measuring apparatus and measurement methods'. CISPR 16 3 MIL STD 462 Notice 3, 1970, pp. 21-23 4 'Instruments, components and accessories for the RFI/ EMC engineer'. Solar Electronics Co., 901 North Highland Avenue, Hollywood, CA 90038, USA
MEASUREMENT DEVICES FOR CONDUCTED EMI 5 Draft BS6667, 1990: Electromagnetic compatibility for industrial process measurement and control equipment, Part 5: surge immunity requirements 6 BORESERO, M., VIZIO, G. and NANO, E.: 'A critical analysis of the immunity methods for and television broadcast receivers'. sound Proceedings of lEE symposium on EMC, UK, 1990, pp. 219-226 7 'A two-part five-day comprehensive training course in EMC' part 2. Don White Consultants, PO Box D, Gainesville, Virginia 22065, USA, page 2.1.28 8 'Calibration & operation manuals for Carnel Labs. Corporation current probes 91550-2 and 941 f 1-1'. Carnel Labs. Corporation, 21434 Osborne St., Canoga Park, California, 91304-1520, USA
71
9 'Recommended test specification for the electromagnetic compatibility of aircraft equipment'. Technical memorandum FS(F)510, RAE Farnborough, Hants, UK 10 1990 product guide. Hewlett Packard, p. 128 11 'Fischer surface current probes'. Fischer Custom Communications, Box 581, Manhattan Beach, CA 90266, USA 12 KWAN, H.K.: 'A theory of operation of the CISPR absorbing' clamp'. Proceedings of lEE symposium on EMC, 1988, pp. 141-143 13 'A two-part five-day comprehensive traInIng course in EMC' part 2. Don White Consultants, PO Box D, Gainesville, Virginia 22065, USA, page 2.2.23
Chapter 5
Introduction to antennas EMC antennas
In the following chapters, antennas that are commonly used in EMC radiated emission and susceptibility testing are examined individually in detail. Particular characteristics are explored and reference made to tests that employ the various types of antenna.
EMC measurements can be complex and inherently unrepeatable. This is particularly true when Hlaking radiated emission or susceptibility measurements in anything other than ideal freespace test conditions. Military EMC standards require measurement to be made in chambers which mayor may not have radio absorbent material (RAM) to damp out wave reflections; if uncontrolled an increase in measurement
5.2 EMC antenna basics
5.2.1 Arbitrary antennas Consider the impact of an electromagnetic wave shown in Figure 5.1, which is defined by E) l! (wavefield impedance), wavefront phase curvature and the direction of the wave normal k, which is incident on an arbitrary conducting surface S from which power can be extracted to a load Z. A linear transfer factor can be established for the voltage delivered to the load in terms of the incident wavefield. This factor is a function of all the specific conditions under which it was obtained. It depends on the detailed nature of the
tests on large items of equipment may have to be made on an open range in the presence of ambient electromagnetic noise, an uncertain ground plane and possible reflections from objects. All these effects can increase measurement uncertainty. Standards covering radiated emission testing of commercial electronic and particularly information usually specify testing on a type of open test site that has to be calibrated before use. Repeatability of test results measured on an open range is however also open to question and continues to stimulate debate in the EMC community. See Chapter 3 References 11, 12, 15-17 and 19. Whether the radiated emission testing is carried out in screened rooms or on an open field test site an tennas used are the componen ts in the measurement chain. What these various antennas measure, and the influence that surroundhave on these measurements, is often not and can lead to considerable and error. antenna theory is complex and mathematical and much has been written on the basic of antennas [1-4] and the of many antennas for communications use from HI" to millimetric wavelengths 7]. It is neither possible nor appropriate to cover such in detail in this text on EM C measurement, therefore emphasis is placed on the aspects of antenna characteristics which engineers and test technicians can use with view to minimising potential measurement errors. An overview of the important basic characteristics of antennas is presented to establish a framework of equations within which an tenna types may be discussed further.
E vim INCIDENT WAVEFIELD
\ Wave Zw
Holm
If (is
SURFACE CURRENTS INDUCED BY INCIDENT WAVE RF POWER EXTRACTION
V
TECHNIQUE
....-----1]
LOAD Z
VOLTAGE DEVELOPED POWER DELIVERED
impE~dorlce I
=V
S (x, y, z) POWER IN WAVEFIELD Pw Ex H W/M2
\ ARBITRARY 3D CONDUCTING SURFACE ( IN REAL ANTENNAS THE STRUCTURE IS CHOSEN TO HAVE SPECIFIC PROPERTIES)
(aj
"YV'l11C'11Y\rl .c>Y'C'i-"l' Y\r-41Y\r<'
Ev/m
......... A.''-''--'".L 'J'L.L.LL"-.':;".L.L'--'L.L'--'
diameter d
Holm
\ Small gap
t-JA. '--""",',.L'--'L-l,.L
BALANCED MATCHED TRANSMISSION LINE
\
DIPOLE CONDUCTING ELEMENTS
(bj 5.1
72
P
Arbitrary antenna and simple dipole ( aj Arbitrary antenna concept ( bj Simple nonarbitrary antenna - dipole
INTRODUCI'ION TO ANTENNAS electromagnetic wavefield mentioned earlier and on the exact form of the conducting surface, the way the power is extracted from it and the complex impedance of the load. The transfer factor is meaningful only for this specific situation and is not applicable to any other general situation where the wavefield is different, the conducting surface is different, or the geometrical relationship between the two has changed. A particular conducting surface or antenna (S) wi th its specific transfer factor may have been designed to be exactly what is required to meet a particular need, say for a communications link. But it may be of little use as a general measurement antenna for EMC work which must be wideband, inexpensive, portable and can yield an absolute measurement of field strength via a calibration curve of its transfer factor which has been derived under controlled conditions.
73
Table 5.1 Radiation fields from Hertzian dipole If the current in the Hertzian dipole is 1 10 cos OJt, the radiated electric field intensi ty E and magnetic field intensity H are given in SI units by
E i e [60n 10 dl sin e sin (OJt - 2nrjA)JjAr H == i¢ [10 dl sin e sin (OJt - 2nr jA) Jj2Ar where r is the distance from the centre of the dipole to the field point (r, e, ¢) and i e and i¢ are unit vectors, respectively, in the directions of increasing e and ¢. The electric vector E is proportional to sin e and the field radiation pattern of the dipole consists simply of a plot of sin e against e. OJ
== angular frequency of driving current
2nf f == frequency A wavelength of radiation dl == elemental length of Hertzian dipole
5.2.2 EMC antennas The nature of antennas used In EMC measurements is quite different from those required for specific functions such as communications. The optimum design of EMC test antennas has received relatively little academic attention to date. The design aim for an EMC antenna is to produce a simple, cheap, rugged, wideband conducting structure that is coupled to a given load impedance such that the output voltage can be related simply to the E (electric) or H (magnetic) field strength in a known manner under general conditions of use. Ideally, the transfer factor, or antenna factor, should be calculable directly from a knowledge of the geometry of the conducting surfaces, as in the case of special D-dot antennas [8J so that an abs9lute measurement of field strength can be made. If this is not possible, the antenna factor should be derivable from a simple set of measurements using known wavefields to yield a reliable calibration curve. There are some simple antenna configurations for which solutions of the induced current and voltage may be calculated for a specified excitation [9J. Equally, the E and H fields produced by the antenna for a given current excitation can also be calculated. The best known of these is that for the electrically short Hertzian dipole and an oscillatory excitation current. For example Benumof [1 OJ gives the formulas in Table 5.1 for the E and H fields from a dipole given an excitation of the form 1 == 10 cos OJt
5.1
where 1 is the time-varying driving current, 10 is the peak driving current, and OJ is the angular frequency.
I t is possible therefore to understand how a simple nonarbitrary conducting structure such as a short dipole behaves, and to use this knowledge to interpret the wavefield it will generate from a given driving current. I t is also possible to calculate the reciprocal behaviour of the dipole output voltage in terms of the incident electromagnetic wavefield which is to be measured.
5.3 Basic antenna paratneters
5.3.1 Gain Gain is a measure of the ability of an antenna by virtue of its specific conducting structure to concen tra te a transmitted (radiated) signal in one direction, or to receive a signal from only one direction, as opposed to all other directions [11 J. Gain is usually expressed in dB relative to a perfect isotropic radiator. Consider Figure 5.2 where two antennas, one isotropic and one with gain, both radiate into a surface at a point P at distance R to deliver a unit power density P d (watts/square metre):
An tenna gain
Thus
input power to isotropic antenna for unit power density at P input power to directive antenna for unit power density at P Pin (W) X
where W == power in watts. isotropic source,
4nR2
gain
5.2
Note that for an
5.3
74
A HANDBOOK FOR EMC TESTING AND MEASUREMENT
ANTENNA SYSTEM CASE 1 - ISOTROPIC DEVICE CASE 2 - DIRECTIVE DEVICE
E
H
Lk
-T~-+---t/
CASE 1 - P in W (ISOTROPIC) CASE 2 - Pin W (DIRECTIVE)
POWER DENSITY ON SURFACE = P d W/M 2
P in (DIRECTIVE) < P in (ISOTROPIC)
of the antenna. For example the physical frontal area of a horn or parabolic dish antenna is related to the energy capture aperture. The two are not equal owing to the way in which the electromagnetic fields are distributed nonuniformly over the physical area. The gain of an antenna with a given physical aperture is dependent on the wavelength of the radiation being transmitted or received. The relationship between the aperture as defined by eqn. 5.8 and gain is also dependent on wavelength:
4n
gain G DIRECTIVE ANTENNA GAIN = P in (ISOTROPIC) (for a given P d ) P in (DIRECTIVE)
Figure 5.2
where <0 is impedance offree space 377 ohms, and E is the magnitude of the wave electric field at a distance R. To produce a 1 Vim field at 1 m from an isotropic antenna an input power of 33 m W is required. For 100V/m a power of 330W is needed. Also 5.5 where H is the magnitude of the wave magnetic field. Thus to produce 1 A/m at 1 m a power of 4800 W is required. For a directive antenna,
4nR E Pin
and
P
5.4
2
Pin
<0
gain
2
2
4nR H
<0
gaIn
X
aperture
4n
5.10
5.11 4n<0
These relationships can be used to derive both the transmitting and receiving antenna factors.
5.3.3 Transmitting antenna factor Consider a feeder cable delivering a power P t to an antenna. The voltage Von the line of impedance < is given by
=
V2
Z
5.12
From eqns. 5.6 and 5.12
V2 5.7
Consider an antenna in an incident field with a power density of P d watts/square metre which delivers a power P watts to a matched load. Then Pd
5.9
5.6
5.3.2 Aperture
==
A2 C x P
== - - - -d
PI
The Lorenz reciprocity theorem [1 OJ shows that the gain figure for a transmitting antenna is the same as that for the identical receiving antenna. However, to better understand the concept of receive gain it is necessary to introduce the idea of antenna aperture.
P
A2
or
2
X
aperture
where A is the wavelength. Using eqns. 5.8 and 5.9 the received power P can now be expressed as
Concept oj antenna gain
and
X
5.8
and the aperture has the dimensions of area. The aperture is not identical with the physical antenna area, nor its projected area onto a plane normal to the incident wave direction. In some cases it can be identified with the physical shape
<
4nR 2 E 2 <0
X
gain
therefore
5.13
The ratio E / V is called the transmISSIon antenna factor (T AF) and relates the field strength E (V/m) produced by the antenna at a distance R metres with an input signal of voltage V across a line of impedance < ohms. This relationship is useful in calculating the field strength incident on an equipment at a distance from a transmitting antenna in a radiated susceptibility test.
5.3.4 Receiving antenna factor Bennett [12J states that 'antenna factors are widely used, yet the literature does not appear to include any detailed mathematical characterisations of such factors', and proceeds to develop useful formulas for some standard dipole antennas.
INTRODUCTION TO ANTENNAS In general, the receIvIng antenna factor is defined as EjV where E is the magnitude of the electric field of the incident wave and V is the voltage delivered from the antenna to the load of impedance Z. It can be shown that
~ = ~ [z :~oainr
5.14
Knowledge of the derivation of these two antenna factors is most important to the EMC engineer as it permits the setting up of experiments to calibrate antennas and derive the factors with confidence. Otherwise the engineer has to rely on the manufacturer's data supplied with proprietary equipment which may contain artifacts of possibly imperfect calibration procedures. Manufacturer's antenna factors are usually reliable and available in graphical form as a function of frequency. Examples for specific antennas are shown later.
characteristics of the antenna and the relationships involving gain and distance which describe the coupling between the antennas will no longer rigidly apply. The antenna radiation pattern, antenna factor and feed point impedance will depart from calibrated values and any measuremen ts then made will be subject to an increased uncertainty. Consider two identical antennas separated by a distance d in Figure 5.3a. There will be a mutual impedance term Zm introduced into the equivalent circuits of the two antennas as shown in Figure 5.3b. The mutual impedance of simple antennas can be calculated from the antenna dimensions and spacing and may be found in standard texts. The circuit equations relating to Figure 5.3b are
lout (Zout
5.3.5 Antenna phase centre
GENERATOR
When an antenna is radiating into free space, the wavefront at great distance (in the far field) is usually assumed to be plane. This is however alvvays an approximation as the radiation is emitted from a structure with a finite size and so the wavefront will have a slight curvature at a finite distance. The apparent centre of the curvature is usually situated on the antenna structure and is called the phase centre. When making calculations or measurements which involve the distance from an antenna to an object, the distance from the phase centre should always be used. This can be important when such distances are comparable with the antenna dimensions, as in the case of some EMC radiated emission measurements where the EDT to antenna separation is only 1 m and the antenna is about half-a-metre long. The position of the phase centre on most antennas is obvious, for example at the feed point of a dipole, but for some frequency independent antennas such as log periodics and log spirals the phase centre will move along the axis of the antenna with freq uency. This can lead to measurement errors and even calibration errors at VHF jDHF when the two-antenna method is used for such antennas with a short 1 m separation.
When operating an antenna close to another similar antenna, ground plane, earth or screened room wall, undesirable mutual coupling will take place between the antennas, or between the antenna and its electrical image in any reflection. Such an effect will alter the normally calibrated
MID - PLANE I
I
+ Za) RECEIVER
I I
I I
I
I
I
V out lout Zout
rdr I
(a)
Zm lout
Zm 19
ANTENNA I LOAD
(b) Figure 5.3
5.3.6 Mutual ante11na coupling
75
Antenna mutual impedance coupling ( a) Identical antennas separated by d ( b) Equivalent circuit Zg == generator impedance Za == antenna impedance Zm == mutual coupling impedance Zout == load impedance Ig == generator current lout == load current
76
A HANDBOOK FOR EMC TESTING AND MEASUREMENT 10,000...-------------------.....,
which leads to
--Induction f i e l d - - - - - F a r field----I
lout = Zm- (Zout
+ ZaJ
(Zg
5.15
+ Za)
<m
3000
<m
As the separation d becomes large the value of tends to zero and the current flowing in the receiving antenna lout as a result of mutual impedance coupling tends to zero. The coupling between the two antennas then reverts to only radiative coupling which is determined by the normal gain equations given earlier. The problem of mutual coupling between an tennas and their images in the earth and other ground planes bedevils EMC measurements and gives rise to much comment [13-18]. This subject is discussed again later in the text concerned with open range and screened room radiative testing.
5.3.7 Wavefield impedance
5.16
Plane-polarised wave
The wavefield impedance close to a source of electromagnetic radiation is determined by the nature of the source. For example, for an electric-field source such as a small dipole oscillating charge, the wave impedance close to the source will be high at several thousand ohms, as E is much greater than H. For a small electrostatically screened oscilIa ting current loop (with minim urn exposed voltages) the magnetic field component predominates and the ratio of EIH is low, therefore the wavefield impedance is low, perhaps only a few tens of ohms close to the source. I t is useful to define the distance from an EM source in terms of the wavelength being radiated
Ea and H<j>U+
I (J)
E
'§
Zo = 120 1t = 377
300
n
3= N
ZE = Zo
100
v'd2+1
--d--
ZH = Zo d
Jd 2 + 1 Zo
= 1201t =
377n
104----------+------------' 10 1.0 0.1 DISTANCE FROM SOURCE d (increments of r = _A_) 21t
Figure 5.5
EV/m HA/m
v
Figure 5.4
I
30
A simple representation of a sinusoidally varying electromagnetic wave is shown in Figure 5.4. The electric field E V 1m and the magnetic field components H Aim are orthogonal and the wave propagation velocity is v m/s. In a vacuum v == c == 2.9979 X 10 8 , normally approximated to 3 X 10 8 m/s. The wave impedance
1000
Variation of waveJield impedance with distance from source
and not as an absolute distance in metres. A graph of wavefield impedance as a function of distance from the source in terms of wavelength, for both an electric and magnetic oscillator, can be seen in Figure 5.5. Note that close to the source, that is at distances of less than )"/2n, both the E- and H-fields vary rapidly as square and cubic terms with distance. Beyond this distance the wavefield impedance from either type of source tends to the same constant value of 377 ohms or 120n. This is known as the free-space wave impedance which occurs in what is called the 'far field' of the source.
5.3.8 Near-field/far-field boundary An understanding of the origin of the Inverse linear, quadratic and cubic dependence of field strength with distance can be obtained by considering the elemental dipole in Figure 5.6, which is of length h and carries a uniform sinusoidally oscillating current of the form
I
==
10 exp (jwt)
5.17
The solution of Maxwell's equations for this antenna element [9J leads to the components for the electric and magnetic fields shown in Table 5.2.
INTRODUCTION TO ANTENNAS
z
Table 5.2 Solution ,[or field components for short dipole
H$ Efl
Er p
H¢
CP-+1). r r
1h
0 ==-
2
4n
Er OSCILLATING DIPOLE CHARGE
~
E()
...---------t----y , I '"
I
"
I I
' .... ,
"
I
'" ......I
Figure 5.6
77
Oscillating dipole element
Eq ua tion 5.19 for the radial electric field E r has only two terms in 1/ r2 and 1/ r3 . This means that E r decays rapidly with distance from the oscillating element and is therefore only importan t close to it. The 1/ r3 term can be identified with the field calculated for an electrostatic dipole. Equation 5.18 for the azimuthal magnetic field shows a l/r and l/r 2 dependence. Close to the curren t element the 1/ r 2 term dominates and it is in phase with the excitation current 10 • It can be identified as the usual magnetic induction field obtained from Ampere's law. Equation 5.20 for the elevation axis E-field contains terms in l/r, l/r 2 , and l/r3. The higher order terms dominate close to the source and are identified as the dipole and induction fields, with the 11 r term being the radiation field which is dominant at large distances from the source. These equations show that the characteristics of the field close to the source are different from those for the field at a position far from the radiating element. The radiation field E and H components are in phase, spatially orthogonal and the ratio of E to H is the free-space wave impedance Zoo The region in which the higher order terms dominate is known as the near field and that where the l/r terms dominate is called the far field. Examination of eqns. 5.18 to 5.20 will show that the l/r 2 terms are of the same magnitude as the l/r terms at a distance of A/2n and this is sometimes referred to as the near-field/far-field boundary. A more comprehensive analysis of near-field transition to the far field is given by Yaghjian [19] and he classifies the boundary distances as shown in Figure 5.7.
SIn
B
5.18
5.19
) casB
==
CWIl 1 Zo).smB -+-.-+r Jwcr 3 r
10 h
2
4n
where Er E() H¢ r 10 h
==
== == Zo == c == J1 == f3 == w ==
5.20
radial electric field elevation electric field azimuthal magnetic field distance from current element current on element length of element impedance of surrounding medium permittivity of surrounding medium permeability of surrounding medium propagation constant of medium angular frequency of current 10
To help clarify equations 5.18, 5.19 and 5.20 note the following relationships
w == 2 nf
5.21
wherefis the frequency in hertz
f3 == 2nlA
5.22
== A == c ==
5.23
Af where
c
c
wavelength speed of electromagnetic wave propagation
1/ Vii8
5.24
Zo
5.25
f3
5.26
Balanis [4] states that the reactive near-field region is defined as that region of field immediately surrounding the antenna where the reactive field dominates, and is taken to exist out to a distance of 3
R
< 0.62 [ ~ ]
5.27
He defines the radiating near-field (Fresnel) region as that region between the reactive near field and the radiating far field wherein radiation fields predominate and where the angular field distribution is depend en t on the distance from the antenna. 'The inner boundary is taken to be that in eqn 5.27 and the outer boundary is at R
==
2D 2
T
5.28
78
A HANDB'OOK FOR EMC TESTING AND MEASUREMENT
" ,\
REGIONS OF STABLE ANGULAR PATTERN AT FAR FIELD DISTANCES
RADIATING \ NEAR \ FIELD ~ _1---...... REACTIVE I NEAH I FIELD I
..
FAR FIELD F (<1>,0) e ikr
o
r
co "0
FIELD
DISTRIBUTION
\
IFRESNEL!
I
I I
I : I I I I
I I
~
o (O.6211"537T) Figure 5.7
FRAUNHOFER
w
I
~
: I
a: w
:
o
I
2 0 2 /A
VARIATION OF PATTERN WITH DISTANCE IN THE FAR FIELD AROUND THE ANTENNA PATTERN FIRST NULL
z -10 a:
~
~ -20
>
RANGE r
~w a:
Classification offield regions around antennas
for D » A. The far-field region can be defined as that region of the field of an antenna where the angular field dis tribu tion is essentially independent of distance from the antenna. Outside the far-field boundary defined by eqn. 5.28 the angular distribution is not entirely independen t of range, but the differences are significant only at angles corresponding to the first null in the pattern. See Figure 5.8 for the patterns at different distances from a paraboloid antenna [4]. Knowledge of the near-field/far-field boundary for antennas used in EMC measurement is very important. Most radiated measurements at box or sub system level are made close to the equipment under test (EDT). Often the antenna being used either to receive or transmit with respect to the EDT is not clearly in the far field. In such a situation the transmitted or incident wave cannot be accurately defined by reference to calibrated far field behaviour. The antenna becomes arbitrary as discussed in Section 5.2.1 and the measured signal is only meaningful in the context of that particular situation, so that no absolute measurements of E or H for the wave can be made. Figure 5.9 shows a graph taken from BS800 (now EN550 14) of the near-field/far-field boundary distance in metres against frequency based on the simple A/2n rule. For those predominantly military tests which call for radiated emission and susceptibility measurements to be made at 1 ill from the EDT it can be seen that this is only valid down to a frequency of 50 MHz. In the case of standards for measuring commercial electronic equipment, such as FCC part 15 or EN55022 where radiated emission measurements are allowable on an open site at a
-30
-40 ~_ _....a.-_""""'_-""'o...&-_--a.........
o Figure 5.8
1t
21t U =( DOd sine
31t
___
41t
Variation of antenna pattern with far-jield distance. Calculated radiation patterns of paraboloid antenna for different distances from antenna. Source: HOLLIS) ].S.) LYON) T.]. and CLAYTON) 1. (eds): ,Microwave antenna measurements). Scientific-Atlanta) Inc.) July 1970
Reproduced by permission of Scientific-Atlanta
N
I
~
>()
zw
::> 100
a w a::: u..
10 018TANCE FROM ANTENNA (m)
Figure 5.9
Relationship of near-fieldlfar-Jield boundary with frequency d == A/2n
INTRODUCTION TO ANTENNAS range of 3, 10 or 30m from the EUT, it can be seen that the measurement is in the far field for frequencies down to 15 MHz, which is below the lowest frequency to be measured in the test (30MHz). The simple A/2rc formula yields values for the far-field distance of only a few centimetres at frequencies above 1 GHz. It may seem that all microwave measurement can automatically be carried out in the far field with a test distance of only 1 m. In practice, this is not the case because the antennas used in the microwave regIme are usually of a large aperture, possibly employing parabolic reflectors. For such antennas the near-field/far-field boundary should be defined by distance
5.29
Usually the additional A is left out of the calculation, but is included here to cover the situation where the maximum aperture dimension D in Figure 5.7 is less than a wavelength. The Rayleigh distance to the far field should properly be measured from the outer boundary of the reactive fields around the antenna [19]. Using the eqn. 5.28, for the example of a 50 cmdiameter dish antenna operating at 10 GHz, the far field is at 17m from the dish and so all usual EMC measurements at this frequency would be very much in the near field giving rise to results which at the least would be difficult to interpret and may be suspect.
5.3.9 Beamwidth The angular width of the main beam between the halfpower points is termed the halfpower or -3 dB beamwidth and is a measure of the degree to which the antenna can confine and concentrate the radiation towards a single point. The beamwidth may be specified in the plane of the electric field or the magnetic field produced by the antenna and the values are not necessarily the same. Simplistically, knowledge of the beam width and a separation distance from the antenna to a point of interest will allow the spot size which can be illuminated by the antenna to be calculated. By reciprocity, this is also the size of the area on an extended EUT from which emission signals will be received most efficiently by such an antenna. Consider for example, an antenna aperture which has a constant illumination function as in Figure 5.10, producing by Fourier transform the far-field radiation pattern shown above it. If the illumination function is a true 'boxcar', where
from d> otherwise I the form
79
to d < D/2, I == constant the radiated power plot will take
- D/2
== 0
5.30 The beamwid th is shown in Figure 5.10 as being measured from the halfpower (-3 dB) points. If one calculates the total power radiated in the main beam between the first zeros or nulls and divide this by the mean radiated power level calculated over all angles the figure obtained is called the directivity of the antenna. Beamwidths can be calculated for real antennas with more physically meaningful current distributions but the mathematics are nontrivial. In Figure 5.11 a one can see the polar diagram for dipoles of various lengths. These are calculated [4] using the current distributions in Figure 5.11 b and the 3 dB beamwidths are given in Table 5.3. Note that the difference between a halfwave and electrically short dipole is small.
Table 5.3 Calculated beamwidths for varzous dipole lengths Dipole length l
«
A
l
A/4
l
A/2 3A/4 A
l l
Beamwidth 3 dB 3 dB 3 dB 3 dB 3 dB
beamwidth beamwidth beamwidth beamwidth beamwidth
== == == == ==
90° 87° 78° 64° 47.8°
Although it is possible to calculate the beam patterns and hence beamwidth for an antenna structure, this is usually only carried out for specific communications antennas. Even in these cases the patterns are always measured in an RF anechoic chamber or on an open test range for confirmation. Antennas for use in EMC testing are measured individually and calibration curves of antenna factor and a beamwidth figure are usually supplied by the manufacturer. Specific data are available for the 3 dB beamwidth of certain common types of antenna which may be used in EMC radiated emission and susceptibility testing. For example, for reasonably high-gain pyramidal horns the RSGB [20] gives a graph of gain (for a 50% efficiency horn) against 3 dB beamwidth as in Figure 5.12. A polar diagram is given for a typical high-gain ~ntenna [21] in Figure 5.13a and a graph of 3 and 10 dB beamwidths as a function of antenna gain in Figure 5.13b. The approximate formula which may be used as a rule of thumb relating gain to beamwidth is [21]
80
A HANDBOOK FOR EMC TESTING AND MEASUREMENT RADIATED POWER dB
Po I ---j----OdB
5.37
3dB
Other useful approximate formulas for horns [23] in general use are
Z 0:
~
0.. UJ
en z
o
0.. -UJ---
en
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60 BIA (deg)
5.38
f)H
68 AlA (deg)
5.39
PEAK SIDE LOBE - LEVEL
2 nd NULL
+7t PATTERN ANGLE RADS-
1 sf NULL
and gain
I
WAVELENGTHATWHICH ANTENNA IS OPERATING
C
Figure 5.10
.
I
Relationship between antenna physical aperture: illumination function and radiated far-jield pattern
3 dB beamwidth ==
I
T
27 00012
5.31
1
where G is the numerical gain (not expressed in dB) and beamwidth is in degrees. This is identical to
3 dB beamwidth
I
164.3jG2 (deg)
5.32
or I
3 dB beamwidth == 2.86jG2 (rad)
5.33
Horns are one of the general antenna types used extensively in EMC testing and useful approxima te expressions for the gain and beamwid th are [22]: G == 10 ABjA 2
5.34
where G is the gain, A is the wavelength, and A and B are the H- and E-plane aperture dimensions as in Figure 5.14. Note that the theoretical gain [20] of a long tapering horn where a plane wave emerges at the front aperture IS
G == 4nABjA 2
5.35
The 3 dB beamwidths in the E- and H-planes are f)
_ E -
42,000 f)E X f)H
5.40
APERTURE IN '" s ---
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~---D-
G
51A B
5.36
where G is the gain not expressed In dB, and f) E and f) H are in degrees. The exact formulas for the gain and beamwidth of an tennas in the far field depend on the detailed current distributions on the conducting surfaces of the particular antenna. Thus the shape of a horn or other antenna (length, aperture size, E and H plane dimensions) will- affect the radiation pattern. This accounts for the small differences between some of the approximate formulas presented, each of which is appropriate for that particular type of antenna. Such formulas if used carefully can indicate the gain and beamwidth for other antennas, bu tare limited to those with significant gain at the wavelength of interest [21]. Thus the EMC engineer is caught in something of trap: a clear idea of the far-field distance and spot size for the antennas to be used is needed but usually the time and resources cannot be devoted to calculating these antenna parameters from first principles. T'he simple formulas for high-gain antennas are all slightly different; care must be taken not to use these for antennas for which they were not derived. There appears to be no clear guidance available as to the limits of applicability of some of the rules of thumb. In reality, the best approach for the EMC engineer who wishes to understand the performance and limitations of the wide range of antennas that may be used, is to make careful measurements, either on an open range or in an anechoic chamber, of the antenna patterns and gain at various distances of interest. This is not a simple nor a quick task, however, and requires special test equipment, facilities and time to make reliable measurements at the range of frequencies covered by each antenna. The correct understanding and use of antennas is one of the most difficult aspects of EMC testing. As such testing becomes more important with increasing business and legal implications the technical uncertainties which surround the
INTRODUCTION TO ANTENNAS 00 30
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Beamwidth against gain for pyramidal horn
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DIPOLE ELEMENTS I_
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(b) Figure 5.11
Dipole antenna patterns and current excitations (a) Dipole pattern responses for various lengths (b) Current excitation patterns for various dipole lengths
Reproduced by permission of' Wiley
halfwave dipole beamwidth is 78° which yields a spot size of 1.6 m at a distance of 1 m. Estimates for other dipoles can also be made using Figure 5.11a. However for a low-gain, dipole-like antenna operating below about 50 MHz and working at 1 m from the EUT, calculating even a crude spot size in this way is undesirable as 1 m < A/2n: the E UT is in the near field of the antenna. For a large-aperture dish reflector an tenna working at microwave frequencies, the near field may extend well beyond the antenna-to-EUT distance as 1 m < 2D IA and again (assuming a measurement at 1 m) the EUT is in the near field of the antenna. In this case the crude spot size may be taken to be the dish diameter D, with the power density falling off at a rate of
24xlD (dB) use of antennas for radiated emiSSion and susceptibility measurements will attract greater interest.
5.3.10 Spot size Using the calculated or measured far-field 3 dB beamwidth and with an antenna to EUT distance of 1 m (as required by MIL STD 461 for example) it is possible to estimate a rough spot size which is approximately equally illuminated by the antenna. Eqn. 5.33 is useful in this resp~ct but should only be applied to high-gain antennas. For low-gain devices such as a dipole it is possible to use some calculated results to indicate the beamwidth and spot size. Using Figure 5.11a the
and
4PIA
where P d is the power density at a distance x from the edge of the beam cylinder, Pmax is the maximum power density in the cylinder, P is the input power to the antenna in watts, A is the area of the reflector of diameter D, and x is the distance normal to the beam with x == 0 at the beam edge [24]. See Figure 5.15. Using the criterion of D« A/2 use far-field distance of A~2n, and if D > A/2 use far-field distance of 2D 1A. Taking into account these comments, the farfield boundary distances have been estimated for typical EMC antennas [25] and are shown in Figure 5.16. The calculated spot sizes (covered by
82
A HANDBOOK FOR EMC TESTING AND MEASUREMENT
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Figure 5.14
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Gain ofpyramidal horn in terms of its dimensions measured in wavelengths
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applicable in the far field. The spot sizes given in Figure 5.1 7 are shown as a broken line over the frequencies for which the EDT is in the near field (Figure 5.16) at 1 m and it is unsafe to rely on farfield formulas such as eqn. 5.33 to derive the spot SIze.
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20
30
40
50
60
ANTENNA GAIN dB
(b) Figure 5.13
Relationship between beamwidth and gain for typical high-gain antennas (a) Typical polar diagram of high-gain antenna ( b) Approximate relationship between antenna gain and beamwidth
Reproduced by permission of RSGB
the 3 dB beamwidth) using the information given in this section for typical EMC antennas [25 J are given in Figure 5.17. The concept of a definable beamwidth, and therefore spot size, is only really
Connor [26J defines the effective length of an an tenna in terms of the non uniform current distribution on its surface. Such a distribution is shown in Figure 5.11 b for a dipole. The relationship between the effective length Le and the physical\ length L is
Le
area under nonuniform current distribution
L
area under uniform peak distribution
The effective length of a long wire antenna for example will be a fraction of its physical length owing to the nonuniform current induced by the wave along its length. The shorter effective length leads to a lower output voltage for a given incident field strength than would be predicted simply using the physical length of the antenna.
5.3.12 Polarisation Balanis [4 J defines the polarisa tion of an antenna
INTRODUCTION TO ANTENNAS f------------
ANTENNA FAR FIELD ---------IS AT 2 0 2 / A Spot size is less than EUT size
/ Pd
T
EUT
83
Figure 5.15 EUT in near field of high-gain antenna D == reflector diameter A == reflector area
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1 EUT is not in the far field Reprod uced by permission or ArLech House Inc.
Power density falls as 24 x / 0 dB for x > 0/2
in a given direction as 'the polarisation of the radiated wave when the antenna is excited'. The polarisation of the wave is defined as that property of the radiated electromagnetic wave describing the time-varying direction and relative magni tude of the electric field vector; specifically, the figure traced as a function of time by the extremity of the E-field vector at a fixed location in space, and the sense in which it is traced, as observed along the direction of the propagation. Thus the wave shown in Figure 5.4 is said to be vertically polarised. Polarisation may be classified as linear, circular, or elliptical. Linear and circular polarisations are special cases of elliptical. The sense of circular and elliptical polarisation can be right hand (clockwise) or left hand (anticlockwise) . In EMC testing many of the antennas such as dipoles, biconics, log periodics and horns are linearly polarised and require measurements to be made in two orthogonal planes, usually vertical and horizon tal. Antennas such as conical log
spirals are circularly polarised and measurement needs to be made.
5.3.13 Bandwidth Bandwidth may be considered as the range of frequencies either side of some central frequency where characteristics such as polar pattern, gain input impedance, sidelobe levels, 3 dB beamwidth or polarisation are within an acceptable value of those at the centre frequency. The bandwidth of an antennna is therefore not defined by a single absolute figure. In the case of antennas used by EMC engineers the bandwidth is usually limited by variation in input impedance which is specified in terms of VSWR (voltage standing wave ratio) with respect to 50 ohms. Engineers should be aware that all the other characteristics of an antenna (such as beamwidth, spot size and far-field boundary) may also change with frequency, and while the VSWR may be the
Figure 5.16 l~r-field distances for selection of typical EMC test antennas
i i
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84
A HANDBOOK FOR EMC TESTING AND MEASUREMENT 2.5 ...- - - - - - - - -.....- - - - - - - - - -.....- - - - - - - - - -....
Figure 5.17 Estimate oj spot sizes Jor typical antennas used in EMC measure- ments at distances oj 1 m
2.0
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obvious and limiting parameter the effect of these other changes on the measurement should not be ignored.
lie between 50 and 300 ohms with a dipole at first resonance having a value of72 ohms.
5.4 References
5.3.14 Input impedance The electrical complex impedance presented by the antenna at its input terminals will contain a component known as the radiation resistance which is related to the power loss radiated away from the antenna. I t is a fictitious resistance [26J which represents the radiative power loss in the antenna equivalent circuit. The radiation resistance should be large compared with any real resistance in the antenna (e.g. element resistance) such that at resonance the efficiency of the device is high and most of the power is lost to the radiation resistance. The antenna feed-point impedance is
2 3 4 5 6 7 8
5.41
9
where ZA is the antenna input impedance, R A is the antenna resistance, X A IS the an tenna reactance, and
10
R A == R r + R L
5.42
where R r is the radiation resistance and R L is the resistive loss of antenna components. An antenna is usually operated at or around resonant frequencies where the input impedance is nearly resistive and has a value which is convenient for coupling to an external load. Typical useful antenna impedances
11
12 13
KING, R.W.P.: 'Theory of linear antennas' (Harvard University Press, 1956) SCHELKUNOFF, S.A. and FRIIS, H.T.: 'Antennas theory & practice' (Wiley, 1952) JOHNSON, R.C. and jASIK, H.: 'Antenna engineering handbook' (McGraw-Hill, NY, 1961) BALANIS, C.A.: 'Antenna theory analysis & design' (Wiley, NY, 1982) 'Radio communications handbook (RSGB, 1982, 5th edn.) Chaps. 12 and 13 IEEE Transactions on Antennas 'Reference data for radio engineers' (Howard W. Sams, 1977) Chaps. 27 D-dot sensor, model HSD-2 & HSD-4. EG & G Electromagnetics, 2450 Alamo Avenue, SE, PO Box 9100, Albuquerque, NM 87199, USA MARVIN, A.C.: 'Practical notes on antennas for EMC engineers'. Study note 2/10/79, British Aerospace Dynamics EMC Group, Filton, Bristol, UK BENUMOF, R.: 'The receiving antenna', Am. ]. Phys. 1984,52(6), pp. 535-538 SARGENT, j.: 'Choosing EMC antennas'. Interference Technology Engineer's Master, 1989, pp. 182-304 BENNETT, W.S.: 'Properly applied antenna factors'. IEEE Trans. EMC-28 (1), 1986 BENNETT, W.S.: 'Antenna to ground plane mutual coupling measurements on open field sites'. Proceedings of IEEE symposium on EMC) 1988, pp. 277-283
INTRODUCTION T'O ANTENNAS 14" KENDALL, C.: '30m-site attenuation improvement by increasing the transmit antenna height'. Proceedings of IEEE symposium on EMC) 1985, pp. 346-350 15. BRENCR, C.E.: 'Antenna differences and their influence on radiated emission measurements'. Proceedings of IEEE symposium on EMC) 1990, pp. 440-443 16 MISHRA, S.R. and KASHYAP, S.: 'Effect of ground plane and charnber walls on antenna input impedance'. Proceedings of IEEE symposium on EMC) 1988,pp. 395-399 17 MISHRA, S.R., KASHYAP, S. and BALABERDA, R.: 'Input impedance of antennas inside enclosures'. Proceedings of IEEE symposium on EMC) 1985, pp. 534-538 18 McCONNELL, R.A.: 'An impedance network model for open field range site attenuation'.
19 20 21 22
23 24 25
26
85
of IEEE on E'MC') 1990, pp. YACHJIAN, A.D.: 'An overview of near-field antenna measurements', IEEE Trans. AP-34 EVANS, D.S. and JESSOP, C.R.: manual' (RSCB, 3rd edn.) p. 8.69 EVANS, D.S. and JESSOP, C.R.: manual' (RSCB, 3rd edn.) p. 8.46 'Reference data for radio W. Sams, 1977) pp. 27-37 Microwave Journal tJu.,,-,~~vU,'_~\._Hh.J KEISER, B.: of compatibility' (ArtechHouse, 1987) 335 EMC facility brochure BT British Dynamics, Filton, Bristol, UK, 1982, p. 8.7 and 1990 issue CONNOR, F.R.: 'Antennas' Arnold, London) p. 2
Chapter 6
Antennas for radiated emission testing An tennas for radiation emission measurement are treated separately from antennas for radiated susceptibility testing to show the importance of different antenna parameters in the two cases. With antennas for emission measurements the key parameters are bandwidth, sensitivity, dynamic range and absence of cross or intermodulation products in the case of active antennas with built in amplifiers. Important parameters for antennas used in susceptibility measurements include bandwidth, gain/power requirement, beamwidth/ spot size, power dissipation, size and mass. Such topics are discussed in the following chapter. This one looks in detail at the types of receiving antennas that are widely used for radiated emission testing. They are discussed in sequence, beginning with the passive monopole which has the least complicated construction.
TRANSFORMER COUPLED OUTPUT FROM LOADING COIL
~50n II I I : I
.1
Figure 6.2
6.1.1 Construction These are amongst the simplest receiving antennas used for EMC measurements. They consist of a cond ucting rod of defined length connected to an impedance matching circuit which usually feeds a 50 ohm cable to a matched 50 ohm input of an EMI receiver. The impedance-matching circuit consists essentially of a base loading coil with sufficient inductance to resonate with the capacitive reactance of the monopole elenlent at frequencies of interest, see Figure 6.1. A more sophisticated matching circuit may use ___ Monopole effective height
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UNBALANCED OUTPUT
Simple impedance matching circuits for monopole antennas
transformer coupling or a tapped loading inductor as shown in Figure 6.2. The design of an EMC antenna is perhaps more complicated than that of a communications antenna by the need to achieve good sensitivity (good effective height) over a wide band of frequencies. This requires that the total frequency range of interest (usually 10 kHz to 30 MHz) for an EMC monopole be segmented into bands with a different impedance matching circuit for each band. MonDpole antennas are used mainly for testing military equipment inside screened rooms where a good antenna ground plane reference-is available. I t is appropriate therefore to demonstrate its use with reference to a n1ilitary standard. MIL-STD 462 calls for the use of a monopole and a typical passive antenna is 41 inches long with an effective height of 0.5 m. Such a device may have up to eight frequency bands as shown in Table 6.1.
6.1 Passive Illonopoles
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Table 6.1 Typical monopole antenna frequency bands 10 kHz to 32 MHz in eight bands Band Band Band Band Band Band Band Band
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RF GROUND PLANE REFERENCE FOR MONOPOLE
Figure 6.1
Simple monopole antenna and base loading coil 86
1: 2: 3: 4: 5: 6: 7: 8:
10 to 250 kHz 250 to 500 kHz 500 kHz to 1 MHz 1 to 2 MHz 2 to 4 :NIHz 4 to 8 l\t1Hz 8 to 16 MHz 16 to 3211Hz
AN1'ENNAS FOR RADIATED EMISSION rrESTING
87
6.1.2 Performance The receiving antenna correction factors for a typical commercially available passive 41-inch monopole [IJ are given in Figure 6.3. An antenna factor of between 24 and 50 dB must be added to the output voltage to obtain the electric field strength in V jm. An example of the use of a passive 41-inch monopole to measure the radiated emissions from an EDT on a ground plane is MIL STD 462 note 3 test RE02-1, see Figure 6.4. The area of maximum emission on the EDT is located by probing its surfaces with close-in field probes and then orientating the EDT so that this area is facing the monopole. The antenna to EDT separation is 1 m and the frequency range of the test is from 14 kHz to 30 MHz. When connected to a low-noise EMI measurement receiver the antenna is sensitive enough to measure field strengths of a few microvolts per metre at 10 MHz. The performance is sufficient to measure signals in the range 14 kHz to 30 MHz at below the specification limits for the RE02 test shown in Figure 6.5. A monopole measures the E- field of the incident wave with a polarisation along the axis of the monopole element. If the rod is vertical then the antenna is most sensitive to a vertically polarised E- field. The ground plane or electrical counter-
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20
22
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FREQUENCY IN MHz
Figure 6.3
}'igure 6.4
Antenna factors for typical passive 41" rod antenna suitable for use with MIL STD 461
Reproduced by permission or Camel Labs Corp.
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Test set-up for measurement of radiated emissions (10 kHz-30 MHz) u)ith monopole antenna MONOPOLE ANTENNA FREQUENCY RANGE -
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Figure 6.5
FrequenC)J coverage and example of sensitivity achievable with monopole antenna
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BAND 1
Counterpoise bonded to
poise for the monopole is at the base of the element and must be large enough to adequately termina te the electric field lines from the rod as illustrated in Figure 6.1 such that the rod to ground plane capacitance does not change significant!y if the ground plane is enlarged further. 1'he rod element uses the ground plane as its voltage reference and the field strength measurement will be in error if too small a ground plane is used. The size of the 41-inch monopole antenna ground plane is specified in MIL STD 461A and is 60 cm square. In MIL STD 462 note 3 the monopole ground plane is bonded to the E UT ground plane bench by a solid copper extension of the counterpoise. See Figure 6.4. Monopole antenna measurements can be in error if the rod element is too close to the conductive screened room walls or ceiling. This arises because too many of the electric field lines associated with the rod capacitance are termina ted other than on the reference ground
88
A HANDBOOK FOR EMC TESTING AND MEASUREMENT receivers owing to the number of narrow resonant bands which the antenna has to maintain its sensitivity. The monopole element itself oilers a relatively high reactive (capacitive) impedance at its base over most of the operating freq uency range below its self-resonance, which occurs at low VHF. If an active amplifying device with a high input were used to buffer the rod to the low 50 ohm ou tpu t, it would be possible to prod uce the required impedance transformation wi thou t loss of Moreover, a frequency shaping network could be included in the circuitry to produce a flat antenna transfer function. In commercially available active monopole antennas such as the Carnel Labs. Corporation model 95010-1 [2J, the first active element is a field-effect transistor with a very high input impedance. This particular active monopole has a specification shown in Table 6.2. I t has a single frequency compensated band over three decades wide with a 0 dB flat transfer function when extended to 50 inches and used with top loading. rrhe addition of the top loading plate increases the capacitance of the rod to the ground plane and increases its over the standard 41-inch configuration. This highly sensitive device can detect narrowband emissions down to below the levels shown in Figure 6.7a and broadband down to those in Figure 6.7b.
SCREENED ROOM CEILING
SIDE WALL
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6.6
ejfect oj j)lacing monof)ole antenna too close to conducting surfaces in screened room
Reprod uced by permission of ICT I ne.
in 6.6. l'his leads to a detuning of the with a consequent in the antenna ",---,..o"",+,£:>r1 in MIL STD 462 the rod nlust greater than 30 crn from and 1 m from the walls of the room. Where these distances should be increased to the maXilTIUm available by in a chamber which is lined with radio absorbent Inaterial (RAM). the effectiveness of RAM at the for which are used is rather limited and the criteria for an unlined room should be ~J..J.."'-,A.'cJ..J..~A.>
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6.2.2 Disadvantages
Trp'r1"'£:>Yl,r""::>('
Power supP0J: A battery pack is required, usually housed within the base box of the antenna which also contains the active circuit. The ElVIe test engineer must make pack checks both before and after each test to be sure that the measurements are made with a properly active circuit. Front end damage: Care must be taken when the rod or moving the antenna that static does not accumulate on the test
LLIJIJ.!.i'",--,"--l.
m.onopoles
active monofJole antenna
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amjJlifier) Carnel Labs. 950101-1 10kHz to 40 111Hz One ±1 dB from 10 to 25 kHz ±0.5 dB from 25 kHz to 40 MHz 50 inch top-loaded rod: 1 m 41 inch rod: 0.5 m 10 Mohm shunted by 8 pF 50 ohms 50 inch loaded rod: 0 dB 41 inch 8 dB on receiver bandwidth and frequency .LJ'-/IJ''-/LI.'-lvil.L
ANTENNAS FOR RADIAl'ED EMISSION TESTING NARROWBAND SENSITIVITY
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r-----..,------r----~--
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FREQUENCY
Active monopole antenna sensitivity
engineer which can be discharged to the rod element and therefore into the gate of the FET. It is relatively easy to damage some active antennas and they should not be moved or touched without grounding the rod to the box. Intermodulation distortion: Any active component only has a limited range of operation where its transfer function is linear. If large signals are present and the device is working on a nonlinear part of its operating curve then spurious distortion and intermod ula tion signals can be created. With the dynamic range of EMC measurements often being in excess of 60 dB it is possible to measure and record small spurious signals at harmonic and intermod ula tion frequencies, if large signals are present out of band. An active antenna with a bandwidth of 40 MHz is also prone to overload and signal distortion when measuring impulsive broadband noise. This will result in observing an incorrect spectrum of the pulsed wavefield being measured. Typical maximum field strengths that must not be exceeded for the 1 m and 0.5 m effective height monopoles [2J are given in Table 6.3.
89
connecting a balanced feeder to the elements across the gap. A dipole can be of any length with respect to the wavelength of an incident wave, from a hundredth of a wavelength to a few wavelengths. Practical dipoles for communications purposes are built to resonate with the wavefield to maximise receiving sensitivity at a fixed frequency. The most common dipole is a half-wavelength long, making each element a quarter-wave long. For EMC measurements, dipoles must be able to receive signals over a very wide band of frequencies from about 30 MHz to 1 GHz. If a single dipole is used which is for example 4 m long, tuned to 38 MHz, t:he frequency response is of the form shown in Figure 6.8. Thus the dipole will only respond well to certain harmonic frequencies in the band of interest and is not very useful for broadband EMC measurements. This problem can be overcome by tuning the dipole to a new resonant length at each frequency of interest in the band being measured. The antenna characteristics such as correction factor and beamwidth at the first resonance can be established and a useful measurement of the wavefield can be made. This is the basis of the tuned-dipole radiated emISSIon test methods which are required by commercial specifications such as FCC part 15j and EN55022/BS6527/ VDE0871. However, tests carried out in this way are time consuming and the method is not well suited to automated scanning emission measurement with swept frequency EMI receivers or spectrum analysers. A. DIPOLE
2"
I
FREQUENCY MHz
200
400
600
co
"C
W
...J
«
Table 6.3 Overload jield levels jor active monopole
() CJ)
-20
>-
0:
Maximum signal input (overload)
One-metre rod: 0 dB ACF Narrowband: 107 dBIlV/m Broadband: 73 dBIlV/m/MHz 41 inch rod: 8 dB A.C.F. Narrowband: 115 dBIlV/m Broadband: 81 dBIlV/m/MHz
«0:
I-
00
0:
«
-40
w CJ) z
0 a..
CJ)
w w
0:
-60
0
6.3 Tuned dipoles 6.3.1 Introduction A dipole antenna is constructed from two long thin co-linear conducting elements with a small gap in the middle. The electrical ou tpu t is luade by
:::>
I-
:.J
a.. :2
«
-80
rzgure 6.8
Typical response oj tuned dipole (4 m long at ),/2). Dipole parameters: length 400.0 cm; diameter 1.0 em; load 100
n
90
A HANDBOOK FOR EMC TESTING AND MEASUREMENT-
6.3.2 Practical tuned dipoles
Table 6.4 Tuned dipole parameters
A practical tunable dipole is shown in Figure 6.9 with telescopic elements which can be extended to be a quarter-wavelength long over the frequencies of interest. At resonance the dipole presents a balanced impedance of 72 ohms across the feed point. This must' usually be transformed by the balun (balanced to unbalanced transformer) to a 50 ohm output for connection to a 50 ohm coaxial cable and thence to the matched input of the EMI meter. The effective length of an infinitesimally thin halfwave tuned dipole in free space is given by Ma and Kanda [3J as
Leff == Lire
6.1
To achieve self-resonance of the dipole (zero reactance) experience shows that it is necessary to make it slightly shorter than a half-wavelength. Schelkunoff [4 J derived the required length in terms of the length-to-diameter ratio of the elements
L/D ratio
L/A at resonance
Dipole R in
50 500 50000
0.47 0.48 0.49
61 66 70
n
L D
dipole elelnent length dipole element diameter dipole feed point impedance wavelength at resonance
R in A
The theoretical antenna factor for a thin resonant dipole with a lossless balun and cable connected to a 50 ohm receiver is given by Ma and Kanda [3J as shown in Figure 6.10, where is the dipole impedance (taken to be 70 ohms).
40 ~---~---T"""-----r-----r----""
al
30
"'C
1 - 0.2257 ]
6.2
L r = (L/2) [ In(L/D) - 1
where D is the diameter of the dipole element, L is tuned length, and L r is required length. The effective length of a dipole near to resonance is given in Reference 3 as
Lejjff .
L tan(reL r I2L)
== - - - - - -
a: 0 t- 20
~
(0.025) (F in MHz)
U
« LL « z z
10
W
t-
Z
0
«
-10
50
20
6.3
re
Figure 6.10
If the dipole is shortened slightly to the required length for resonance (eqn. 6.2), the theoretical resistance R at resonance depends on the length-todiameter ratio. Typical values are given in Table 6.4.
100
2000r------------------.., L / A. = 0.8 L / D RATIO = 2000
R 50n INPUT
BALUN LOSSES \
H
T
\ ~~~f~s
~
I
1
Figure 6.9
70n 1000 CJ)
o
:><:
[8J
w
()
z
.
EMI METER
«
~
w
a:: \
II
~
:2
BALUN c<1-q-
EXTENSION ARM
\
:r:
L!::====~~========: '::::1
Z=72Q
1000
Reproduced by permission of NBS
2"
50n COAXIAL CABLE
500
Theoretical antenna Jactor Jor thin resonant dipole
FIRST RESONANCE A. DIPOLE
TELESCOPIC ELEMENTS
200
FREQUENCY MHz
-1000
SWR "LOSSES"
OUTPUT CONNECTOR V out
ANTENNA FACTOR =
--L/A. = 1.0 -2000---------------
E Vout
Tuned and rrlatched halfwave dipole oj variable length Jor EMC measurements
DIPOLE COMPLEX IMPEDANCE Z
Figure 6.11
c
R+j X
Complex impedance oj thin dipole as Junction oj element length/wave length
Reproduced hy permission of
BS
ANTENNAS FOR RADIATED EMISSION TESTING
The plot in Figure 6.11 shows the complex impedance as a function of dipole length to wavelength with the first useful resonance close to 0.5 and the second close to 1.5.
~t--I--~--I I I
~ O
25
Z
20
~
o
Model OM -105 A - T1 ....----20 - 200 MHz
o
« ~w
0 -5
I
l
l
SHIELDED CASE
I
:
J __
j
,---_
I
'\ "
"'"
'J " l l
,_)(_)i-t(
I
I I
_I
___ ~
_
I I I I I
J
BALUN RFOUTPUT
(aj A r--"4A----1-4--j
\
TELESCOPING ROD
FARFIELD ANTENNA CORRECTION FACTOR antenna 3m. above ground ." (\\
\\~
~p(\o
~\~Y;. 0 \0 ~e ~t\1-
r---~
s \u(\e \0 '21 .
I I
Q\~\~s oo~(\
t5
15 ~ 10 ~ 5
r------ I I ._---J I ( ----..,/ I I I () tj () I I\:..>..)_'*l_"
TELESCOPING ROD
"'-BALUN
~ ~ 35
a: 30
I I I
I
\
I
I
6.3.3 Commercial EMC tuned dipoles Consider the performance of some commercially available tuned dipoles for EMC testing to meet civil and military standards. The Carnel Labs. Corporation models DM~105A (T1, T2 and T3) are a set of three dipoles [4] which can be tuned from 20 MHz to 1 GHz. They are supplied with a frequency calibrated 'tape measure' which can be used to set the element lengths q uickly and correctly without calculation. The antenna factor for the far-field performance is shown in Figure 6.12. The solid line A is the antenna factor for the dipole tuned to each individual frequency. It is untuned below 27 MHz which is the frequency at which the telescopic elements are fully extended. At frequencies below 27 MHz the efficiency of the dipole starts to fall as its effective length decreases, it also becomes mismatched with the inclusion of capacitive reactance, see Figure 6.11.
91
\e(\9J'
oo~(\
\u(\e~() ~t\1-~ ,~~ - trCC 'o~
t-
"\Oc\O Model OM - 105 A ! T2
~\~"Y;. ':\0(\(\0 ~e~\cO~ S ~
;(,."(\f?P
\
~
FREQUENCY IN MHz
Antenna factors of tuned dipoles for use with FCC Pt. 15j (tuned down to 30 MHz) and EN55022 (tuned down to 80 MHz)
For FCC part 15j testing of information technology equipment the dipole can be tuned down to the lower test frequency of 30 MHz. When testing to the European close equivalents of EN55022/BS6527/VDE0871/CISPR22 the dipole is tuned at all frequencies down to 80 MHz, but is then fixed at the resonant length for that frequency while testing is continued down to 30 MHz. In such a case the antenna correction factor for the dipole would be that shown by the broken line in Figure 6.12. The lower solid line B is the theoretical antenna factor derived from the NBS calculations in Figure 6.10. The increased
I
----I
RFOUTPUT
140 - 400 MHz
~ -102'-!-O-......3.... 0 - - S 0 - - - 1...... 0 0 - - -I-O----5......0..... 0--10--'00 20
Figure 6.12
I I
I
L __
Model OM -105 A - T3 400 - 1000 MHz
~
r--'I
I
(b) Figure 6.13
Examples of baluns (a) Wound balunfor use at H F/ VHF (b) Stub balun for use at UHF
Reproduced by permission of' Camel Labs. Corp.
losses in the commercial antenna over the theoretical one are due mainly to balun losses. A typical balun configuration used for the lower frequencies up to 200 MHz is shown in Figure 6.13a. Similar choke designs can be found in Reference 5. A suitable VHF /DHF balun for tuned dipoles is shown in Figure 6.13b and further details are given in Reference 6.
6.3.4 Radiated emission testing Testing to meet EMC standards for commercial eq uipmen t accounts for the major use of tuned dipoles, but they are sometimes used to determine 'antenna induced voltage' called up by military standards. The civil tests are commonly carried
92
A HANDBOOK FOR EMC TESTING AND MEASUREMENT
~-------MAJOR DIAMETER;:::
/"
~~
/ ",/
2d
------~I
--------1-- ---I~~' MINOR DIAMETER
""-
;:::~
/ /
...........
~
\
\
/
\ \
II
DIPOLE OR - OTHER ANTENNA
'"
"'~ Figure 6.14
/
I
...........
\
-------------..;...-
~///
Plan view of open-field test site. Boundary oj area defined by ellipse. Area to be jree oj reflecting objects e.g. buildings)fences) trees) poles) etc.
Reproduc~d by permission of BSI
ou t on an open-si te tes t range wi th the tuned dipole antenna at a distance d from the EDT. Depending on the specification and the equipment class this distance can be 3, 10 or 30 m. The site must have no reflecting objects within an ellipse with a major diameter of 2d, see Figure 6.14. There must also be a conductive ground plane of a specified minimum size to provide unvarying ground interaction properties. The dipoles and EUT may be set up as in Figure 6.15. There are anum ber of potential disadvan tages which the EMC engineer must be aware of when conducting radiated emission testing with tuned dipoles. The tests can take a long time, as the dipole has to be adjusted at each frequency of in terest in the emission spectrum of the E UT. Some specifications call for testing 'with both horizon tal and vertical polarisation, and the
height of the antenna must be changed at each frequency of interest to determine the maximum emission level from the EDT. For the FCC tests at low frequencies the manipulation of the large dipoles becomes a problem and the lower element (vertical polarisa tion) gets very close to the ground. This problem is not so severe for the EN55022 test \vhere the dipole length is restricted to that for resonance at 80 MHz (elements about 1 m long). The most serious problems arise as to what the rather large antenna actually measures. There are considerable mutual impedance problems due to ground reflections [7-9] and the extended nature of the FCC type antenna integrates the field variations along its length: this leads to a field strength estimate that differs from those made with the EN and CISPR (80 MHz) antenna or a compact broadband biconic (where permitted) [10]. Experienced test engineers are a ware of some of these measurement problems and are careful not to become over confident about the validity of a technique simply as a result of its being specified in a national standard. Tuned dipoles are easily sensitive enough to make EMC emission measurements when used with a low-noise EMI receiver and quasipeak detector. Signa~ .levels set for radiated interference limits in standards such as EN55022 are not difficult to achieve. These are 30 dBflV/m (QP) from 30-230 MHz 37 dBfl\T/ m (QP) from 230-1000 MHz Ambient radio noise experienced when making measurements on an open test range can be a problem and this can mask the signal from the EDT. There is little that can be done to minimise such a problem other than siting the test area in a low radio noise location.
6.4 Electrically short dipoles 6.4.1 Special short calibration dipoles
---1/:;;;;;
~METAL GROUND PLANE EUT HEIGHT TYPICALLY 1m. ANTENNA HEIGHT VARIES BETWEEN 1 ·4m.
Calibration dipoles are not widely used for EMC testing but a knowledge of their existence and some idea of how they work is essential for the professional EMC engineer. There are two types of calibration antennas which measure electric and magnetic fields. They are based on short dipoles and small loops. This section deals only with these special dipoles used for standard E-field measurement and calibration.
EQUIPMENT SITED ON A REFLECTION-FREE OPEN AREA TEST SITE
Figure 6.15
Use oj tuned dipoles to measure emissions Jrom computer equipment carried out on open-area test site
6.4.2 Roberts dipoles Wilmar Roberts was assistant chief of the FCC laboratory (1967) when he undertook the
ANTENNAS FOR RADIATED EMISSION TESTING
z
::::> -I
« co
4 COMMERCIAL ANTENNA
3
/
'" CJ) 2 CJ) 0
-I
ROBERTS ANTENNA /
co ,.., 0
20
30
50
100
200
300
500
1000
500
1000
FREQUENCY (MHz)
5 4
a::
~
CJ)
>
3 2 ROBERTS ANTENNA
1 20
30
50
100
200
300
FREQUENCY (MHz)
Figure 6.16
VSWR and losses Jor typical commercial balun in tunable dipole Jor EMC measurements and Roberts balun Jor calibration dipole
development of standard dipoles which have an tenna factors close to those for a theoretical tuned dipole [11]. Their chief characteristics are a low VS WR and low balun losses over a wide range of frequencies [12, 13]. The balun loss and VSWR performance of the Roberts dipole are shown in Figure 6.16 compared with commercially available tuned dipoles such as in Section 6.3.3.
6.4.3 Small nonresonant dipoles Electrically short dipoles can be used for some EMC radiated emission testing. BS 727 in paraH3. 3 permits the use of a dipole shorter than half a wavelength but longer than a tenth of a wavelength to be used for testing commercial electronic equipment. Short-dipole antenna correction factors (50 ohm load) are provided in graphical form in Figure 7 of Appendix H in that document. Workers at NBS (National Bureau of Standards) in the USA produced a number of designs for calibration dipoles [14J and spherical dipoles have been used at the National Physical Laboratory (NPL) in the UK. It is not intended to deal in depth with this topic here, save to say that the approach is based on two characteristics of di pole behaviour. The first is to reduce the dipole length to a few centimetres or less [15 J which has anum ber of effects. The device averages the field over only a small region. The short dipole is almost purely capacitive and presents a high reactive impedance and is not capable of delivering the currents one
93
would expect froin a resonant dipole into a standard 50 ohm load. I ts effective height is small and the device is insensitive compared with a resonant dipole. The second is to connect a high impedance balanced DC voltmeter across the short dipole which has a low turn-on-voltage Schottky diode connected across the feed point. The balanced connection to the display meter is made using a resistive filter line constructed from carbon impregnated plastic in a nylon jacket. Each line has a resistance of about 6000hm/cm. Thus a short dipole with a practical sensitivity is achieved by using the high-impedance detector/meter and the wavefield is not disturbed in the vicinity of the dipole by the balanced connections due to its high resistance. Although a short dipole is capable of making an accurate field measurement almost at a single poin t in the wavefield it has two draw backs for use other than for controlled calibration type testing. I t is insensitive and requires a field strength of a few hundreds of m V /m for operation. I t is also frequency insensitive (all spectral information being lost in the diode conversion to DC). I t cannot be used where more than one signal is present at a time.
6.4.4 Microscopic dipole probes Berger, Kumara and Matloubi describe a special high field strength probe manufactured by Electromechanics Co [16J which is based on the NBS small tapered resistive dipole work by Kanda [1 7J. The device uses diode detection and highly resistive leads but makes use of resistive dipole elements. It can operate fronl 1 MHz to 10GHz within 1 dB accuracy at field strengths from 1 V 1m to 1 kV 1m. The dipole elements, detector and resistive leads can be seen in Figure 6.1 7.
""
TAPERED RESISTIVE DIPOLE ELEMENTS
DIODE RECTI FI ER
...........
BALANCED HIGH IMPEDANCE CARBON FILAMENTS CONDUCT DC FROM THE DIODE TO A MILLIVOLTMETER
Figure 6.17
Advanced lnicroscopic resistively tapered dipole with built-in diode detector. The microscopic dipole probe sensor shown next to a sewing needle Jor comparison oj size
94
A HANDBOOK FOR EMC TESTING AND MEASUREMENT z
6.5 Biconic dipoles
FIELDS PRODUCED AT POINTP E, H (r, 8,$)
6.5.1 Introduction CONICAL ELEMENTSOF APEX ANGLE <X
The biconic dipole is a broadband derivative of the long thin resonant dipole. Balanis [18J gives the theoretical basis of operation of a biconic dipole in detail. The treatment considers conical di pole elemen ts of infini te length and calculates the radiation pattern and feed-point impedance as a function of cone angle. See Figure 6.18. Schelkunoff [19J devised a transmISSIon line equivalent method of calculating the feed-point impedance for finite length small angle biconic elemen ts. The resistance and reactance of the feed-point impedance calculated by Jasik [20J are plotted in Figure 6.19 and show how the antenna becomes more broadband as the cone halfangle Increases. Approximations to true biconic antennas can be made in the form of triangular sheet 'bow tie', or bow-tie wire simulations as in Figure 6.20. A wire bicone with approximately eight intersecting wires has a performance almost equivalent to a solid sheet bicone. The intersecting wire bicone is the one most often used in EMC testing as it is light, rugged and compact with elements about 1 m long covering the frequency range from 20 to 200 MHz.
P
.......
.......
.......
.......
....... .......
'.I
x
Basis for theoretical calculation of radiation fields and feed-point impedance jor infinitely long bicone elements
Figure 6.18
6.5.2 Commercial biconic antennas Typical commercial biconic antennas used in EM C testing are the Emco 3108 [21 J and the Carnel Labs. Corporation 94455-1 [22]. These antennas consist of a pair of open stiff wire frame bicone elements and a balun as in Figure 6.21. For an example of the gain and antenna factor of a commercial biconic antenna see the figures for
...--....----.----.--... 10,000
Figure 6.19 Calculated feed-point impedance ofjinite length bicone relating angle to broadband response
THIN DIPOLE
CONE ANGLES
I
'I
I,
~'\
I,
en
n'\
I \
I I If
~
J:
o
1,000
.5
a: w
\ •\ ;\\,\
SEE Fig. 6.18
:E J:
o
'."-, II"f\\ ~ W
,
I
0
'\
U." \I.~\, ~ \ II II ~~
~ ~
Ci5 w
a:
..... a..
\
100
::>
I
~
o Reproduced by permission of'McGravv-Hill
en
II \ I. I \
~\'
()
a/2= 5° ----. a 12 = 11° - - a /2 =1/100°
1 7£:' A. 2
6
8
1st BICONE RESONANCE
10
0
1
~
2
4 1t
II A.
. I
I
.....
,~ ~
8 StCONE LENGTH IN WAVELENGTHS SOURCE: JASIK
ANTENNAS FOR RADIATED EMISSION TESTING
95
Table 6.5 Gain and antenna jactor for wire bicone antenna
BOW-TIE
Figure 6.20
Frequency
Gain
Antenna factor*
(MHz) 30 40 50 60 70 80 90 100 120 140 160 180 200 220 240 260 280 300
0.05 0.07 0.15 0.50 0.59 1.06 1.06 1.05 0.50 0.93 1.19 1.34 1.33 1.16 0.90 0.97 1.17 1.00
(dB) 12.8 13.5 12.5 8.8 9.5 8.1 9.07 10.0 14.8 13.5 13.6 14.1 15.0 16.4 18.3 18.7 18.5 19.8
WIRE APPROXIMATION
Antenna shapes derived from basic solid bicone
*Specification compliance testing -factor (1.0 m spacing) to be added to receiver meter reading in dbpV to convert to field in tensi ty in dBp V Im (Emco 3108)
Figure 6.21
ANTENNA FACTOR OF WIRE BICONIC ANTENNA COMPARED WITH TUNED DIPOLES
Typical wire bicone antenna used in EMC testing. Biconical antenna Emco 3108
§00~
EMC03108 BICONE
15
~ t; dJLE
T2 DIPOLE /
10
5 0
I-
~
DIPOLE - - CARNEL LABS TUNED DIPOLES
-5 L . - - _ . L - _ - - ' -_ _- - - I
20
30
50
_ _____l
~I
100
200
400
FREQUENCY IN MHz
Figure 6.22
Antenna factor of wire biconic antenna compared with tuned dipoles
20 z o i=cn
f:d"O
a::z
~~
() 0
«I-
";/,..
....... 10
z~
W
«
,,;
\
,,;
---_...
--- ........,,
............ ....
,,;
....., . ./ /
zu.. IZ
...
1m ANTENNA FACTOR,,;"; ,,;
15
FAR FIELD ANTENNA FACTOR
5 O~---L_--'-_---'-_--A-._......I-_.....L.-_...L-_~---'
20
6.5.3 Use of biconic antennas Some biconic antennas are designed specifically to drawings given in MIL-STD 461A and are used extensively for test methods specified in MILSTD 462 from 20 or 30 MHz up to 200 or 300 MHz depending on the capability of specific
......
25
i= &3 ~ 20
Reproduced by permission of Emco
the Emco 3108 given in Table 6.5. Note that the antenna factor is for 1 m spacing between calibration antennas so that it may be used for normal military standard emission testing, and is not necessarily the far field antenna factor, which should be used for 10 and 30 m testing in commercial standards. In Figure 6.22 the '1 m close-in' antenna factor for the biconic is plotted with the '3 m close-in' antenna factor for the T1 and T2 tuned dipoles. The shorter element length of the biconic results in a less efficient antenna below 80 MHz. The far field and 1 m antenna factors for the Carnel Labs. Corporation 94455-1 are given in Figure 6.23. When the antenna is used at 1 m from the EDT it appears to be predominantly a few dB less sensitive than for reception of far-field signals.
30 r---r------,---~---.,----
z
o
40
60
80
100
120
140
160
180
200
FREQUENCY IN MHz
}'igure 6.23
Far-Jield and 1 m antenna factors for typical wire biconic antenna used in EMC testing. Biconic antenna: G'Iarnel Labs. 94455-1
Reproduced by permission of Carne! Labs Corp.
96
A HANDBOOK FOR EMC TESTING AND MEASUREMENT
bicone models. They can also be used for some tests specified by civil standards for commercial electronic equipment such as information technology. For example, EN55022 (Section 10.2) permi ts the use of aerials other than tuned dipoles providing that the results can be correlated with a balanced tuned dipole with an acceptable degree of accuracy. More explicit req uiremen ts for the use of complex broadband aerials is given in BS727 (para H4.2). Biconic antennas can be used with a low-noise 50 ohm EMI meter to make sensitive measurements over frequencies from 20 to 300 MHz to detect signals below the severe levels set in MIL STD 461 C (using a peak detector function) as shown in Figure 6.5. The chief advantage of a broadband biconic antenna is that it provides good sensitivity over a wide band and is easy to use, unlike tunable dipoles, and enables tests to be made quickly with automated scanning receivers or spectrum analysers.
6.6 Wideband antennas
6.6.1 Introduction Before the 1950s antennas with broadband input impedances and pattern characteristics covered frequency ratios of about 2: 1. The advanced designs which came later extended bandwidths to 40: 1 or more [23]. These designs were referred to as frequency independent and they had the property that their geometries were specified by angles. Antenna characteristics such as impedance, beam pattern, polarisation etc. remain unchanged when the physical size of the antenna changes, provided that the operating frequency also changes by the same ratio. Thus the antenna characteristics are invariant if the electrical size of the antennna does not change. If the shape of an an tenna is therefore completely specified by angles then its performance is independen t of frequency [24]. An infinite biconic dipole is one such antenna which is only specified by the cone angle as in Figure 6.18. To make practical truncated structures it is necessary that the current on the structure decreases with increasing distance from the feed point. If the current beyond a certain distance is negligible the structure may be truncated and removed. Such an antenna has a low-end cut-off frequency above which the an tenna characteristics are the same as those for the infinite structure. The upper cut-off frequency is determined by the size of the feed point which must be less than A/8, where A is the upper cut-off wavelength.
Although the biconic antenna or transmISSIon line is defined by an angle, its current distribution does not fall away to zero with distance and it cannot be successfully truncated to form a frequency independent antenna. Other antenna shapes which are specified by angles do have current distributions which decay with distance and can exhibit frequency independent properties.
6.6.2 Log-periodic antenna If an antenna is designed so that its physical structure is periodic with the logarithm of frequency, and the variation of structure is small or negligible over a single period, then a practical freq uency independen t an tenna can be constructed [25]. For a more detailed explanation see Balanis [23]. There are many interesting and bizarre shapes which fulfil these requirements and a variety of useful communications antennas have been built from planar and wire log-periodic structures, planar and wire trapezoidal toothed log periodics and log-periodic slots. The configurations most widely used for performing EMC tests are log-periodic dipole arrays and the conical log spiral. Usually the first is linearly polarised and the second is always circularly polarised.
6.7 Log-periodic dipole antenna Consider a set of dipoles with characteristics and spacings which change in a log-periodic manner as in Figure 6.24. Define 6.4 and lJ.
==
tan
-1
In
-
6.5
Xn
Ideally, the dipole gap sizes and the element diameters should also change in a sinlilar fashion. However, if they do not, and constant values are used, it has little impact on the practical performance of the antenna. If the elements are closely spaced and all fed in phase from the narrow end then an endfire beam propagates backwards in the direction of the larger elements. If each successive dipole is fed in antiphase as in Figure 6.24b the endfire beam emerges forwards from the short end of the array. In either case, the antenna must have a balanced feed which is usually impractical. An unbalanced coaxial feed cable may be used by staggering the elements and connecting the coaxial inner conductor as shown in Figure 6.24c. This is the configuration that is most familiar to EMC test engineers as a number of commercially available antennas are of this form.
ANTENNAS FOR RADIATED EMISSION TESTING
97
1.0 0.9 0.8
>-
()
zw <3
u::
0.7
I 1
0.6
u.. w ll R 0.5
L
C)
z
(a)
~ « 0::
0.4
Ci
BALANCED FEED ELEMENTS OF t PHASE ...
0.3
T = 0.89
a =45°
0.2
t
0.1 O_:::;;;;;"a.._...s..-
Beam' produced at this end
o
(b)
0.1
0.2
.&--
0.3 0.4
0.5
_
0.6 0.7
0.8
0.9 1.0
L/A
Figure 6.25
Radiating efficiency of log-periodic dipole array. L width oj array) a half angle oj apex
+8 ......- - - - - - - - - - - - - - -... +7
COAXIAL LINE INPUT
+6 +5
+4 +3 +2
Figure 6.24
-....-.
50
UNBALANCED FEED Beam formed at this end
GAIN (
_....-.
_
.....
100 300 500 700 900 1100 1300 1500
FREQUENCY MHz
c)
Log-periodic dipole array antenna (a) Array dimensions (b) Balanced antiphase Jeed (c ) Unbalanced coaxial feed
~~=--....:;;;a
a:
~
(J)
>
700 800 1100 1300 1500
50
FREQUENCY MHz
Reproduced by permission of Wiley
170 ......- - - - - - - - - - - - - -........ (J)
The radiating efficiency of a log-periodic dipole array is given in Figure 6.25 showing how this changes with the largest dipole dimension [26]. The antenna achieves 85 010 efficiency with the largest dipole element of 0.6 A. Therefore these antennas may be compact, at least not significantly larger than a halfwave dipole at the lowest freq uency of interest. Typical gain, beamwidth and VSWR (50 ohms) for a log-periodic dipole array operating from 50 MHz to 1 GHz are given by [27] and shown in Figure 6.26. A drawing of a typical antenna which can be used for both EMC radiated emission and susceptibility measurements is given in Figure 6.27 and its performance is illustrated in Table 6.6. The gain, VSWR (50 ohm) and antenna factor as a function of frequency from 150 MHz to 1.1 G Hz for this typical antenna are given in Figure 6.28. This type of sensitive wideband antenna is ideal
@ Cl
150
130
J:
b
110
~
90 80 70 60
~
«w
CO CO "0
(I)
50
40 ......-50~...............1... 00-3..00-500 .......-7... 00-800 .......-11...00-1... 30-0-...150D
BEAMWIDTH
Figure 6.26
FREQUENCY MHz
Typical performance parameters Jor logperiodic antenna
for EMC measurement as scanning receivers can be used to make rapid measurements which satisfy a wide range of military and civil standards. I t is well capable of measuring field strengths (in its operating band) below the severe limits specified for the MIL STD 461C RE02 test given in Figure 6.5.
98
A HANDBOOK FOR EMC TESTING AND MEASUREMENT
co "0
8.0 7.0
25
6.0 ~O
~
0::
o
t-
15
40"
TYPICAL ANTENNA FACTOR
C/)
10
==:
a: ~
C/)
>
z
t-
o
LO
« z W
:r: o
U
it
2.5
5
Z
«
2.0 1.5 1.0
.-..-
.-..
_ .....
a 100 200 300 400 500 600 700 800 900 1000 1100 FREQUENCY MHz
Figure 6.28
Gain, antennaJactor and VSWRJor typical commercial log-periodic antenna.
Reproduced by permission or Amplfier Research
6.8 Conical log.-spiral antenna
Figure 6.27
o
0
o
0
0
0
0
0
0
0 00000
o
0
o
0
0
0
0
0
0
0 00000
Dimensions oj typical log-periodic antenna used in EMC measurements (150 MJ-Iz1GHz)
Reproduced by permission or Amplifier Research
Table 6.6 Performance qf commercial log periodic Frequency range Power input (maximum) 150-250 MHz 250--500 MHz 500-1000 MHz Power gain (over isotropic) Gain flatness Impedance VSWR maxlmull1 average Beamwidth (average) E plane H plane Front to back ratio (minimum) Connector Size(wxhxd) AR model ATlOOO
150 to 1000 MHz 2000 W 1500 W 750 W 6.5 dB min 7.5 dB av ±1.0 dB 50 ohms nom
Another example of a frequency independent antenna, the structure of which is defined by angles, is the conical spiral, a non planar extension of a spiral-plate geometry as shown in Figure 6.29. The length L determines the lowest frequency which can be propagated, and at frequencies above this the pattern and impedance are frequency independent. 1~he ends of the spirals are tapered in thickness to provide a better impedance match. rrhis type of antenna is circularly polarised. When the conducting spiral lies on the surface of a cone, the antenna has only a single pattern lobe towards the apex of the cone with a maximum along the axis. Typical conical log-spiral antennas (Stoddart 93490-1 & 93491 used for EMC measurements are shown in Figure 6.30 with antenna factors in Figure 6.31.
L
1.8: 1 1.5: 1
type C female 102 x 13 x 91 cm (40 x 5 x 36 in)
Figure 6.29
Spiral plate antenna configuration. L determines lowest Jrequenc_y that is radiated efficiently. Feed point is at centre
ANTENNAS FOR RADIATED EMISSION TESTING EXAMPLES OF COMMERCIAL LOG CONICAL SPIRAL ANTENNAS
Log spiral pattern conductor printed onto conical surface
1
4 0cm
1
COAXIAL CONNECTOR
~36an
~I
r-13cm~ 1·10GHz
200 MHz· 1 GHz ego CARNEL LABS MODEL 9349Q.1
Figure 6.30 co ~ ex:
o~ ~
5 ~ ~
ex:
oo
Model no. 93490-1
24 23 22 21 20
Frequency coverage:
19 18
~
16
«
Examples oj commercial log conical spiral antennas
27 26 25
« z
m
ego CARNEL LABS MODEL 93491-2
200 MHz - 1 GHz
17
200
300
400
500
600
700
800
900 1000
_ _-
_ _- _
FREQUENCY MHz
52
co ~
50
~
48 46
ex:
r---oy---oy----r---.,--~-
_ _-
Model no. 93491-2
~ o 44
~
42
o
40
z
~
~
38
~
36
8
34
~
30
~ z ~
Frequency coverage: 1 -100Hz
32 28· 26 _ _ 1
""""_"'"
. a . . - _ . a . . - _ ~ _ . . . L . - _ . . . . . I . -
2
3
4
5
6
7
8
9
10
FREQUENCY GHz
Figure 6.31
Typical log conical spiral antenna factors 200 MHz-l GHz and 1-10 GHz
Reproduced by permission or Camel Labs Corp.
99
The antenna factors are determined by the MIll STD 461 (para 5.2.7) which is based on SAE ARP-958 two antenna method using short separation distances and may not produce an antenna factor calibration which is suitable for far-field measurements. Often the manufacturers recommend that the far-field distance is calculated for these log conical spiral antennas by uSIng
far-field distance
where D is the largest diameter of the an tenna, or 5A whichever is less. Thus for an antenna of diameter of 25 cm the far-field distance at 200 MHz is only a few centimetres. Sometimes it is not made clear where this distance should be measured froIn on the antenna. The phase centre of the antenna (defined in Chapter 5) should be used as the assumed source of radia tion. I t is then logical to use this as the point from· which distances should be measured. However, the phase centre moves along the an tenna axis as a function of freq uency and the measured an tenna factors correspond to the various positions of the phase centre and not to a standard distance of 1 m. Theoretical calculations and measurements by Marvin [28J have resulted in an expression for the distance of the phase centre from the apex of the antenna as a function of wavelength. This can then be used to calcula te the correction to the gain figures obtained by the SAE ARP-958 calibration method. The gain figures for the standard (ARP958) and corrected calibration method which takes accoun t of phase-centre movemen t, are given in Table 6.7 for an Emco model 3103 conical log spiral antenna. Measured antenna patterns are shown in Figure 6.32 for the vertically polarised E-field at 200, 500 and 900 MHz. A typical beamwidth for this type of antenna is 90 0 and the spot size at 1 m in midband is about 2 m. The VSWR (50 ohms) for the model 3103 is typical of this type of antenna and is usually less than 2: 1. See Figure 6.33. Like all wideband frequency independent an tennas conical log-spiral an tennas are easy to use and automated measurements may be made quickly. With these circularly polarised antennas there is no need to evaluate horizontal and vertical polarisations separately. If the polarisation components must be measured separately a linearly polarised log-periodic dipole antenna could be used.
100
A HANDBOOK FOR EMC TESTING AND MEASUREMENT
Table 6.7 Standard gain for conical log spiral antenna (measured at 1 m) and corrected gain taking into account phase centre movement with frequency Frequency (MHz)
Standard measured gain (linear)
Corrected gain (linear)
Antenna factor based on standard gain (dB)
100 200 300 400 500 600 700 800 900 1000
0.21 1.67 2.23 2.11 2.35 2.24 1.85 2.11 1.89 2.10
2.91 3.26 2.79 2.92 2.67 2.12 2.37 2.07 2.26
17.0 14.0 16.3 19.0 20.5 22.3 24.5 25.1 26.6 27.0
Emco Model 3103
200 MHz
6.9 Horn antennas
500 MHz 900 MHz
EMC03013
Figure 6.32
1800
Example of variation with frequency of conical log spiral antenna polar diagram (Emco 3103)
Reproduced by permission or EMCO
3.0
ex: ~
~ 2.0 1.0
100 200 300 400
500 600 700 800 900 1000
FREQUENCY MHz
Figure 6.33
Typical VSWR for comm.ercial conical log spiral antenna (Emco 3103)
Reproduced by permission or EMCO
At microwave frequencies where waveguides are of practical dimensions, it is possible to radiate energy from the open end _of the guide used as a crude antenna. This type of antenna is almost never used in EM C testing owing to its low directivity [29]. Open waveguides are however sometimes used as feeds to illuminate parabolic reflector antennas [30J. A horn antenna can be considered as a tapering extension to a waveguide which provides an increased aperture size leading to improved directivity. The basic theory of horn antennas can be found in almost any standard text on antennas including [29-33J. Various types of horn antenna which are based on rectangular and circular waveguides are shown in Figure 6.34. The type most frequently used in EMC measurements is the pyramidal horn. I t is cheap to construct and its design is well understood. Consider the pyramidal horn in Figure 6.35 defined by its principal dimensions as shown. The radiation pattern of a horn is determined by the amplitude and phase distribution of E- and Hfields across the respective aperture planes. The difference in phase over the aperture denoted by r in Figure 6.35 is a result of the principle of equality of path length (Fermat'-s principle). For adequate performance most horns have this parameter r of less than 0.25L in the E-plane and about 0.4L in the H-plane [34 J. Expressions for the gain and 3 dB beamwidth of pyramidal horns have been given in Chapter 5 equations 5.34 to 5.40. Sometimes horn an tennas are used in the near field when making EMC measurements and work by Ma and Kanda [35J has resulted in an addition to the Shelkunoff gain eqn. 6.34 which should be used in the near field:
ANTENNAS FOR RADIATED EMISSION TESTING RECTANGULAR HORNS
OJ
101
0,
"'0
en
0::
0
z .....
-()
-1
zz
0::0 0-
:r:t; :::>
Exponentially tapered pyramid
Pyramidal
0
w ex:
4
3
2
5
6
DISTANCE FROM HORN m.
Figure 6.36
Typical near-field gain reductionfactorsfor pyramidal horn at 1 GHz (Ma and Kanda)
Reprocl ucecl by permission or NBS
horn operating at 1 GHz over an antenna to EUT distance of 1 to 6 m. Silver [32] gives approximate formulas for the 10 dB beamwidth of typical horns with average flare angles of 20°
Sectoral E - plane
CIRCULAR HORNS
8E
==
Exponentially tapered cone
Figure 6.34
88A
13 deg. 31
Various possible types of horn antenna
for
79A
B/ A <
+A deg.
2.5
for A/A < 3
6.7
6.8
The radiation pattern from a typical horn with
Reproduced by permission 01' McGraw-Hili
~ \
0°
/
,
I
I
PYRAMIDAL HORN
__- - & - - - - - - - + - 2 7 0 °
900~------",-~
r ,/
E PLANE VIEW
H PLANE VIEW
Figure 6.35
180
Figure 6.37
Reproduced by permission or McGraw-Hill
G
32AB
~x RHR E
210 0
1500
E- and H-plane views of rectangular pyramidal horn antenna
6.6
where R H and R E are the gain reduction factors due to the E- and H-plane flares. These factors are calculated in Reference 35 and expressed graphically in Figure 6.36 for the example of a
0
Exalnple of E- and H-plane beamwidths and polar diagrams for pyramidal horn antenna (Source: Balanis). E-plane beamwidth > H-plane beamwidth. 11 == 12 == 6A, A == 12A, B == 6A, a == O.5A_, b == O.25A, E-plane, - - - H-plane
Reproduced by permission or Wiley
102
A HANDBOOK FOR EMC TESTING AND MEASUREMENT
CD
21
"C
Z
20
~
19
2
3
4
5
I
FREQUENCY GHz
; ;~~----r---.---~ 3
4
5
6
7
8
9
FREQUENCY GHz
;
I
10
:~_r----r-\I'lG-\G
""--'8l--""'O \
8
9
10
11
12
13
14
15
16
17
18
FREQUENCY GHz
Figure 6.38
Frequency coverage for horns with standard gain 20 dB) for various waveguide sizes. Typical beamwidth of horns is 18° with sidelobes at -13 dB. Data for commercial horns manufactured by [/lann Microwave
Reproduced by permission of Flan Microwave
Table 6.8 Example of UHF horn for EMC use specification Frequency range Power input (max) Power gain
400-1000 MHz 1000 W 10 dB min typically increasing to 15 dB at 1000 MHz 50 ohms nom 2.5:1 max 1.5:1 av 9.1 kg (20 lb) 56.4 x 79.3 x 73.7 cm (22.2 X 31.2 X 29.0 in)
Impedance VSWR Weight (max) Size (w x h x d)
Manufactured by A.R. model no AT 4001
dirnensions of those in Figure 6.35 is gIven In Figure 6.37 [31 J. A wide range of waveguide horns are commercially available and typical gain against frequency data [36J for various wave guide sizes are given in Figure 6.38:Note that the bandwidth of the horns (and waveguides) is rather small, at less than one octave. At lower frequencies horns can still be used but are usually not fed by a long waveguide. A coaxial cable is employed and connected to a stub in a short section of waveguide directly attached to the horn. The specification of a commercially available horn [37J for use in EMC testing operating in the frequency range 400 MHz to 1 GHz is given in Table 6.8, with typical gain and VSWR curves in Figure 6.39.
6.10 Ridged guide horn antennas
8
;;; 2.5 :2 J:
oo
2
1.0
~ 1.5
TYPICAL UHF HORN ANTENNA FOR USE IN EMC TESTING
~ 400 500
600
700
800
900 1000
FREQUENCY MHz
Figure 6.39
Example of gain and VSWRfor commercially available UHF horn antenna. A.R. model AT 4001
Reproduced by permission of Amplifier Research
A single or double ridge can be introduced into a rectangular waveguide as shown in Figure 6.40. Marcuvitz [38J gives tables of modified cut-off wavelengths for propagating modes and shows that the eq uivalent transverse network for the dominant waveguide mode is a junction capacitance shunted by two H-mode transmission lines with open- and short-circuit terminations. The effect of the central ridge is to load the waveguide and increase its useful bandwidth by lowering the cut-off frequency of the dominant mode [34, 39]. A thin ridge or fin is also effective in producing the loading of a central ridge. If such a fin is extended from the waveguide section into a pyramidal horn, as in Figure 6.41, the
ANTENNAS FOR RADIATED EMISSION rrESTING
103
CROSS SECTION OF WAVE GUIDE
2
T b
L E FIELD IN WAVEGUIDE
SINGLE RIDGE GUIDE
Ylo
0:::
~>
1.5
DOUBLE RIDGE GUIDE
2
Yo
3
4
5
6
7
8 9 10 11 12 13 14 15 16 FREQUENCY GHz
17 18
EQUIVALENT CIRCUIT
0;0
~ypical
p--'igure 6.43
A.C T
1
VSWR againstjrequency .for commercial ridged guide horn antenna. Electro-Metrics RGA-100
-1
Reproduced by permission or Electro-Metrics
Figure 6.40
Single and double ridged wave guide laboratory with a large number of antennas of limited bandwidths. The gain against frequency performance of a commercial wide band ridged guide horn [41 J is given in Figure 6.42 together with the receiving antenna factor. A typical VSWR plot for this antenna is given in Figure 6.43 and shows a rather uneven match to 50 ohms at the lower frequencies.
Reproduced by permission or McGraw-Hill
qp . /
WIDEBAND
COAXIAL FEED
~I""'.
SHORT SECTION OF DOUBLE RIDGED WAVEGUIDE
Figure 6.41
"""I
6.11 Reflector antennas
PYRAMIDAL HORN CONTAINING TWO FINS
Construction oj double ridged pyramidal wideband horn antenna
Reproduced by permission or McGraw-Hill
useful bandwidth of the antenna can be increased many times [34, 40]. This increase in bandwid th is very useful to the EMC engineer who wishes to cover the required measurement spectrum with as few changes of antenna and connections to the EMI receiver as possible. This will save on testing time and reduce the initial capital cost of equipping the test
15 14
en -0
Z
:;: 0
en -0
ANTENNA FACTOR
13
Reflector antennas operate successfully when their aperture dimensions are many wavelengths across. They can be constructed in a number of different ways as shown in Figure 6.44 and explanations of their design can be found in standard antenna texts. Of particular interest to the EMC engineer is the parabolic reflector antenna, as it is widely used from about four to above 18 GHz for both radiated emission and susceptibility testing. Geometrical optics based on ray tracing may be used to study certain aspects of these antennas, while a full analysis requires the use of electromagnetic field theory [42J. A parabolic surface defined in Figure 6.45 has the property of being able to convert a divergent beam into a near parallel one with the minimum of aberration. This is clearly useful for point-topoint communications where narrow beamwidths
0:::
40 0
I-
12
0
11
35
LL
10
z zW
9
30 ~
I-
8
7
FEED
2
4
Figure 6.42
12 10 FREQUENCY GHz
8
14
16
18
Example oj gain and antenna Jactor jor double ridged guide horn antenna. ElectroMetrics RGA-IOO
Reproduced by permission or Electro-Metrics
PLANE
f'igure 6.44
CORNER
CURVED (fronHed)
Geon1etry oj reflector antennas
104
A HANDBOOK FOR EMC TESTING AND MEASUREMENT
- 3 dB BEAMWIDTH
y2
= 4
fx
o
-6
-8
-4
-2
0
2
6
4
8
OFF AXIS ANGLE (DEGREES)
Figure 6.45
Geometry ojparabolic
Figure 6.46
r~flector
are used. But in EMC testing parabolic antennas are used to increase directivity and gain over a wide frequency range using a simple horn feed so that small signal levels, such as those in Figure 6.5, can be measured with a good signal-to-noise ratio. The apparent aperture of a parabolic reflector is given in Reference 43 as
A
==
0.54 x S
Typical polar diagram offront-jed parabolic r~ector antenna. Parameters: JID == 0.82) D == 25cm)f== 20.5cm) Jrequency == 35 GHz
Reproduced by permission of Wiley
18" diam. parabolic reflector, 7" focal length 91892 - 1
24 23
6.9
FEED HORN
where A is the apparent aperture area and S is the physical aperture area, and the factor 0.54 is due to the nonuniform illumination of the reflector. The gain of this type of an tenna is
G
==
0.54 (nDIA)2
6.10
where D is the physical aperture diameter. The 3 dB beamwidth for a parabolic reflector was given in eqn. 5.31 and can be used to drive the following
83dB
7
X
10 4
== - - JD
CO
0
"'0
a:
0
z w
I-
20
:b~ II
0)
II
~
~
II
LO
U
« z
~
.....:
0
I-
Lt
-
21
to
II
~
co
~
-
I ........
co
19
if
~
Z
«
co
"'0
f co
18
CD
N
1
2
3
4
PJ II
0
<:>
II
0
co
"0
110
et)
16
LO
00 "'0 C\J co N II
"'0
17
15
oq-
--1 J t- -t CD
~
6.11
where 8 is the -3 dB beamwidth,Jis frequency in MHz, and D the physical diameter of parabolic in feet. An example of a typical radiation pattern produced by a high-gain parabolic reflector antenna which might be used in a communication link is given in Figure 6.46 [44]. Notice that the -3 dB beamwidth is of the order of 10. 1'ypical of conlmercial parabolic reflector antennas produced for EMC testing is the Carnel Labs. Corporation 91892-1 18-inch diameter reflector with horn feeds 91890-1 (4.4-7.3 GHz) and 91891-2 (7.3-12 GHz). The antenna factors, gains and beamwidths (far field) are given in Figure 6.47. The beamwidths are much larger than for communications antennas with values of 4 to 9°.
FEED HORN
91'890-1 91891-2 4.6-7.3GHz 7.3-12 GHz
22
~
5
6 7 8 910 12
20
FREQUENCY GHz
Figure 6.47
Antenna Jactor) gain and beamwidth Jor lJ)pical parabolic r~ector antenna used in EMG~ testing
Care must be taken when using these antennas in standard EMC measurements, as the equipment under test possibly at a distance of 1 m is almost always in the near field, with all the a ttendan t problems of uncertainty in wave impedance, gain, antenna factor, beamwidth and spot size. The manufacturers of commercial antennas for EMC measurement do not always specify the method used to calibrate the antenna
ANTENNAS FOR RADIATED EMISSION TESTING and derive the antenna factor and the EMC engineer is often left to wonder if the figures su pplied are related to the far-field performance or tor aIm measurement. Until manufacturers produce full traceable performance data with their antennas, it is safest for the EMC test specialist to calibrate antennas using the methods approved by the appropriate EMC standards. In the UK, antennas can be submitted to an independent calibration serVice such as the National Physical Laboratory.
105
140 120 CD
'U
0::
100
0
I-
~ LL z
0
(jj 0::
80 Factor for dB I J,JV I M 50.Q input
-
60
W
> z
40
0 0
Factor for dB I PT 50.Q input
20 0 10
6.12 Magnetic field antennas
100
1k
100k
10k
FREQUENCY Hz
6.12.1 Introduction
Figure 6.48
Loop antennas can be used to measure the strength of the magnetic component of the electromagnetic wavefield produced by an interference source. The design of loops varies with the application and frequency range. In all cases the loops are electrostatically shielded by enclosing the wire turns inside a tubular conducting shield which is broken at some point around the periphery to prevent the shield acting as a shorting turn. Loops are used primarily at low frequencies from a few Hz to tens or hundreds of kHz. For example the MIL STD 462 N3 RE01 (30 Hz30 kHz) and RE04 (20 Hz-50 kHz) tests specify the use of a 13.3 cm-diameter loop over these frequencies. Larger loops, but with areas of less than 1 square metre, are used for example to measure field strengths from 100 kHz to 30 MHz in accordance with VDE 0877 pt2. Such measurements are called for in VDE 0871/EN550 11 in connection with industrial, scientific and medical (ISM) equipment.
6.12.2 Passive loops Loop antennas generate an output voltage by magnetic induction and the relationship of open circuit EMF (below loop resonance) and magnetic field strength is
e
==
flr flo
N A (j) H
6.12
where e == open-circuit output voltage flr rela tive permeability of core flo permeability of free space N == number of turns A == area of the loop (j) angular frequency H == magnetic field strength Taking as an example the small loop, which is well specified in the US MIL STD 461/2, the following characteristics apply:
Antenna eonversionfaetorsfor MIL STn loop (13.3 em dia.)
Electrostastically shielded Diameter: 13.3 cm Area: 139 cm 2 Turns: 36 Wire type: 7-41 Litz. A typical comn1ercial loop antenna of this type is the Solar Electronics 7334-1 [45] which has a transfer characteristic shown in Figure 6.48. Notice that the lower curve on the graph relates EMI meter "indicated voltage to magnetic field strength in dB picoteslas and the upper curve relates output voltage to E-field strength in dB fl V 1m (assuming a free-space wave impedance). Great care should be exercised in using a loop antenna to derive E-field strengths as most measurements at these low frequencies are made in the near field where the wave in1pedance is not likely to be that of free space. As an example of using a small loop look at the test method of MIL STD 462 N3 REO 1. This req uires that the loop is placed 7 cm from the face of the EUT with the plane of the loop parallel to
«
-l
00 W
I-
0
()
140
en
130
a:
'U
:r:
120
I-
110
zw
100
<:>
0:: I00 0 -l
w
IT:
()
f=
w
90 80 70 60 50
z
<:>
«
::E
10
100
1k
10k
100k
FREQUENCY Hz
Figure 6.49
M agnetie field emission lirnits for MIL STn RE 01 test in dB pieotesla. Limit,for RE01
106
A HANDBOOK FOR EMC TESTING AND MEASUREMENT co
CL
"'0
80
0:
0 .... ()
« LL
w
60
()
z « 0
Z
0 OO
W C-
40
a::
~
> z 20
0
0
LU
«
0
...J
~ son
U
« z z ....zLU « co 0
...J
0
600n 100kO
-20 0.01
0.1
1.0
10
100 250
FREQUENCY IN KILOHERTZ
}-'igure 6.51
Antenna conversion factors for large loop 76.2 cm diameter
6.12.3 Active loops Figure 6.50
Example of typical loop antenna emission nleasurement (MIL SYD 461j2 RE04). Plane of loop should be 90° to plane of test sample face
the face. The EUT is then examined to find the position of maximum radiation at each frequency of interest and the levels are recorded and compared with the permissible limit which is shown in Figure 6.49. Small loop antennas may also be used at greater distances from the EUT. Such a test configuration is called for MIL STD 462 N3 RE04 and is shown in Figure 6.50 with loop positioned at 1 m from the EUT. Larger passive loops are also used for measuring magnetic field strengths in this way. An example is the Carnel Labs. Corporation 94608-1 [46] which has the following specification: Electrostatically shielded Frequency range: 20 Hz to 250 kHz (calibrated) usable up to 30 MHz Diameter: 76.2 cm (30 in) Area: 0.456 m 2 Turns: 11 Load impedance: 50, 600 or 100 kO The antenna conversion factor for this loop is given in Figure 6.51. It could be used in VDE testing but would require additional calibration up to 30 MHz. Another example of a large diameter loop antenna which can be used for VDE or TEMPEST (secure communications) testing is the Electro-Metrics ALP-70 [47] which has a diameter of 63.5 cm (25 in) and is calibrated from 10 kHz to 30 MHz.
To ex tend the frequency range of a loop antenna without either incurring a large number of band switches, or reducing the number of turns (and the sensitivity) to avoid loop resonance, while at the same time equalising the freq uency response a t low freq uencies, it is necessary to develop an active loop antenna. An active broadband amplifier is included in the antenna in the same way as for the active E-field monopole. Thus the same comments apply with regard to battery power and overload distortion (see Section 6.2.2). An example of a commercially available calibrated active loop is the Electro-Metrics ALR-30A [48]. Tl1is device has a diameter of 43 cm (17 in) and is calibrated from 9 kHz to 30MHz.
6.12.4 Loop calibration There are a number of ways of calibrating loop antennas [45, 49] but the most common approach is the coplanar two-loop method. This is shown in Figure 6.52 where the generating loop produces a known field strength along the axis of the loop to be calibrated by measuring the voltage developed across a calibrated resistor to derive the driving loop current. A similar calibration technique is specified in IEEE Std 302 [50] where the loops are situated 1 m apart. Investigations by Millen [49] have shown that there are wide variations by up to 7 dB in the measurements of magnetic fields made by different commercial loop antennas in testj calibration situations. The greatest problems occur near the loop resonances. He concludes that loops do not produce the same output voltages in uniform and nonuniform fields and that it is not
ANTENNAS FOR RADIATED EMISSION TESTING POWER SIGNAL GENERATOR or SIGNAL GENERATOR and AMPLIFIER 1 Q PRECISION RESISTOR IN SERIES WITH LOOP
107
~ 12 em-----1
LOOP BEING CALIBRATED (RE 01)
TRANSMITTING LOOP ( RS 01)
I
I
I
I
13.25" 33.57 em.
~
\
I
0.3 em I
LOOP (n = 10 tums) OF 00.16 INSULATED
-L . . . . - - - - - - - -
"!T
COPPER WIRE
5 em
-"-----
llo---_
--
_=___
(a) Figure 6.53
L
J
1.0n PRECISION RESISTOR
(b) Figure 6.52
Co-planar two-loop calibration setup ( a) Illustration oj'layout (b) Circuit schematic
Reprocl ucecl by permission of Solar Electronics
valid to calibrate loop antennas in a nonuniform field, such as that which would be produced with two separated coplanar loops. To achieve the most accurate measurements, loop antennas should be calibrated in a uniform field which could be provided by a Helmholtz coil. Currently this would require the test engineer to calibrate his own loops, which will take time and involve some equipment costs. Whether this is done or not, engineers should be aware of these calibration problems, and should expect an additional measurement uncertainty due to calibration factors when making magnetic field measurements.
6.12.5 Magnetic field susceptibility tests A typical magnetic field susceptibility test is that specified in MIL STD 461/2 as RSOI (30Hz30 kHz) where a small 12 em-diameter loop is wound with ten turns of insulated copper wire as shown in Figure 6.53. In the test the face of the radia ting loop is placed 5 em from the face of the EDT being investigated. The alternating voltage across a calibrated resistor in series with the loop is measured to derive the current in the loop to produce the required field strengths.
~~~E~~~DUCTING B is measured here
Loop specifiedfor magnetic field radiated susceptibility test ( MIL S T D 461/2 RSO1). For this configuration oj loop: B == 5 x 10- 5 T/ A at measurement face. Loop self-resonance occurs at above 100 kHz
:5en
w
b
usa.
LIMIT FOR RS 01
~
~
~160
en
8 10-
~140 I:§ 120
1A
1
10-2 10-3
~ 100
I-
~ 80 w 60 ~ 40
10-5 ~ 10-£ ~
20
o ow
o
i=
w
z
":E c(
10
ff
:c
10-4 ::
r::
~
~
g
100
1k
10k
100k
a: :5 aw a:
FREQUENCY Hz
Figure 6.54
Limits of magnetic field strength Jor MIL STD 461C pt. 4 RS 01 susceptibility test
The test limits are given in MIL STD 462 N3 directly in dB,uV across the calibrated resistor. In later versions (MIL STD 461 C pt.4 the field strength limits which the EDT must withstand are given in Figure 6.54, together with the approximate loop driving current required to produce the field. The use of various loop antennas in lowfrequency EMC field measurement illustrates that generally, military EMC standards require a wider range of tests, over a broader range of freq uencies and using a wider range of antennas than do standards relating to commercial electronic equipment. I t is inevitable therefore that tests in military EMC standards have been used to illustrate the use of the widest range of antennas, sensors and measurement techniques.
108
A HANDBOOK FOR EMC TESTING AND MEASUREMENT
6.13 References
2
3
4
5 6 7
8
9
10
11
12 13
14
15
16
17
18
Passive Monopole Antenna 94592-1, Carnel Labs. Corporation, 21434 Osborne Street, Canoga Park, CA 91304, USA Active Monopole Antenna 95010-1, Carnel Labs. Corporation, 21434 Osborne Street, Canoga Park, CA 91304, USA MA, M.T. and KANDA, M.: 'Electromagnetic compatibility and interference metrology'. NBS technical note 1099, NBS,. Boulder, CO 80303, USA, july 1986, p. 76 Carnel Labs. Corporation instruction manuals for tuned dipoles DM-I05A-Tl/T2 & T3, Carnel Labs. Corp., 21434 Osborne Street, Canoga Park, CA 91304, USA 'RSGB radio communications handbook'. Potters Bar, Herts, UK, 1982, 5th edn., p. 12.42 'RSGB radio communications handbook'. Potters Bar, Herts, UK, 1982, 5th edn., p. 13.6 McCONNELL, R.A.: 'An impedance network model for open field range site attenuation'. Proceedings of IEEE symposium on EMC, 1990, pp. 435-439 KENDALL, C.: '30 m site attenuation improvement by increasing the transmit antenna's height'. Proceedings of IEEE symposium on EMC, 1985, pp. 346-350 BENNET, W.C.: 'Antenna to ground plane mutual coupling measurements on open field sites'. Proceedings of IEEE symposium on EMC, 1988, pp. 277-283 BRENCH, C.E.: 'Antenna differences and their influence on radiated emission measurements'. Proceedings of IEEE symposium on EMC, 1990, pp. 440-443 DASH, G.: 'A reference antenna method for performing site attenuation tests'. Proceedings of IEEE symposium on EMC, 1985, pp. 607-611 'A new wideband balun'. FCC project 3235-16 dated 3/4/57 'Construction and testing of antenna balun assemblies for general purpose use by the Field Engineering Bureau'. FCC project 3235-25, dated 10/11/67 MA, M.T. and KANDA, M.: 'Electromagnetic compatibility and interference metrology'. NBS technical note 1099, sections 6 & 7, NBS, Boulder, CO 80303, USA CAMELL, D.G., LARSEN, E.B. and ANSON, W.J.: 'NBS calibration procedures for horizontal dipole antennas'. Proceedings of IEEE symposium on EMC, 1988, pp. 390-394 BERGER, H.S., KUMARA, V. and MATLOUBI, K.: 'Considerations in the design of a broad band E-field sensing system'. Proceedings of IEEE symposium on EMC, 1988, pp. 383-389 KANDA, M.: 'An isotropic electric field probe with tapered resistive dipoles for broad band use, 100kHz-18GHz'. Proceedings of IEEE symposium on EMC, 1986 BALANIS, C.A.: 'Antenna theory, analysis & design' (Wiley, NY, USA) chap. 8, pp. 322-330
19 SCHELKUNOFF, S.A.: 'Electromagnetic waves' (Van Nostrand, NY, 1943) chap. 11 20 jASIK, H.: 'Antenna engineering handbook' (McGraw-Hill, NY, 1961) chap. 3 21 Emco Biconical Antenna 3108 instruction manual. Electro-Mechanics Company, PO Box 1546, Austin, TX 78767, USA 22 Carnel Labs. Corporation biconic antenna 94455-1 instruction manual. Carnel Labs. Corp., 21434 Osborne Street, Canoga Park, CA 91304, USA 23 BALANIS, C.A.: 'Antenna theory, analysis and design' (Wiley, NY, USA) chap. 10 pp. 413-435 V.H.: 'Frequency independent 24 RUMSEY, antennas'. IRE National Convention record, part 1, 1957, pp. 114-118 25 'Reference data for radio engineers' (Howard W Sams) chap. 27, p. 27-17 26 ISBELL, D.E.: 'Log periodic dipole arrays' IRE, 1960, AP-8 (3), Fig. 7 27 'Antennas, antenna masts and mounting adaptors'. American Electronic Laboratories Inc, Lansdale, Montgomeryville, PA 18936, USA, catalogue 7.5M-7-79 28 MARVIN, A.C.: 'Gain and phase centre evaluation of the EMCO conical log spiral antenna'. BAe EMC study note 005, 29/1/79, British Aerospace Dynamics, Filton, Bristol, UK 29 COLLIN, R.E.: 'Antennas and radio wave propagation' (McGraw-Hill) p. 182 30 RUDGE, A.W., MILNE, K., OLVER, A.D. and KNIGHT, P. (Eds.): Handbook of antenna design, Volume 1. (Peter Peregrinus, London, 1982) chap. 4 31 BALANIS, C.A.: 'Antenna theory, analysis and design' (Wiley, NY, USA) chap. 12, pp. 532-571 32 SILVER, S.: 'Microwave antenna theory and design' (Peter Peregrinus, London, 1984) section 10 33 SOUTHWORTH, G.C.: 'Principles and applications of waveguide transmission' (Van Nostrand, 1950) section 10.1 34 KRAUS,j.D.: 'Antennas' (McGraw-Hill, 2nd edn.) p.647 35 MA, M.T. and KANDA, M.: 'Electromagnetic compatibility and interference metrology'. NBS technical note 1099, NBS, Boulder, CO 80303, USA, chap. 5.3, pp. 67-72 36 'Standard gain horns' data sheet. Flann Microwave Instruments, Dunmere Road, Bodmin, Cornwall, UK 37 Antenna data sheet for AT 4001. Amplifier research (USA), EMV Ltd, 11 Drakes Mews, Crownhill, Milton Keynes, UK 38 MA.RCUVITZ, N.: 'Waveguide handbook' (McGraw-Hill) section 8.6, p. 399 39 COHN, S.B.: 'Properties of the ridge waveguide'. Proc. IRE, August 1947, 35, pp. 783-789 40 WALTON, K.L. and SUNDBERG, V.C.: 'Broadband ridged horn design', Microwave ]., March 1964,7, pp. 96-101 41 Operating instructions for RGA-I00. ElectroMetrics, 100 Church St, Amsterdam, NY 12010, USA 42 CONNOR, F.R.: 'Antennas'. (Edward Arnold, London, 1984) section 5.3, pp. 53-57
ANTENNAS FOR RADIATED EMISSION TESTING 43 'Reference data for radio engineers' (Howard W Sams, 1977) chap. 28, p. 28-20 44 BALANIS, C.A.: 'Antenna theory, analysis and design' (Wiley, NY, USA) chap. 13, p. 616 45 'Calibration of loop antennas, RFljEMC Instruments, components and accessories for the RFI jEM C engineer'. Solar Electronics Co, 901 North Highland Ave., Hollywood, CA 90038, USA 46 Carnel Labs. Corporation loop antenna 94608-1. Carnel Labs. Corp., 21434 Osborne Street, Canoga Park, California 91304, USA 47 Electro-Metrics loop antenna ALP-70, Electro-
109
Metrics Ltd, I vel Rd, Shefford, Beds., SG 17 5JU,
UK 48 Electro-Metrics loop antenna ALR-30A, ElectroMetrics Ltd, Ivel Rd, Shefford, Beds., SG17 5JU,
UK 49 MILLEN, E.: 'A comparison of loop antennas'. Proceedings of IEEE symposium on EMC, 1990, pp. 451-455 50 IEEE Std 302-1969: Standard methods for measuring electromagnetic field strength for freg uencies below 1000 MHz In radio wave propagation. IEEE, 345 East 47th St, NY, NY 10017, USA
Chapter 7
Use of antennas for radiated
susceptibility testing 7.1 Introduction
7.1.2 Standards requiring immunity tests
The types of antenna commonly used for RF radiated susceptibility testing are treated separately from antennas used in emISSIon measuremen ts as the an tenna parameters relating to reception and transmission are differen t.
Radiated susceptibility testing is carried out regularly and extensively on military equipments before going into service. Each country, and sometimes each armed service in that country, has a specification and set of test methods which will direct the testing in a detailed way. Important EMC testing standards covering military equipment are listed in Chapter 2. Many EMC standards relating to commercial electronic equipment do not as yet require radiated susceptibility or immunity testing to be carried out as part of product certification. Examples of exceptions being IEC 801-31 BS666 7-3 for ind ustrial process con trol equipment, EN55101-3 for information technology equipment and NWML0320 for certain items of metrology eq ui pmen t. There is also a European harmonised generic immunity standard EN50082-1 relating to any domestic, commercial or light industrial equipment not covered by a prod uct specific imm uni ty standard. Radiated susceptibility testing is one of the most expensive aspects of EMC assessment. This is mainly due to the capital cost of the range of high power broadband RF amplifiers needed to drive the variety of an tennas used to cover frequencies up to 1 GHz (civil) and 18 or 40 GHz (military). Free-field tests must be conducted in a metalscreened room of sufficient size to house the EDT and test antenna as shown in Figure 7.1. Reflections should be suppressed whenever possible by using radio absorbent material on the walls and ceiling inside the room as shown in Figure 7.2. The screened room and radio absorbent material are also costly items and increase the capital investment which must be made by the test laboratory if radiated susceptibility testing is to be performed. In what follows, antennas commonly used in free-field EMC radiated susceptibility testing are described individually and typical field strengths produced for various input RF powers indicated. This is followed by a similar treatment for bounded-wave devices.
7.1.1 Types of antennas used in susceptibility testing Antennas for EMC radiated susceptibility or immunity testing. fall into two classes: free wavefield and. bounded wavefield, addressed separately in this chapter. Generally, free-field antennas are used for tests on large systems or subsystems, and on units at frequencies above 30 MHz. Bounded-wave devices such as parallel plate lines are generally used for testing small units about 30 cm high at frequencies below 300 MHz. There are of course exceptions to these generalities and bounded-wave devices for example can be used to test large vehicle systems at kV 1m field strengths in NEMP (nuclear electromagnetic pulse) measurements. Small units or components can be tested in bounded-wave devices (with extended frequency coverage up to 1 GHz) such as Crawford cells or GTEM (gigahertz transverse electromagnetic mode) cells. Free-field antennas are not as efficient at producing high field strengths (over a large volume) as bounded-wave devices are over a small volume. In many cases of conformance testing, the use of free-field antennas or bounded-wave devices is specified over a given frequency range by the EMC standard which applies to the EDT. For those development tests where the EMC engineer has some discretion in choosing antennas, the more efficient boundedwave devices will probably be chosen if the size of the EDT and test frequency range permits. The bounded-wave radiators are less costly, and wi th some closed devices wi th no RF leakage such as a Crawford cell, the tests may be carried out in an ordinary laboratory rather than a shielded room. 110
USE OF ANTENNAS FOR RADIATED SUSCEPTIBILITY TESTING
III
where Pd is the power density in Wjm 2 and Pin is the input power to the antenna in watts. Relating the E-field strength to the wavefield power density by the impedance of free space, the input power required for various E-field strengths in Table 7.1. is calculated from 7.4 In the case of a practical antenna with a slightly lossy balun, such as the Carnel Labs DM-I05A TljT2jT3 which were discussed in Chapter 6, where the gain is given as 0 dB from 27 to 1000 MHz, eqn. 7.4 becomes
E
SIGNAL SOURCE & POWER AMPLIFIER
2
== 30
X
Pin
7.5
Table 7.1 Input power required for various field strengths
Figure 7.1
Field strength at 1 m
Standard test set up in screened room for radiated susceptibility testing
Vjm 1 10 100 1000
Reproduced by permission of BSI
ANECHOIC CHAMBER
POWER SUPPLY
I.
TRANSMITIING ANTENNA
1
i
:_~ Figure 7.2
Radio absorbent material lining inside oj screened room used for radiated susceptibility testing
Reproduced by permission of BSI
FREE-FIELD ANTENNAS
7.2 Tuned halfwave dipoles Lossless tuned halfwave dipoles approximate effective area A given by
A
0.13A 2 GA
[IJ
have
an
A
--
so gaIn
G
0.13 x 4
G
1.63 or 2.1 dB
4n
[2J
Lossless
DM-I05A TljT3
W
W
0.02 2.04 204 20449
0.033 3.33 333 33332
Reasonable field strengths of around 10 V jm at 1 m can be obtained for a few watts input RF power. Thus the dipole antenna is quite efficient when used at resonance. It would be impractical to use a dipole with some loss (such as the DM105A T IjT3 designed primarily for reception) to generate fields above about 100 V jm as the balun and other lossy components would begin to heat and sustain damage. The problem with using tuned dipoles for EMC radiated susceptibility testing is that relatively efficient performance with a low VSWR is restricted to the frequencies close to the dipole resonance. The antenna therefore has to be tuned at each frequency of interest when testing across the required band and this would be very time consuming. I t is for this reason that tuned halfwave dipoles are rarely used In practical immunity testing.
7.1
2
and also
Power input
7.2
Using eqn. 5.2 the approximate power density on an EUT at 1 m from the lossless tuned halfwave dipole is calculated as 7.3
7.3 Biconic dipoles This type of broadband antenna is described in Chapter 6. It is often used for both radiated emission and susceptibility testing in the frequency range 20 to 300 MHz because of its wide bandwidth which is achieved without any length adjustment. This makes it convenient to use during swept frequency testing which reduces test times and therefore keeps costs to a minimum.
112
A HANDBOOK FOR EMC TESTING- AND MEASUREMENT
Care must be taken to ensure that biconic antennas that are suitable for radiated emission testing are also capable of handling the RF power for radiated susceptibility testing. Some receiving antennas may have baluns which are too lossy or have built-in resistive networks to aid matching or the production of a flat frequency response. In such cases the power dissipated in these lossy elements could lead to overheating and damage. High-power biconic antennas such as the Emco 3108 (discussed in Chapter 6) are specially developed for radiated susceptibility testing. As an example of the performance of such an antenna, the power requirements for given field strengths at 1 m are shown in Table 7.2.
Table 7.2 Approximate power requirements against frequency for field strengths at 1 m spacing Frequency MHz 30 40 50 60 70 80 90 100 120 140 160 180 200 220 240 260 280 300
Typical gaIn
0.04 0.08 0.13 0.38 0.59 0.94 1.00 0.98 0.69 0.76 0.88 1.31 1.62 1.42 1.04 0.90 0.80 1.02
frequency is also given and reference is made to a commercial log-periodic dipole antenna produced by Amplifier Research, Model AT 1000. The average gain (calibrated at 1 m) for this typical antenna is 7.5 dB ± 1 dB and if a small loss is assumed of 0.5 dB for connectors etc., it is reasonable to use an average gain figure of 7 dB or a factor of five. In a similar manner to deriving eqn. 7.3, for this log-periodic antenna the relationship between input power and wavefield power density at 1 m is
5Pin
7.6
0.398Pin
4n
where Pd is the power density at 1 m and in a similar manner to eqn. 7.4 for the log-periodic antenna:
E2
377
E2
150 Pin
X
0.398 Pin
Field strength 1 Vim 5V/m 10V/m 20V/m
W 0.83 0.42 0.26 0.088 0.056 0.035 0.033 0.034 0.048 0.044 0.038 0.025 0.021 0.023 0.032 0.037 0.042 0.033
W 21 11 6.5 2.2 1.4 0.88 0.83 0.85 1.2 1.1 0.95 0.63 0.53 0.58 0.80 0.93 1.1 0.83
W 83 42 26 8.8 5.6 3.5 3.3 3.4 4.8 4.4 3.8 2.5 2.1 2.3 3.2 3.7 4.2 3.3
W 332 168 104 35 22 14 13.2 13.6 19.2 17.6 15.2 10 8.4 9.2 12.8 14.8 16.8 13.2
I t is now possible to calculate the approximate input power required to produce a given field strength at 1 m from this log-periodic antenna. Figure 7.3 shows the relationship in graphical form for field strengths from 10 to 100 V 1m. This low loss and robust commercial antenna for example, can produce field strengths at 1 m of up to 500 V 1m and handle input powers of over 1.5kW. 50---....-----r----.----~-----.
J : - 40
I-w
00:: zlWW
o::~
30
1- . . . . . (J)(J)
01-
20
...J...J
wO
u:~ 10 0
Emco 3108 high power biconical antenna
At 200 MHz this antenna is almost as efficient as a lossless tuned dipole, but is less so at the edges of the band over which it operates. At 30 MHz where it is electrically short and inefficient (being physically only 1.3 m long) it requires 40 times as much power for the same E-field as a lossless tuned dipole.
7.7
0
2.0
The simplified theory of operation and typical construction of this type of antenna is discussed in Chapter 6. Typical gain as a function of
10.0
120 J: -
100
~~
80
I-W
~~ ~ 00 60 a~
rf! ~
U-_
40 20
7.4 Log-periodic dipoles
4.0 8.0 6.0 POWER (WATTS)
30 40 POWER ( WATTS)
50
60
LOG PERIODIC ANTENNA AT 1000 (150 MHz - 1 GHz)
Figure 7.3
Power against field strength at l,m for typical log-periodic dipole antenna
Reproduced by permission of' Amplifier Research
USE OF ANTENNAS FOR RADIATED SUSCEPTIBILITY TESTING
7.5 Conical log-spiral antennas This type of antenna is also discussed in Chapter 6 and typical gain against frequency figures are given. The typical midband gain for the commercially available Emco 3103 (calibrated' for 1 m) is about a factor 2.3 or 3.6 dB. Deriving the relationship between E-field at 1 m and antenna input power, 7.8 Typical input powers for various field strengths are given for this conical log-spiral antenna in Table 7.3. Table 7.3 Power against field strength for typical logconical spiral antenna at 1 m spacing
Frequency MHz 100 200 300 400 500 600 700 800 900 1000
Gain
Field strength 1 Vim 5V/m 10V/m 20V/m
0.13 1. 72 2.07 2.43 2.35 2.38 2.08 2.17 1.89 1.46
W 0.256 0.019 0.016 0.014 0.014 0.014 0.016 0.015 0.018 0.023
W 6.409 0.484 0.403 0.343 0.355 0.350 0.401 0.384 0.441 0.571
W 25.6 1.9 1.6 1.4 1.4 1.4 1.6 1.5 1.8 2.3
W 102.4 7.6 6.4 5.6 5.6 5.6 6.4 6.0 7.2 9.2
113
produce a truly frequency-independent value for the constant which relates electric field strength to antenna input power as in eqn. 7.8 for example. However, as a guide, take a single gain value of 10 dB at a frequency of 580 MHz and use it to derive the value of E at 1 m as 7.9 This yields an input power requirement of 83 W for a field strength of 200 V 1m at 1 m and at a frequency of 580 MHz. It is evident from this simple calculation that the use of a horn antenna of this type is an efficient way of producing high field strengths at 1 m suitable for EMC radiated susceptibility testing. Graphs of field strengths which can be produced at 1 m by the AT4001 horn antenna for a number of input powers can be seen in Figure 7.4. For standard .gain waveguide horns such as those referred to in Figure 6.38 the average gain for a given waveguide size is about 20 dB ± 1 dB. Deriving the approximate relationship between field strength and input power as in Section 7.2, 7.10 Thus for a field strength of 10 V 1m at 1 m only 0.33 W is required. Care must be taken when. using simple far-field gain formulas such as eqn. 5.2 on which these calculations are based, when considering high gain antennas which may have extended near fields out to beyond the usual antenna to EDT distance of 1 m.
Emco 3103 conical log-spiral antenna
Although the manufacturer shows the antenna being usable down to 100 MHz the VSWR as shown in Figure 6.33 rises to almost 4: 1 at this frequency. If it were used to create a significant field strength of say 20 Vim at 1 m (requiring an input power of around 100 W) care would be needed to avoid reflected power causing damage to the RF power amplifier output stage. For this reason many modern commercial amplifiers for use in EMC testing have output stage protection in addition to current trips to prevent reflected power damage.
1000 800 W
ex: rw
600
400
~ (f)
r....J
200
0
> ::c r-
100 80
Z
60
ex: r(f)
40
0
W
0
....J
W
7.6 Horn antennas The theory and design of horn antennas is discussed in Chapter 6 and typical gain against frequency figures are stated for pyramidal horns. An example of a large low-frequency horn used for EMC radiated susceptibility testing is the AR AT400 1. 1'he gain varies from 10 dB at 400 MHz to 15 dB at 1 GHz (Figure 6.39). It is therefore not possible to
u:
20
10 ~---"'_~_..I--_.L--~_~_-' 300 400 500 600 700 800 900 1000
FREQUENCY MHz
Figure 7.4
Field strength at 1 m produced as function of frequency for various input power levels from pyramidal horn antenna
Reproduced by permission or Amplier Research
114
A HANDBOOK FOR EMC TESTING AND MEASUREMENT
7.7 Parabolic reflector antennas The characteristics of the parabolic reflector an tenna are also discussed in Chapter 6 and typical gain and beamwidth figures are given for commercial antennas which are widely used for EMC measurements at microwave frequencies. As an example, the Carnel Labs 91892-1 18-inch diameter reflector and two horn feeds (91890-1, 4.4-7.3 GHz and 91891-2, 7.3-12 GHz) are shown in Figure 7.5. Table 7.4 lists the antenna gain values, gives the field strength/power constants and indicates the approximate relationship between field strength at 1 m and required in pu t power to the antenna feed horns at the upper and lower freq uencies of the bands covered.
rl_
HORN FEED (7.3 - 12 GHz)
i -1 4cm
Figure 7.5
/~
HORN FEED ( 4.4 - 7.3 GHz )
COAXIAL INPUT CONNECTOR
Parabolic reflector antenna andfeed horns for use in radiated susceptibility testing
Table 7.4 Power against field strength table for an 18inch diameter parabolic reflector antenna Frequency 4.4GHz Gain dB Gain (x)
£2
7C Const.
7.3 GHz
7.8 Radiated im.m.unity field strength requirem.ents
7.8.1 Requirements for commercial products
REFLECTOR DISH
18" diam.
antenna at, say, 7.4 GHz, it would result in a very high field strength at 1 m from the dish of approximately 1.9 kV/m. The near field/far field boundary for the 18-inch (46 cm) parabolic reflector antenna at a frequency of 7.3 GHz calculated from eqn. 5.28 is approximately 10m. An EDT at 1 m from the antenna is clearly in the near field and care must be exercised in applying simple formulas to derive the field strength for a given input power to the antenna. The experienced EMC test engineer will where possible, calibrate his own antennas at the various ranges of interest and derive a more precise set of power tables which can be used with confidence.
12GHz
23 200
28 600
32 1600
fiOOO
18000
48000
The approximate relationships derived for the electric field strength (at 1 m) as a function of antenna input power and the tables of typical val ues produced from them, have been set ou t for a number of antennas commonly used in EMC radiated susceptibility testing: The tables are calculated for field strengths in the range 1 to 100 V /m as this covers the values commonly used in immunity testing to meet most civil and military standards. The field strength specified in the civil standard IEC80 1-3/BS6667-3 (Susceptibility to radiated electromagnetic energy of industrial process measurement and control equipment) over the frequency range 27-500 MHz is normally restricted to 10 V /m or less. This is typical of current standards applicable to commercially produced electronic equipment. These levels may increase in the future as the commercial and domestic EM environmen t becomes more severe, as has been the case in the military field over the last ten years.
7.8.2 Requirements for civil aircraft Field strength at 1 m Vim 1 10 100
Required input power 4.4GHz
7.3 GHz
12GHz
mW 0.16 16.6 1660
mW 0.05 5.5 555
mW 0.02 2.08 208
Octave-bandwidth travelling wave tube power amplifiers are available which can produce up to 200 W output at these frequencies. If this power level were applied to the parabolic reflector
Concurrent with the introduction of digital cockpit instrumentation and fly-by-wire technology, the proposed field strengths which civil aircraft must withstand increased dramatically at the end of the 1980s [3]. The new draft certification requiremen ts are being proposed for aircraft to demonstrate that they can operate safely in a high-intensity radiated field (HIRF). This has implications for both equipment (EMC standard DO 160 ch.20) and system level testing. An example of the draft RF environment (world wide) is given in Figure 7.6 and shows worst-case peak field strengths of over 10kV /m.
USE OF ANTENNAS FOR RADIATED SUSCEPTIBILITY TESTING E
> ....
I
"zex:w
100,000 10,000
-"'1.-"'"1.'LII r -
1,000
L_J
....00
100
0 -' w
10
u::
1 .01
1
.1
10
100
1,000 10,000 100,000
FREQUENCY MHz
--PEAK
------AVERAGE
Draft worldwide severe RF environJnent (116190 )
Figure 7.6
115
limits in the HF band have been addressed [4] and nonradiative methods of susceptibility testing have been devised using high power Bel current probe injection techniques [5-8] and direct injection of RF into the structures of items for test. The recently introduced BCI (bulk current injection) techniques will be dealt with In Chapter 9. In the following section the design and use of susceptibility antennas for use at HF and below are discussed and the problems of generating other than modest field strengths are explored.
7.9 E-field generators 7.8.3 Mili tary req uirements In the military EMC world, MIL STD 461 also shows this trend towards higher field strengths specified for radiated susceptibility tests. Figure 7.7 gives the levels required by MIL STD 461A/B for the RS03 test when applied to equipments from all three services. In general the values are below 100 V 1m bu t in special cases (some aircraft on carrier decks) the limit is raised to 200 V 1m. I t can be seen by extrapolating from Table 7.2 (biconic antenna) that to test even small equipments to this level would require power amplifiers with outputs of 1.3 kW at 100 MHz and 16.8 kW at 40 MHz. Such powerful test equipment would be very expensive and special ultra-Iow-Ioss antennas would be needed to survive this power input. In general the situation becomes worse at lower frequencies where antennas have a lower power gain, and becomes critical in the HF band where inefficient 'fringe field transmission line' antennas (discussed subsequently) are commonly used. It is at present impossible to carry out for a reasonable cost a 200 V 1m radiated susceptibility test through the HF band on a complete system which has dimensions of more than a few metres. The problems of susceptibility testing to the RS03 10 VIm (Navy & USAF) 300
/
40 Vim (Navy) -
(all services)
--L......
10
I
(all services)
pi Jo----ool
31
HIGH LOCAL liE" FIELD
I
\
30-
0.3
An E-field generator, as its name implies, is not exactly an antenna. It is a compact radiating device which produces a localised high electric field in its vicinity. It does not have a well-defined radiation pattern and does not produce an intentional beam of any _kind. These devices sometimes exploit the fringe radiation from a short open transmission line which is energised to a high voltage from a powerful amplifier via a broadband RF step-up transformer. A sketch of a transmission line antenna is given in Figure 7.8 and shows the transmission line
200 V1m for non-metallic aircraft and above-decklf~selage equipment
\
100~
7.9.1 Construction
_
(Army & USAF) 1 Vim (Army)
,.",
10 kHz 100 kHz
1 MHz
I
I
10 MHz 100 MHz
I 1 GHz
I
POSITIONING OF
10 GHz 100 GHz
II
IIE FIELD GENERATOR AND EUT
FREQUENCY
Figure 7.7
Radiated susceptibility limits for us services as given in MIL STD 461 A and B (Test RS03)
Figure 7.8
Typical construction oj high E-Jield generator
116
A HANDBOOK FOR EMC TESTING AND MEASUREMENT
rails, the impedance transformer/balun and the high power low inductance loads. In some designs the load is external to theE-field generator. The EDT is usually placed 1 m to the side of the device in the fringe field of the short transmission line. The live rails, or plates, can often be extended to the side of the main an tenna to provide a higher field region between them. A reasonable impedance match is made to the power amplifier by the use of the high power low inductance RF absorbing resistors across the rails and VSWRs of around 2.5 to 3: 1 can be achieved prior to the EDT being pu t in place or the antenna being operated in a small screened room. Generators constructed in this way are not efficient but have a wide bandwidth, from 10 kHz to 30 MHz, as the loading on the amplifier is essentially resistive. A wide-bandwidth transformer is used to step up the RF voltage from the 50 ohm input by up to a factor ten to increase the voltage across the transrnission line and thereby increase the E-field produced between the rails and around the device. A circuit diagram and electrical parameters for a typical genera tor are shown in Figure 7.9.
A. R. Model AT 3000
Figure 7.10
Reproduced by permission
~2kW
~1
3 : 1 RF TRANSFORMER (WIDE BAND)
kV
500n HIGH POWER LOAD
1
d = 1m
I
I
E FIELD ~
1 kV 1m
SHORT TRANSMISSION LINE "RAILS
Figure 7.9
Typical construction of high-jield strength local E-field generator
Compact highfield strength/high power local E-field generator
II
or Amplfier Research
they become self resonant. The field strength can vary considerably with frequency for a constant input power and it is usually impractical to expect to produce a given field strength at a particular point close to the generator simply by measuring the device input power and extrapolating from a calibration curve. An example of the variability of field strength with frequency for a constant 1 kW input power is given for a moqel EFG3 when used in a reflecting screened room in Figure 7.11. E-field generators are therefore normally used in conjunction with a small broad band (10 kHz300 MHz) E-field monopole detector which may
150
7.9.2 Practical devices Several commercial designs of wide band E-field generators are available. Typical of these are the IFI EFG3 [9J (10 kHz-220 MHz) and the AR AT3000 [10J (10kHz-30MHz). The generators for laboratory use are about 1 m X 1 m X 8 cm and are mounted on a nonconductive stand or tripod. They may have extending arms or rods which enhances the field between them. A diagram of the AR A T3000 is given in Figure 7.10. Although E-field generators can operate over a wide frequency band they do not always have a well behaved frequency response at VHF where
E ; 100
o...J W
u:
w
50
0.01
.1
1
10
100 200
FREQUENCY MHz
Figure 7.11
Example of variation in field strength for constant input power of unlevelled E-jield generator inside shielded chamber
Reprod uced by permission
or BAe Dynamics
lISE OF ANTENNAS FOR RADIATED SUSCEPT'IBILITY TESTING HIGH POWER E FIELD GENERATOR 10 - 100 V / m constant field strength
1====;:::::====::1
SMALL BATIERY POWERED _________E FIELD MONOPOLE SENSOR FIBRE OPTIC LINK
~/
L== ==1
RF POWER AMPLIFIER
100 W - 2 kW VARIABLE OUTPUT POWER
J?igure 7.12
E-field generator used to produce known levelled E-field againstJrequency.
be coupled via a fibre optic link to a broadband RF levelling preamplifier as in Figure 7.12. The input power from the main amplifier is then automatically controlled to produce a constant E-field at the position of the sensor. If the maximum output power required from the main amplifier is greater than that which can be supplied, the levelling will not take place and the E-field will fall below that demanded. The VSWR of the Efield generator varies with frequency and may exceed the limits into which the amplifier can drive the power. Under these circumstances the protection devices in the amplifier will trip out and shut off the power. This is more likely to occur when a large EDT is being tested, or when the E-field generator is working in a small screened room with insufficient spacing to the walls, floor or ceiling. Even with the limitations described, at first sight the levelling technique overcomes the problem of the highly variable frequency response of the Efield generator. However, when the EDT is placed in the field and the E- field sensor is placed on, or close to it, this will still result in a different levelled field. That is, the levelling now takes place taking into account the diffraction field from the EDT. The diffraction fields of the EDT have thus been compensated for by the levelling loop and the field strength to which the EDT is subjected is not the same as that which would be derived by using the levelling loop without the EDT present. To overcome this problem it is possible to use computer-based instrumentation to record the input power needed as a function of frequency to produce the levelled field without the EDT present and to replay these power settings when the EUT is in place. This will then subject the EDT to a known calibrated field, providing it is
117
not so large as to significantly alter the intrinsic performance of the E-field generator mutual impedance coupling to the active elements. The exact manner in which these tests are conducted is still to some extent a matter for the EMC test engineer. E-field generators produce fields with different impedances at different frequencies as the EUT is almost always in the reactive near field of the device. The wave impedance is usually greater than that of free space, particularly at low frequencies from 10 kHz to a few tens of MHz. Thus the susceptibility of the ED'T may not be that which it would be if subjected to a free-field plane wave with the same field strength. Indeed, using these generators it is often difficult to know exactly what wavefield the EDT has been subjected to, apart from knowing the one parameter which is measured (E-field). I t is possible to produce reasonably uniform fields over a large volume by constructing devices up to 3 m high and 2 m wide. An example of such a large E-field generator is the AR A T300 1 (10 kHz-20 MHz) which can be used to subject systems up to the size of small vehicles to high Efields. This commercial device has a frequency response which is shown in Figure 7.13. E- field ge~<;ra tors are very inefIicient devices and require a few kW ofRF input power to generate a few hundred V 1m fields over a relatively small test volume of around a cubic metre. This makes the test method extremely expensive as it requires a broadband kW RF power amplifier. More efficient bounded-wave devices for conducting EMC radiated susceptibility tests on small equipments are considered subsequently. FIELD STRENGTH BETWEEN ELEMENTS
sao r__--.---..------,--,---r---,--Ir----.---r---r-,-----, W
"S 400
~ .~300
~ :g 200 -- 3':
(J)o
1-0
c5 ~ 100 >"5
so 0'----.0L... -.01--.....J......--L.1-..J..2--.S1---1L--~-~--'7":::----=-=-----=-!.50 os 2 1 FREQUENCY MHz
GENERATOR SPECIFICATION (A.R. AT 3001 ) Power input, cw maximum 3000 watts Frequency range 10kHz· 20 MHz Impedance 50 ohms, VSWR 2.5:1 maximum, 1.5:1 average Electric field intensity ( at 2500 watt input) 200 vim minimum between elements RF irlput IconnlBctor ' 'yp" v female
Figure 7.13
Field strength Jor given power as function ojfrequency, Jor large commercially available E-field generator (EUT size 2x3x2m)
Reproduced by permission or Amplifier Research
118
A HANDBOOK FOR EMC TESTING AND MEASUREMENT
7.10 Long wire lines 7.10.1 Advantages Another form of E-field generator or antenna that is capable of producing high field strengths is the long wire line. This technique has the advantage over short transmission line E-field generators in that they operate in the dominant TEM (transverse electromagnetic mode) using the conductive enclosure in which they are erected as the outer sheath of a coaxial line [11 J. This allows some understanding of the field pattern and the wavefield impedance at points around the wire and in the vicinity of the EDT. The equipment being tested is placed under the wire in the screened room and it should be small compared with the size of the room (less than a quarter of the linear dimensions of the room). Long wire lines can therefore be used for generating high field strengths on relatively large objects (0.5-1 m 3 ) if used in a good sized screened enclosure. Test objects of this volume would normally be too big to be tested in a parallel plate line or other bounded wave device (discussed later in this chapter). The long wire line can only be used below about 30 MHz [11 J in the average sized screened room used for example for EMC susceptibility testing to MIL STD 461 (5 x 4 x 3 n1), as higher order modes will be set up at frequencies above this and the ordered nature of the TEM field will be destroyed.
7.1 0.2 Use in testing military equipment The MIL STD 462 example of an EMC radiated susceptibility shown in Figure 7.14.
notice 3 [12J is the best standard which calls for a test using a long wire line as The line is suspended tautly
between two insulators mounted on the opposite walls of the screened room, below the ceiling level, at between 1/4 and 1/3 of the height of the room. A coaxial feed is constructed from a 1 in. diameter copper pipe with a 16 AWG bare wire centre conductor. The outer of the coaxial line is grounded to the wall of the room at its base and the signal generator or power amplifier connection is also made to the coaxial feeder at this point. I t is preferable to have the signal generator or power amplifier outside the room (for convenience and safe operation) but the length of the connection to the feeder should be kept as short as possible and certainly less than one tenth of a wavelength at 30 MHz (1 m). If the amplifier or signal generator is inside the screened enclosure and the coaxial feeder centre conductor is extended to connect to the amplifier output, its length must be kept to less than 1 foot (30 cm) [11 J. There is a careful procedure which must be followed in MIL STD 462 N3 to derive the values for the terminating resistors (R 2 and R 3 in Figure 7.14) for any given long wire installation. These resistors correctly load the feeder and long wire line so that the generator can feed RF power to a well matched line to radiate in the room at frequencies up to 30 MHz. Once the noninductive loading resistors have been determined the same val ues maybe used each time the line is erected to conduct a test. The final configuration is shown in Figure 7.15 and the equation relating the Efield produced at the test sample to the driving voltage at the base of the feeder (which is the transmitting antenna factor, T AF) is
7.11 where E L is the driving voltage, E the E-field on the centre line of the EDT and kd the attenuation constant (TAF), 7.12
INSULATORS
~
R3
J
COAXIAL FEEDER Field strength at EUTis EV/m
DRIVING VOLTAGE EL
Figure 7.14
Typical screened room layout for using long wire antenna (MIL STD 461)
Figure 7.15
Geometry for long wire line relating field strength at E UT to driving voltage
USE OF ANTENNAS FOR RADIATED SUSCEPTIBILITY TESTING
where d) d1 and d2 are the distances shown in Figure 7.15, and Z is the characteristic impedance of the long wire line (R 2 ) . The a tten uation constant (TAF) is independent of frequency resulting in the straightforward performance of the radiated susceptibility test when using a long wire line. Although the T AF will be a good guide to the relationship between the E-field generated at the EUT and the driving voltage or input power (unlike the case of E-field genera tors) it is advisable to have a small fibre optically coupled E- field sensor [ 13J at the E UT location to measure the actual E-field generated. The long wire line maybe considered as a freefield radiating antenna, wi thin the confines of the screened room. I t is also equally tenable to consider it as a simple TEM bounded-wave device with the screened room itself forming the ou ter conductor of a coaxial line. BOUNDED-WAVE DEVICES Bounded-wave devices is the name given to a class of EM-field generating devices where the radiation is largely confined by the radiating structure. This tends to make these devices efficient but only in generating fields over a small volume.
7.11 Parallel-plate line
contribute to the upper frequency of simple lines for use in an EMC laboratory, being limited to about 30 MHz. The larger the EUT the larger must be the line to accommodate it, and thus the lower must be the upper frequency limit of the line. Very large parallel-plate lines have been built which approximately simulate predominantly low-frequency (less than 10 MHz) nuclear electromagnetic pulse. Complete vehicles can be irradiated in such lines at high field strengths of around 50 kV 1m.
7.11.2 Line impedance The characteristic impedance of a parallel plate is the key design parameter. I t determines the voltage across the plates for a given input power and therefore determines (crudely) the E-field as this is the plate voltage divided by plate separation. The characteristic impedance itself is determined by the physical configuration of the line: 7.13 where Z is characteristic impedance of the parallel-plate line, L the inductance per unit length, and C the capacitance per unit length. It can be shown [15, 16J that for an air-filled line the approximate impedance is
7.11.1 Properties The parallel-plate line is the simplest of the bounded-wave devices and a line of about 0.5 m separation is capable of producing high field strengths of up to a 100 V 1m for moderate input RF powers of less than 100 W. There are two main limitations in its use for EMC susceptibility testing: (i)
(ii)
The height of the EUT is restricted to about one third of the line spacing. 'This means that for practical laboratory lines of less than 1 m separation [14J the EUT must have dimensions of less than 30 cm. For the parallel-plate line specified in MIL STD 462 (notices 1 and 3) with a plate separation of 18 inches (46cm) the EUT can have no dimension larger than 6 inches (15 cm) if it is to be tested in all three axes. The useful frequency range of the susceptibility test is limited by the cut-off frequency for the line where higher order modes begin to propagate in addition to the simple TEM. The overall VSWR of the line is determined by the InJection and load rna tching sections at either end of the parallel section and by the variation of line ilTIpedance with frequency. All these factors
119
Z == Zo
X
hlw
7.14
if h « w (i.e. h < O.lw), where
h == plate separation w == plate width Z characteristic line impedance Zo == impedance of free space (377 ohms).
7.11.3 Construction The construction of a simple parallel-plate which is suitable for testing to MIL STD RS03 is shown in Figure 7.16. I t can be seen the line is a sizable laboratory device which
line 462 that can
POWER INPUT END
Figure 7.16
Parallel stripline jor radiated susceptibility test RS03 US MIL STn 46112
120
A HANDBOOK FOR EMC TESTING AND MEASUREMENT
SOURCE
PARALLEL PLATE LINE
LOAD
50n SIGNAL SOURCE
520
800
(b) Parallel plate line
(0) Insertion loss
=4.2dB
7.17
46.4Q
(c) Insertion loss
=30dB
son
test a small item of equipment with dimensions less than 6 inches (15 cm) . The dimensions of the line lead to a characteristic of 83 ohms and to drive it froln a conventional 50 ohm signal source an impedance network is required. This -is -shown in 7.17 a and has an insertion loss of 4.2 dB. . line termination is shown in Figure 7.1 7c and IS a 83 ohm load with a voltage divider attenuator with a 50 ohm output for connection to an EMI meter which enables the RF voltage across the to be measured. A suitable attenuator in the line to the EMI Ineter has a loss 30dB. A calibration factor for the stripline is shown 7.18. A field strength of 100 V jm can be with 25 W dissipated in the load which 65 W from the 50 ohm signal source. A 150 ohm impedance parallel-plate line with a 0.8 In separation for use in commercial to the DIN 45 305 [14] standard is shown in Figure 7.19 with the and load circuits. This line tapered sections from the to the source and load resistive networks to
122
n
Q: :Q61~: Z=150Q
Matching networks at input and load ends of RS03 parallel stripline
SOURCE
SOURCE PAD
Figure 7.19
LINE
Example ofparallel-plate line usedfor testing commercial equipment (DIN 45305)
reduce the effects of sharp impedance discontinuities which adversely affect the VSWR. ~ more sophistica ted line [1 7] is required for testIng commercial equipment to IEe801-3j BS6667-3 in the frequency range 27-500 MHz and is shown in Figure 7.20. The test volume of the line is defined by a cube of 0.8 m side. The maximum recommended EDT size is a cube with a 25 cm si?e. 1~he lin~ has two tapers each 0.8 m long but stIll uses reSIstor pads for the source and load matching networks. The condition associated with eqn. 7.. 14 clearly does not hold (as the plate separatlon and wid th are identical) and the characteristic impedance cannot be calculated using this TERMINAL STRIP
WOODEN PLANK
SUPPLY t INPUT & OUTPUT CABLES ( 3 + 2 + 2 wires)
~
All cables are twisted
METAL BOX WITH FILTERS
TERMINAL STRIP (see fig. 7.21 )
I 24 r---,--,---r--,---,----,--r---.---.---
~
22
~
20 18 16
~ ~
QE
TERMINAL STRIP (9 tags)
HF MEASURING PROBE
uI 00
14 iI ~ 12
EARTH
~~ 10 t-
&1
6
W
4
..J
BLOCKS OF WOOD (400 x 200 x 125 mm)
8
SUPPORT OF FOAM PLASTIC
2 0o~_~--=----:~~-~--'--...1.--L--l--J
1
2
3
4
5
6
7
8
9
10
V - VOLTAGE ACROSS LOAD ( VOLTS)
7.18
calibration chart for RS03 1WIL 461 parallel stripline
Figure 7.20
FRAMEWORK OF WOODEN BEAMS ( section 50 x 50 mm )
Parallel-plate line with tapered source and load sections IEC/ 801-3 for testing industrial electronics
Reproduced by permission of BSI
USE OF ANTENNAS FOR RADIATED SUSCEPTIBILITY TEST'ING
simple formula. The value of the matched load is 135 ohms which is close to the line impedance. rrhis particular line is specified for use up to 500 MHz, which is high considering the large plate separation. I t is fed in such a way that the live plate is the lower one which sees the ground plate above it and the actual ground below it is separated by 0.4 m. This is a crude coaxial configuration which can help to extend the upper frequency of the line. The design does not include distributed RF absorbing materials to provide RF damping which are sometimes used in highperformance lines. Care would be needed when carrying out tests in this line and the test engineer should monitor the actual E-field components in the line, preferably at points along its length, as the test frequency is changed. I t is interesting that this IEC80 1-3 stripline makes provision to extract the EDT cables via a set of filters placed above the top (grounded) plate, and then run them down the tapered input section to the inpu t RF connector. They are then twisted around the RF coaxial cable for some distance away from the line before being connected to the EDT support equipment. This configuration helps to minimise the interaction between the EDT cables and the fields in the line so that the test will predominantly reveal the susceptibility of the box of electronics rather than its connecting cable. The EDT box is tested in three axes to determine the worst-case susceptibility. Schemes have been suggested [ 18J to reduce the testing time by mounting the EDT at 45° to the plates in both longitudinal and transverse axes and then rotating it through 360° to expose all facets of the EDT to the maximum field. Such novel approaches have not yet been included in test standards.
7.11.4 Complex lines I t is possible to design more sophisticated parallelplate lines using long and carefully tapered transmission lines or wave launchers of complex geometry. A 6 m long tapered line with aIm separation and multiple strip conductors instead of plates (to reduce transverse currents) has been built [19J to operate at frequencies above 30 MHz. A large 1 m plate separation design using a wave launcher is specified in AFSC DH 1-4 [20J as shown in Figure 7.21. I t uses a distri bu ted load made from conductive plastic sheets with 377 ohm/square. Because it uses no lumped resistor matching pads it is extremely efficient and can produce a consistently high field strength as a function of freq uency. A field strength of 10 V /m can be produced for just 0.83 W from DC to 20MHz. See Figure 7.22.
121
WAVE LAUNCHER DETAIL BNC orN TYPE CONNECTOR
~
6m~
1m
~
TERMINATION comprising 3 separate layers of 377 n /0 conductive plastic film, joined at ends and connected to plates
INPUT CONNECTOR WAVE LAUNCHER
;'-6 copper on Perspex 11
Figure 7.21
Sophisticated parallel stripline "Loith wave launcher US AFSG'V DH 1-4
Reproduced by permission of' US DoD
i
1000
~
100
0::
10
w ~
a.
I-
~ ~
for 10 V / m
1 ...
-'
.1 W = E2 = 100 = 0.833 W 120 .01 .001 L...--_---'-_ _. . I - _ - . . . L_ _- L -_ _L - - _ - - ' - _ - - - - - I 10 100 1k 10k 100 k i M 10M 100 M FREQUENCY Hz AFSC DH 1 - 4 LINE
F'igure 7.22
Zc
Power inputfor 10 Vim field in efficient parallel-plate line. No source or load resistive pads
Reproduced by permission of' US DoD
7.11.5 Field uniformity and VSWR All parallel-plate lines suffer a degradation in field uniformity and VSWR when an EDT is introduced into the line. A study made by Anke and Busch [15] has produced an estima te of the VSWR seen at the input of the parallel plate line specified in MIL STD 462 when EDT objects of various sizes are introduced into it. Let the wid th of the line be Wand the height of the line be H. Now let the height of the object be h and its wid th be equal to that of the line, and define R == W/H and r == W/(H h). Then hili == 1 - R/r and the VSWR at the input to the line is given in Figure 7.23. The effective value of R for the MIL STD line is about 1.5 and the standard req uires that 'in no case shall the test sample be closer than 10 cm to the top plate'. Thus the ratio of the maximum height of the object to the height of the line (h/H) is 0.78 and r == 6.8.
A HANDBOOK FOR EMC TESTING AND MEASUREMENT
122
5r-----~-----,...---::I.-----,------.
a:
~ 2.5
line performance! However, a strong VSWR effect would be noticed at about 50 MHz with this EDT in the line where the impedance reaches over 200 ohms compared with the ideal value of 83 ohms.
>
7. 11.6 Use in screened room
o
(R~~~5)
0.5
~---.
LIMITATION OF MIL STD 46f
Figure 7.23
VSWR at input to parallel-plate line as Junction oj relative height oj E UT to line. Let W == width ojplate line, H == height ojplate line, h == height oj EUT. Define R == WIH, r == WI (H-h) , hlH == l-Rlr
Tracing these values onto the R == 1.5 curve (for the MIL STD line) the worst-case VSWR for an EDT which is only 10 cm less than the 46 cm pIa te height will be close to 3: 1. Anke and Busch [15J measured the input impedance of the MIL STD line as a function of frequency up to 100 MHz and revealed an increase above the nominal 83 ohms which starts at 8 MHz and peaks with a value of 154 ohms at 20 MHz. See the heavy line trace marked 2 in Figure 7.24. This shows that the relatively simplistic design of the MIL STD line is rather inadequate at the high end of its specified operating frequency range of 30 MHz. When an EDT is placed in the line which is 50 cm long X 50 cm wide and 30 cm high, the measured input impedance of the line changes to that shown as the light trace 1 in Figure 7.24. Within the 30 MHz frequency range the insertion of the EDT actually improves the 200..-------,-------y---r--...".-----.,
W
z ::Ja
A parallel-plate line will radiate a significant amount of energy away from itself and must be used inside a shielded enclosure for all but the lowest power tests. Porter [21] has conducted experiments to determine the effect of a screened room on the performance of a parallel-plate line derived from that specified for testing con1mercial electronic equipment in IEC80 1-3, as discussed earlier. A particular a.pplica tion of this line has been incorporated in a report [22] and is used by the DTI to satisfy the requirements of ECE Reg.13 (annex 13) [23] and 85/647 EEC annex x [24] with regard to the immunity of antilock braking devices. The screened enclosure in which a parallel-plate line is operating behaves as a cavity resonator [25] with frequencies given by:
J
V(a/x)2
+ (b/y)2 + (c/::l
7.15
120
150
100
c:
~.g 100
~~
.....I
2VJi8
where a, b aQd c are integers (one of which may be zero) and x, y and z are the dimensions of the cavity, with f1 the permeability of free space and 8 the permittivity of free space. The screened room in which the experiments were conducted was 6.7 x 2.6 x 2.2 m and had a fundamental resonance at 62 MIlz. A 10 V 1m field calibration of the line was therefore subject to progressive changes above this value as the modes set up in the screened room interact with the line, see Figure 7.25. By placing RF absorbing material in and around the line, together with a stack of
wN ~ 25
.....I
== _1_
E
~~
w
ex: 83
n
..... w
line
~
80 60
(j)
50
..... ...J 0
40
>
OL-0.1
--'1
20 .....Io-_ _
10
~
30
_
____I
100
FREQUENCY MHz
Figure 7.24
Variation oj line input impedance with Jrequency, with EUT present and absent. - - - - with EUT - - - without EUT
0 1
10
100
500
FREQUENCY MHz
Figure 7.25
Parallel-plate line calibration: field strength Jor constant input voltage, without absorbent material Jor damping
USE OF ANTENNAS FOR RADIATED SUSCEPTIBILITY TESTING
123
are called TEM cells. They are however usually quite small and can only be used for susceptibility testing at component or board level.
SCREENED ROOM"
___ RF ABSORBING BLOCKS
1-2.23 m
7.12 TEM cells
I
PLATE LINE
i
I I
I
1.9 m-:----+-J~-2.6 m
7.12.1 Basic construction
1 1 - - - 1 a i..:af-.· i
I I I
~I~ DOOR
1.3m
··~i,~
I \ ABSORBER STACK
1-----------6.7 m - - - - - - - - -
Figure 7.26
Disposition oj RF absorbing blocks around parallel-plate line to control standing waves in screened room
absorber at a specific location in this particular room, shown in Figure 7.26, it was possible to control the interaction between the room and the line to produce the field strength calibration for a constant 50W input power shown in Figure 7.27. While there are still variations in the plot, they are much reduced over the original disturbances in Figure 7.25 and show the line being useful up to almost 500 MHz. These examples of the use of parallel-plate transmission lines indicate the type of effects which the EMC test engineer must be aware of when using them to perform radiated susceptibility testing. In general, these efficient and economical devices are best suited to testing small objects at high field strengths, with simple lines being limited to frequencies below about 30 MHz. There are more sophisticated transmission-line devices which are totally enclosed, operate up to higher frequencies and can be used outside a screened room. These 120 LU
z
~
80
J:
o '-Q)
.....
~Q5
60
o
40
E
a: .............. (f)~ 0 ..J
LU
iI
>
20 0
7. 12.2 erawford cell performance Because the cell is based on a coaxial line with an expanding cen tre conductor surrounded by a carefully designed tapered rectangular conducting box it has an almost constant impedance for frequencies up to around 1 GHz. Figure 7.28 shows the construction of a Crawford Cell and indicates the small volume into which the EUT can be placed. Keiser [29J gives the characteristic impedance of a square section Crawford cell as
Z ==
100
:::i
A TEM cell is constructed by gradually expanding the size of a coaxial transmission line to dimensions which are large enough for a small EDT to be placed between the inner and outer conductors without significantly altering the properties of the line. The Crawford cell [26-28J was designed by M.L. Crawford at the National Bureau of Standards in the USA as a means of establishing standard uniform electromagnetic fields in a shielded environment [29]. Compared with a parallel-plate line the Crawford or TEM cell has two advantages: operation at higher field strengths for the same input power, and operation up to higher frequencies. TEM cells have the disadvantage that they can only accommodate small objects for test without being scaled up to sizes where the SWR becomes a problem. 'Typical sizes for test objects are 15 x 5 x 10 cm and are much smaller than those which can be tested in open parallel-plate lines.
1
10
100
500
FREQUENCY MHz
Figure 7.27
Controlled field strength obtained in parallel-plate line Jor 50 W input power with absorber material in place around line and in screened room
ft;[
94.15
we]
b(1 - t/ b)
ohms
7.16
+ 0.088 £1'
where Z == characteristic impedance of line, C == capacitance/unit length (pF /cm), £1' == relative permittivity, and dimensions w) band t relate to those of the cell componen ts shown in Figure 7.29. Keiser [29J also gives figures for typical cut-off frequencies above which more complex modes will propagate in the cell. These are given in Table 7.5 as an indication of the upper limit of simple TEM operation for a typical line (with C == 0.087 pF /m.). Other investigations into TEM-cell cut-off frequencies have also been reported [30, 31].
124
A HANDBOOK FOR EMC TESTING AND MEASUREMENT TEST VOLUME
At frequencies well below the cell cut-off frequency Spiegel et al. [32J have calculated (using quasistatic approximations) the electric field strength as a function of position above and below the centre conductor or septum. These calculations were made for the 30 cm-square TEM cell in use at the National Bureau of Standards, and then compared with measurements which had previously been made. The plots in Figure 7.30 show how the field varies across the working space above and below the septum.
7.12.3 Wave impedance in TEM cell Top view
- - - - - - - - - - . , . .-----OUTER BOX INNER PLATE
\
OUTPUT CONNECTOR Side view
~6~~~~~L - :j~TVOLUME SEPTUM PLATE
Figure 7.28
INSULATING LOW LOSS SPACERS
Ijpical Crawford cells
Reproduced by permission or Amplifier Research
b
i
+I~---I TI
OUTER BOX SEPTUM PLATE
I
-w~·1
1--
1--1-- -
}'igure 7.29
b
7.12.4 Field distortions in TEM cell
------.tal
Cross section dimensions of square section Crawford cell
Table 7.5 Dimensions and cut-offfrequencies of a Crawford cell
b
w
cm 150 50 30
cm 124 41 25
Cut-off frequency cm 0.157 0.157 0.157
Because the structure of a TEM cell is so simple and the wavefield is well behaved below the cu t-off freq uency, few engineers have investigated both the electric and magnetic fields present in the working space [32J. This has led to the assumption that the E-field can be determined from a consideration of the cell as a parallel-plate capacitor and the H-field is equal to the E-field value divided by 377 ohms. Thus the H-field determination relies on the relationship between E and H for free-space propagation. For most applications this rough treatment is sufficient, however at low freq uencies this approach is progressively unsatisfactory [32]. The issue of wave impedance in either a TEM cell or a parallel-plate line is an important one for the EMC engineer, as the value of the H-field components can have a significant effect on the induced currents in the EDT, and thus affect its susceptibility. If the same object were to be tested in apIa te line and in a free field at the same frequency and E-field strength, there is no guarantee that the immunity of the object to the two wavefields would be identical, as the H-field in the line may not be related to the E-field value by the impedance of free space.
MHz 100 300 500
When an EDT is placed into the TEM cell it distorts the field in the line and loads it at some point along its length. Because the field strength changes from the unloaded condition it is then difficult to know what value to ascribe to a susceptibility which might be observed for the object under test. Foo et al. [33J have performed 2D finite element EM computations for TEM cells with various dielectric and conductive objects inside them. As a guide to the percentage change in E-field which will occur in a TEM cell the calculations are performed around a semicircular cylindrical EDT, and result in the data shown in Figure 7.31.
USE OF ANTENNAS FOR RADIATED SUSCEPTIBILITY TESTING o _-r---.--...,..-.,...-r---r--r--r-......--r--I"'"""""T--'--""-"'"
:c
0.1 0.2
I-
~ ~
CROSS SECTION OF A SQUARE CRAWFORD CELL
0.3
30em-....- - - -
0.4 ~ 0.5 Cl 0.6 ~ 0.7 ~ 0.8
u:I u:
125
Figure 7.30 Variation offield strength with position inside square cross section Crawford cell
26.25 cm
§ a.
22.5 cm
~.8 0.9
«i= rrJ
10
0
.
1.1 1.2 1.3 1.4
a::
18.25 cm SEPTUM PLATE
---- --------
15em
--C::=::::=:::J
1.4 ~
1.3 1.2
<:)
rIi
11.25 cm
1.1
~ E 1.0 CfJ.2 0.9
g gW
7.5cm
0.8
C/)
iI ~ 0.7 UJ 03 0.6
2::.0
0.5
rrJ
0.4 0.3 0.2
~
a::
3.75cm Oem 30 em
0.1 I
o
I
wide---~~I
I
6 8 10 12 14 16 18 20 22 24 26 28 30
2 4
DISTANCE FROM SIDE WALL
The plots show the percentage field distortion at the centre of the cell (above the septum) for cylinders of various radii and dielectric constants. The upper heavy curve is for a
I
1-----0
conducting cylinder and this result is most appropriate for the type of metal box object which is usually subjected to EMC susceptibility testing. I t is clear that significant distortions of about 50% of the assumed field value are encountered by a conductive object which is 1/4 the height of the working part of the cell.
r-
;::)
w
~ ~
%
7.12.5 Other uses of TEM cells
124
o
0-
r-
z
100
(5
SEPTUM PLATE
0-
~
78
o
....J
W
u::
w
58 conductor
~
z o
41
i= 0:
o ~
CS #.
33 20
25
15 10
8
0.15 b
.2 b
.25 b
.3 b
.35 b
.4 b
NORMALISED RADIUS OF CYLINDER r
Figure 7.31
Effect oj EUT in TEM cell. Inset: upper half oj TEM cell
.45 b
In addition to the conventional use of TEM cells for high field strength wide bandwidth EMC susceptibility testing of small items, these devices are used for other purposes. The measurement of the shielding properties of materials including gaskets has been undertaken using TEM cells [34, 35]. Special TEM cells have also been used to test the susceptibility. of circuits on printed circuit boards to fields with deliberately high wave impedances to sim ula te the conditions pertaining to near field coupling at circuit board level [36]. TEM cells are especially useful in the field of investigating the biological effects of intense EM radiation [37, 38]. These cells can generate accurately known fields at very high levels using simple apparatus and for a reasonable cost. A graph of field strength against input power for a cornmercial TEM cell [52J is given in Figure 7.32.
126
A HANDBOOK FOR EMC TESTING AND MEASUREMENT
7.13 GTEM cells w
0-
U5 E
z->
J:-
..... w C)~ z::> w-J 0: 0
~>
ot...Jw WW u::t-
1000 900 800 700 600 500 400 300 200 100 .1
7.13.1 Description
50 100
10
5
500
INPUT POWER ( WATTS)
Figure 7.32
Field strength against input RF power for commercially available Crawford cell. Specification AR TC0500: frequency range) de to 500 MHz)· power input) maximum) 500 W)· impedance of cell) 50 ohms)· VSWR) maximum) 1.2:1 to 250 MHz) 1.4:1 to 500 MHz)· cell total width) 1 m)· cell total height) 30 em)· cell total depth) 50 em)· septum depth) 37 em)· maximum dimensions of device under test (w x h x d)) 15 x 5 x 10 em
Reproduced by permission of Amplifier Research
7.12.6 Asymmetric TEM cells Asymmetric TEM cells have been designed [40J to increase the working space where the EUT can be accommodated for a given overall cell size. In a normal sg uare cell the E UT height is limited to about one sixth of the cell overall height. The septum is offset in the asymmetric design as shown in Figure 7.33 leaving a greater space beneath it. The field distortion that is produced as a result can be mitigated by making the cell wider, but these changes usually result in compromising the cell cutoff freg uency. The construction and operational parameters associated with TEM cells have been discussed, and examples offactors which EMC engineers should be aware of, such as bandwidth and field distortion, have been presented. These low cost, simple to operate devices are widely used for EMC susceptibility testing and for other related purposes. They can produce high field strengths over wide bandwidths but are usually limited to small test objects.
The GTEM or gigahertz transverse electromagnetic mode cell is a high-frequency variant of the TEM cells discussed. It is a single-taper development of an asymmetrical TEM cell with an offset septum plate for increased working volume. I t also has both a current load connected to the septum and distributed wave termination in the form of a RAM wall at the end of the enclosing taper. I t may be viewed as a careful combination of aspects of a TEM cell and an anechoic chamber. l'hese features endow the design with the useful properties of a large working volume and high freg uency performance in addition to the normal features of Crawford-type TEM cells.
7.13.2 Typical construction The concept has been the subject of a patent [41 J and has been variously reported by the originators [42-44 J. A vertical cross section of a GTEM cell is shown in Figure 7.34. It is constructed as a tapered section of a rectangular 50 ohm transmission line. At the apex of the cell is a precision made transition from coaxial cable to the transmission line. The travel time for any signal path from the source to the load at the opposite end of the cell is the same, as the RAM-loaded end wall is curved. Also in a GTEM cell, there are no shape discontinuities, such as exist in Crawford cells, which can act as sources of diffracted non-TEM radiation. As th-e radiation travels along the line, the strength of the E-field varies as l/distance in a similar manner to that for a spherical plane wave. The offset septum plate is terminated by a distributed noninductive resistive load across the large end of the cell and provides a return path for the current via the conducting outer case. CLOSED METAL RESISTIVE MATCH FOR RETURN CURRENT ON SEPTUM
END WALL
OFFSET SEPTUM OFFSET SEPTUM PLATE
---+I
I
300 750
4
IN/PUT CONNECTOR \
IT
T
I
~
r 60°-+-9°°+6001
~
t--
-----j
1350
Example of asymmetric TEM cell
\
\
\
720
all dimensions in mm
Figure 7.33
r
\
~ \
\
\ A TAPERED 50 n -----TRANSMISSION LINE
~ 15% SECTOR OF SPHERICAL WAVE FRONT ( ie. almost a plane wave I
Figure 7.34
RADIO ABSORBENT MATERIAL
- acts as a high frequency load for the wave
Schematic diagram of GTEM cell
USE OF ANI'ENNAS FOR RADIATED SUSCEPTIBILITY TESTING The performance of a GTEM cell with aIm maximum floor to septum height produced within Asea Brown Boveri by Hansen et al. [43] is shown in Figure 7.35. The flatness of the E- and H-field
I l-
e:> z
1 T
W 0::--1 1---1 (J)W
E FIELD
-
~W L1.1-
we:>
>z
--
-
H FIELD
~-
-- - - - ..-..-. ....
~
--I
W
0::
10
100
1000
FREQUENCY MHz SEPTUM HEIGHT - 1 m
Figure 7.35
Flat response oj E- and H-Jields in GTEM cell
1.20....---------,.------------. SEPTUM
"
1.00
~ 0.80 e:> jjj
I
frequency response of ±3 dB is outstandingly good over the frequency range up to 1 GHz. The ± 1 dB E-field contour for aIm GTEM cell is shown in Figure 7.36 with an indication of the size of an EDT which has dimensions of 1/3 of the septum height and cell width. It is clear that an EDT of this size is immersed in an almost uniform field.
7.13.3 Power requirements
i
aO 10dB --1~
---
1-000..
127
The field strength which can be generated in a GTEM cell as a function of input power and cell size has been determined [43] and is shown in Figure 7.37. For aIm final septum height it required just less than 1 kW to generate 200 V/n1 over the test object volume with a uniformity of ±l dB. Very large GTEM cells with final septum heights of 5 m have been constructed for automobile EMC testing [43] but it can be seen by extrapolating from Figure 7.37 that RF amplifiers capable of delivering more than 10kW are needed to produce high field strengths (200 V 1m) in such a large volume. A three-dimensional view of a GTEM cell suitable for immunity testing of medium sized electronic units with sides up to 50 cm long, such as PCs (personal computers) and peripherals, is shown in Figure 7.38.
0.60
--J --J
+-1 dB envelope
W
() 0.40
Offset septum leaves a large volume for testing RF TRANSPARENT END WALL _____
0.20 0.00
SEPTUM LOAD
1--
RF SEALED OUTER STRUCTURE eliminates leakage
_
o
-1
CELL WIDTH
Figure 7.36
Field uniformity contours in G TEM cell. Inset: An EUT with dimensions oj cell working size
\
i
WAVE LAUNCHING SECTIONS for both CW & pulsed testing
INPUT CONNECTOR
1,000'
Figure 7.38
o::E
w ....
6>
i!: ~ o=> ffi5 0::>
1-0 ClJz S2
9
PYRAMIDS MADE FROM RADIO ABSORBENT MATERIAL • act as a good wideband load to wave field
General viezRJ of G TEM cell with E UT in place.[or testing (G T EM is a trademark of Emco)
100,
7. 13.4 G 1-'£ M cells for emission tes ting 10
~g§
u.~
1 ......_ ......._ ........_ - - a . _ - - - I t . . . -
o
0.5
1
1.5
2
_
2.5
3
3.5
INNER CONDUCTOR HEIGHT m
Figure 7.37
Field strength as Junction oj working volume height in G TEM cell Jor various input RF power levels
It has been suggested [43, 44J that GTEM cells can also be successfully used for radiated emission testing of suitably sized objects. The EDT is positioned as for a susceptibility test but the input connector to the cell now becomes the ou tpu t which is conl)ected to an EMI meter. The sensitivity of the cell used in this way is reported to be high [43J and successful measurements have been made and compared with VD~ and FCC measurements made on OATS (open area test sites) .
128
A HANDBOOK FOR EMC TESTING AND MEASUREMENT
The great advantage of making emiSSion measurements in this way is the absence of ambient signals which so often confuse open-site measurements and slow down the testing. Using a GTEM cell also obviates the need for the large number of measurement antennas described in Chapter 6, and as no antenna changes are necessary the testing is accomplished more quickly. Further work will need to be done to gain general acceptance for this GTEM method of emission measurement, but it would appear to be a very promising cost-effective technique.
7.14 References
2 3
4
5 6
7.13.5 Pulse testing Because the GTEM cell has such a flat frequency response over a wide band it is ideally suited to making distortion-free fast pulse measurements. These cells can be used to propagate simulated NEMP waveforms over test objects at full threat field levels as required by MIL STD 461 C test RS05 (50 kV 1m). The peak field strength obtainable as a function of final septum height for a lOOkV input pulse amplitude is given in Figure 7.39 [44J.
7
8
9 10
-
1000
E
11
>
~
w ::> -'
~ ~ w
12
100
D-
C
-' w
u: o
a: I-
~
W
13 10
0.5
--11-_ _- - - - ' ' _ _
2
14
_
3
4
5
TEST CHAMBER HEIGHT m
Figure 7.39
Pulsed E-Jield strength against test volume height Jor pulse generator amplitude oj lOOkV
15
16
The GTEM cell is the result of progressive develop men ts in design from simple parallel-plate lines, through TEM Crawford cells and asymmetric TEM cells combined with some properties of RF anechoic chambers to yield a multipurpose reliable piece of test equipment for modern EMC testing. Its advocates point out that it is less costly and more useful than anechoic chambers for small test items. I t appears to be a very versatile equipment which may become widely used in future years.
17 18
19
20
'Reference data for radio engineers'. (Howard W. Sams, 1977) p. 27-8 'Reference data for radio engineers'. (Howard W. Sams) p. 28-20 BULL, D.A. and CARTER, N.J.: 'Testing civil aircraft and equipment to the new external RF environmental conditions'. Proceedings of IEEE symposium on EMC, 1990, pp. 194-203 JOFFE, E.B.: 'Are RS03 limits in the HF band realistic?' Proceedings of IEEE symposium on EMC, 1990, pp. 196-201 DEF STAN 59-41, DCS02, Ministry of Defence, UK CARTER, N.J., REDMAN, M. and WILLIS, P.E.: 'Validation of new aircraft clearance procedures'. Proceedings of IEEE symposium on EMC, 1988, pp.117-124 KERSHAW, D.P. and WEBSTER, M.J.: 'Evaluation of the bulk current injection technique'. Presented at IEEE symposium on EMC, 1990, 14 unnumbered pages not bound into proceedings BURBIDGE, R.F., EDWARDS, D.J., RAILTON, C.J. and WILLIAMS, D.J.: 'Aspects of the bulk current immunity test'. Proceedings of IEEE symposium on EMC, 1990, pp. 162-168 Model EFG3 E-field generator operating instructions. Instruments for Industry, Inc, Nj, USA Model AT 3000 E-field generator, Amplifier Research, 160 School House Rd, Souderton, PA 18964-9990, USA WHITE, D.R.J.: 'Handbook series on electromagnetic interference and compatibility, volume 2: EMI test methods and procedures'. Don White Consultants, Germantown, Maryland, USA MIL ST'D 462 notice 3 (EL). Army Department, Washington, DC 20360, USA 9 Feb 1971, pp.118-124 E-field sensors EFS 1/2/3. Instruments for Industry Inc, Nj, USA DIN 45 305 part 302: Methods of measurement on radio receivers for various classes of emission; Methods of checking the immunity fronl interference fields of radio receivers. Beuth-Verlag, Berlin 30, Germany ANKE, D. and BUSCH, D.: 'Parallel-plate antennas: field distortion caused by test objects'. Institution of Electronic & Radio engineers, 1985, 55, (6) pp. 210-216 'Reference data for radio engineers'. (Howard W. Sams) p. 24-22 '0.8 m parallel-plate line'. ERA report 80-135, ERA, Leatherhead, Surrey, UK BRONAUGH, E.L.: 'Simplifying EMI immunity (susceptibility) testing in TEM cells'. Proceedings of IEEE symposium on EMC, 1990, pp. 488--491 MARVIN, A.C. and THURLOW, M.: 'A 1 m separation strip line using tapered impedance transformer sections'. British Aerospace Dynamics, Filton, Bristol, UK 'Design handbook, electromagnetic cOlTIpatibility'. Department of Defense, Washington DC, USA, AFSC DH 1-4
USE OF ANTENNAS FOR RADIATED SUSCEPTIBILITY TESTING 21 PORTER, R.S.: 'A high field strength low cost component susceptibility test facility'. Proceedings of IEEE symposium on EMC, 1990, pp. 227-231 22 MADDOCKS, A.J.: 'Draft specification for the measurement of immunity of road vehicle anti-lock braking system to electromagnetic radiation'. Project report 5043/4R6/4, 1983, ERA, Leatherhead, Surrey, UK 23 'Uniform provisions concerning the approval of vehicles with regard to braking'. ECE Reg. 13/05, UN Economic Commission for Europe, 1988 24 'On the approximation of the laws of member states relating to the braking devices of certain categories of motor vehicles and of the trailers'. EC Directive 71/320 amended by 85/647EEC, Commission of the European Communities, 1985 25 HARRINGTON, R.F.: 'Introduction to electromagnetic engineering'. (McGraw-Hill, New York, 1958) 26 CRAWFORD, M.L.: 'The generation of standard EM fields using TEM transmission cells'. IEEE Trans., 1974, EMC-16, p. 189 27 CRAWFORD, M.L.: 'Measurement of EM radiation from electronic equipment using TEM transmission cells, NBS international report 73-303, 1973 28 CRAWFORD, M.L. and WORKMAN, j.L.: 'Using a TEM cell for measurement of electronic equipment'. NBS technical note 1013, 1979 29 KEISER, B.: 'Principles of electromagnetic compatibility'. (Artech House, 1987, 3rd edn.) p. 346 30 HILL, D.A.: 'Bandwidth limitations of TEM cells due to resonances'. ]. Microwave Power, 1983, 18, pp. 182-195 31 WElL, C.M., JOINES, W.T. and KINN, j.B.: 'Frequency range of large scale TEM mode rectangular strip lines'. Microwave ]., 1981, 24, pp. 93-100 32 SPIEGEL, R.J., JOINES, W.T., BLACKMAN, C.F. and WOOD, A.W.: 'A method for calculating the electric and magnetic fields in TEM cells at ELF'. IEEE Trans., 1987, EMC-29 (4) 33 FOO, S.L., COSTACHE, G.l. and STUCHLY, S.S.: 'Analysis of electromagnetic fields in loaded TEM cells by finite element method'. Proceedings of IEEE symposium on EMC, 1988, pp. 6-8
129
34 CATRYSSE, j.: 'A new test cell for the characterisation of shielding materials In the far field'. Proceedings of IEEE symposium on EMC, 1990, pp. 62-67 35 BROWN, j.T.: 'Using TEM cells for shielding performance evaluation'. Proceedings of IEEE symposium on EMC, 1990, pp. 495-499 36 DAS, S.K., VENKATESAN, V. and SINHA, B.K.: 'A technique of electromagnetic interference measurement with high impedance electric and low impedance magnetic fields inside a TEM cell'. Proceedings of IEEE symposium on EMC, 1990, pp. 367-369 37 BLACKMAN, C.F. et at.: 'Induction of calciumion effiux from brain tissue by radio freq uency radiation: Effects of modulation frequency and field strength'. Radio Sci., 1979, 14, (6S), pp. 9398 38 HILL, D.A.: 'Human whole body radio frequency absorption studies using TEM transmission cell exposure system'. IEEE Trans. 1982, MTT-30, pp. 1847-1854 39 TC 0500 TEM cell. Amplifier Research, 160 School House Rd, Souderton, PA 18964-9990, USA 40 VERHAGEN, V.H.A.E.: 'Analysis of an asymmetric TEM cell for immunity testing'. Proceedings of IEEE symposium on EMC, 1990, pp. 157-161 41 HANSEN, D. and KOENIGSTEIN, D.: Patent CH 670 174 A5: Vorrichtung zur EMI-Prufung electronischer Gerate, 1989 42 KOENIG·STEIN, D. and HANSEN, D.: 'A new family of TEM cells with enlarged bandwidth and optimized working volume'. Proceedings of 7th international symposium on EMC, March 1987, pp. 127-132 43 HANSEN, D., WILSON, P., KOENIGSTEIN, D. and SCHAER, H.: 'A broadband alternative EMC test chamber based on a TEM cell and anechoic chamber hybrid'. Proceedings of IEEE symposium on EMC, 1989, vol. 1, pp. 133-137 44 GARBE, H. and HANSEN, D.: 'The GTEM cell concept; Applications of this new EMC test environment to radiated emission and susceptibility measurements'. Proceedings of IEEE symposium on EMC, 1990, pp. 152-156
Chapter 8
Receivers, analysers and • nneasurennentequlpnnent 8.1 Introduction
Section 8.1.1 is discussed in two groups: that used for emission testing and that used for susceptibility testing
This chapter discusses the types of electronic test equipment commonly used in EMC emission and immunity testing over the frequency range from a few hertz to tens of gigahertz.
INSTRUMENTATION FOR EMISSION TESTING
8.1.1 Outline of equipment
8.2 EMI receivers
EM C emission testing is carried ou t using a sensor or pickup device connected to a receiver, spectrum analyser or other item of measurement equipment which gives a voltage reading that can then be converted to the quantity being measured via the sensor calibration or transfer function. 1'he receivers, or EMI meters as they are sometimes called, are complex items of RF technology which in some cases have been especially designed for EM C test work. Susceptibility testing requires the use of a range of CW, modulated CW and pulsed signal sources. High output powers are often needed and are produced using broadband high-power RF amplifiers. Other equipment which is commonly used directly in RF susceptibility testing includes directional couplers, power circulators, highpower RF broadband loads, diode detectors, RF power meters and frequency meters. The practising EMC test engineer must be familiar with the performance capabilities and limitations of all this laboratory electronic instrumentation, and be able to use it correctly to carry out testing to a wide variety of EMC standards. The managers of companies engaged in test work should appreciate the sophisticated nature of EMC test equipment and be able to evaluate its technical merits, the capital outlay required, probable life, calibration and maintenance costs and reliability to make cost-effective investment decisions. The customers of EMC test facilities should be familiar with the type of instrumen ts used to make measurements on their test specimens and able to appreciate how the high cost of this equipment contributes to the facility tariff.
8.2.1 Design requirements EMI receivers are frequency tunable audio, RF and microwave variable bandwidth voltmeters which can measure and display the absolute amplitude of a complex unknown input signal. The receivers are usually superheterodyne equipments which have been designed carefully to measure the amplitude of CW, broadband noise and impulsive noise signals accurately using a wide range of intermediate frequency and post-detector bandwidths. The key design features of a typical midfreq uency range (10 kHz-30 MHz) EMI receiver are Wide tunable frequency range, up to three or four decades High sensi tivi tyflow noise figure, < 10 dB (0.01 flV in 100 Hz) Good input VSWR < 1.5: 1 Good gain flatness across band « ± 2 dB) Good absolute measurement accuracy (uncertainty < ± 2 dB) Built-in switchable calibration sources for CW and impulse signals to enable substitution measurements to be made Good out-of-band signal rejection > 100 dB (using inpu t bandpass and tracking filters) Careful mixer design giving overload signal warning and low harmonic distortion, intermodulation, LO leakage, image responses and spurious signal responses of > 70 dB down Wide-range input and IF attenuators with coupled action LO output with usable power> - 20 dBm Low parameter drift with time/temperature (e.g. frequency and amplitude) Good dynamic range of 0-60 dB in a single range, and 0-120 dB with attenuators Good dynamic range with impulsive signals 060 dB with preselection
8.1.2 Groups of equipment EMC test equipment, including that mentioned in 130
RECEIVERS, ANALYSERS AND MEASUREMENT EQUIPMENT
vVide selection of measurement IF bandwidths 1 kHz-10 MHz with known 3 and 6 dB bandwidths and shape factors Wide selection of post-detector bandwidths, 1 Hz-100kHz Selection of detector functions Peak Slideback peak Quasipeak Average BFO FM Manual and sweep frequency tuning Automatic tuning via a digital data bus Automatic frequency control (AFC), switchable Selection of output ports (amplitude) Linear IF Log IF (0-70 dB) Linear video (0-5 MHz) Log video (0-5 MHz, 0-70 dB) Panel meter Audio output Plotter ou tpu t Data bus output Selection of output ports (frequency) Panel meter Plotter x-drive Panoramic display x-drive (spectrum analyser display) Remote control All main functions and ou tpu ts should be accessible via a common standard data bus (e.g. IEEE 488) The equipment should be rugged, lightweight (man portable), battery powered (as an option) and should have a good EMI-shielded case (better than 100 dB) . EMI meters which cover the low audio-frequency range below 10 kHz and microwave receivers above 1 GHz may not be able to satisfy all the parameter requirements listed, as the circuit
INPUT PORT
ep
131 VIDEO OUTPUT
AUDIO OUTPUT
VOLTMETER
L-BUFFERED OUTPUTS FREQUENCY
Figure 8.1
Simplified EMI receiver block diagram
designs will be different for these equipments froD1 the midrange HF jVHF jUHF instruments. For example, it has been extremely difficult to provide good front end preselection filters in a small lightweight unit for the low frequency receivers. The more complex input filtering and multiple heterodyning designs used in microwave receivers can affect the sensitivity, input VSWR and gain flatness which may not be as good as the midfrequency range receivers. The design of EMI receivers with all the features listed above is a considerable task and manufacturers must inevitably make compromises between some of the design parameters. An insight into the design process and some appreciation of the engineering which justifies the high cost of these sophisticated receivers may be gained by reference to Coney and Erickson [1 J. A simple block diagram of a basic EMI receiver is shown in Figure 8.1. A more detailed diagram of a typical receiver is given in Figure 8.2 and details of an RF. front-end based on a commercial receiver [2J which makes use of a number of separate octavewide receiver modules is shown in Figure 8.3. These modules are switched into the signal path to cover the appropriate frequency being analysed. The restricted bandwidth of the individual modules pern1its a design with good out-of-band signal rejection and intermodulation suppression.
Figure 8.2 Block diagram of typical EMI receiver
PLOTTER OUTPUT x DRIVE (FREQUENCY)
LINEAR IF OUTPUT
LOG IF OUTPUT
OUTPUT LEVEL METER
Reproduced by permission Corp.
or
Camel Labs
132
A HANDBOOK FOR EMC TESTING AND MEASUREMENT
Figure 8.3 Example offront end of commercial EMf recezver IF OUTPUT
Reproduced by permission of Camel Labs Corp.
' - - - - - - - « AFC
8.2.2 Selectivity and sensitivity
power, then
High signal rejection to frequencies other than the one being measured is the key performance feature of an EMI receiver or meter. The spectrum presented to the input port is likely to be completely unknown, and may contain a time-varying mixture of high- and low-level narrow-band signals, together with bursts of random and impulsive noise. Without narrow bandpass or tracking filters at the input to the receiver, the first RF amplifier or mixer stage will be overloaded [1 J and a forest of spurious signals will be generated, leading to an inaccurate assessment of the spectrum. A good review of the circuitry and operation of EMI meters can be found in White (volume 2) [3J and is not repeated here. I t is necessary however to have some idea of the sensitivity which EMI meters can achieve. Receiver sensitivity is defined with reference to the equivalent noise power translated to the receiver input port, i.e. the output or indicated noise power divided by the receiver gain for a given set of parameters such as frequency, attenuation, IF and post-detector bandwid tho The noise power is defined as
N == KTB x F where N
==
Smin dBm == -114+FdB + 10log 1o B For a 50 ohm input impedance, SdB,uV
noise power in W
K == Boltzmann's constant,
1.38 x 10- 23 W IK/Hz T == temperature of receiver front end (typ. 21° C == 293K) B == receiver bandwidth in Hz F == receiver noise factor (a multiple), F dB is the receiver noise figure If the receiver sensItIvIty is defined as that signal power which is equal to the referred input noise
==
SdBm
+ 107
8.3
Examples of receiver sensitivity against bandwidth for various receiver noise figures are given in Figure 8.4. The narrowband sensItIvIty for a receiver is defined with regard to the random or thermal noise power referred to its input port as given in eqn. 8.1. Such noise is incoherent and does not have a predictable phase relationship between incremental frequency components of the noise waveform. The comparison of intrinsic receiver noise power and random noise or CW inpu t signals results in the definitions of S, the receiver sensitivity. If the input signal is broadband coherent
>-
1--
20
-87
~s !:: 0. 10
>-
-97 ~
(/)Z
rna
(/)0
8.1
8.2
a::ll')
UJ-
»
W::1,
i= -107~
0
-117
ffi !g
-20
-127
0 w
OeD
~
'0
~E
-10
-137 a::
-30 -40 1 kHz
>
-L.-.--..J-147 10 MHz 100 MHz
..I.-_ _--I-_ _---L_ _
10 kHz
100 kHz
1 MHz
RECEIVER BANDWIDTH
Figure 8.4
Receiver sensity as function of bandwidth for narrowband signals and various noise figures. F == receiver noise figure
Reproduced by permisssion of ICT Inc.
RECEIVERS, ANALYSERS AND MEASUREMENT'
133
r:
50,-~-----~---,.----r-----
8.2.3.2 Quasipeak detector (various
i=
40
LU
30
rrhe function of this detector is to an output reading which correlates well with the ~..L assessed annoyance when listening to discontinuous impulsive noise which may be heard from a broadcast radio receiver. There are a number of different specifications for quasipeak the two most widely used are the ANSI and CISPR (16) standards. 1'he rise and fall times are different and given in Table 2.5 in Chapter 2. Graphs of indicated output amplitude as a function of pulse repetition frequency for peak, CISPR, ANSI and RMS detectors are given in Figure 8.6 for a 6dB bandwidth of 9kHz (CISPR 16) [5J. 'The equivalent graphs for the CISPR 120 kHz bandwid th are given in Figure 8.7 Quasipeak detectors are almost used for EMC emission measurements of commercial electronic equipment being tested to standards such as US FCC, German VDE, British BS or the harmonised European EN series of standards.
:>
(jj Z C/)
oJL-<.'J I ' u " ' J L L V ...
O:N
~~ u>
20
F =35 dB F =30dB
LU-
~~ 10 0'0 Z
« OJ
o
o «
o0:
-10
OJ
F =25dB F=20dB
-20 L..-_ _- ' -_ _--'100 Hz 1 kHz 10 kHz
F = 15 dB F = 10 dB F=5dB F=3dB L..-_ _- ' -_ _--I F = 0 dB
100 kHz
1 MHz
10 MHz
RECEIVER BANDWIDTH
Figure 8.5
Receiver sensitivity as junction of bandwidth Jor broadband signals and various noise figures. F == Receiver noise figure
Reproduced by permission of leT Inc.
impulsive noise, then there is a defined phase relationship between adj acen t incremental freq uencies. In this case, the receiver will measure a different rate of increase of impulsive signal power to receiver noise power as the bandwidth is increased. This leads to an increasing sensitivi ty to the coherent signal as the receiver bandwidth is increased, provided that the signal bandwid th is always greater than the receiver bandwidth. White [3J gives the following definition of receiver sensitivity to impulsive noise:
0:
o I-
o
ILU ~tu 1- 0 ~
5
-----AM~:===-_.-1!J1IJIIIIt
0 ..... I
-10
a:~ ~« UO
LUI-
-7+F
lOlogloB
8.4
tu~
~
o
This assumes that the coherent signal power In a given bandwid th increases as the square of bandwid tho Examples of receiver sensitivity to coherent broad band signals as a function of bandwidth and noise figure are given in Figure 8.5.
8.2.3 Detectors In addition to the high sensItIvIty, high dynamic range and good unwanted signal rejection, EMI receivers have a number of special detectors which meet the requirements of various EMC standards and test specifications. The main types of measurement detectors are as follows.
-40
....J
W
0::
10
100
1000
10,000
IMPULSE REPETITION RATE (Hz)
Figure 8.6
Output Jrom quasipeak (OJ?) detectors as junction oj impulse repetition rate. -6 dB bandwidth == 9 kHz
Reproduced by permission of ICT Inc.
0:
oI-
PEAK
o I----------------~_-_t U I-W -6 ::::>1o..LU 1-0 -12 ::::>~ 0« -18 o:W
8.2.3.1 Peak detector This function allows the measurement of the peak value of a time varying signal to be measured during a preset window or gate tilue and displayed as the RMS value of an equivalent sinewave. Typical gate times are in the region of 3 ms to 3 seconds. A brief discussion of the factors which determine optimum gate times for various measureluents may be found in Reference 4. Most MIL STD 462 emission measurements for example are made using a peak detector.
00.. -24 -30
1-«
°0 ~I- -36 WW
0> -42 o..i= -48
0:) W
0:
-54
-60 ~_ ____L. 1 0.1
_ I _ _ - - - - - I - - - - - L - - - -.....
10
100
1000
IMPULSE REPETITION RATE ( Hz )
Figure 8.7
Output Jrom quasipeak (OJ?) detectors as junction oj impulse repetition rate. -6 dB bandwidth == 120 kHz
Reproduced by permission or ICT Inc.
134
A HANDBOOK FOR EMC TESTING AND MEASUREMENT
8.2.3.3 Slideback peak detector This function is available on some receivers and enables the operator to manually set a threshold level which causes the amplitude meter to read when a corresponding signal level is encountered. If the peak value of an incoming time varying signal reaches this threshold an audible tone may be sounded to attract the operator's attention. By successively reducing the threshold level the operator can determine the amplitude of the peak signal level. By tuning the receiver he can ascertain the frequency dependence of the peak signal. l'he time-varying triggering of the slideback peak detector by a complex signal can help the EMI engineer to identify the source of some interference signals to particular functions and circuits.
8.2.3.4 Average detector This detector produces an output which is proportional to the average of the modulus of a sinusoidal signal. I t is mostly used for determining the levels of CW carrier signals and is sometimes called the field intensi ty detector.
8.2.3.5 AMjFM detectors Also included in most EMI receivers are a simple audio AM detector and an FM detector with an output bandwidth which may be up to 10 MHz wide to assist the operator in identifying modulation types and probable EMI signal sources. 8.2.~ Commercially available EMI receIvers
There are a number of commercial EMI receivers available and examples of frequency coverage against model type are given in Figure 8.8. EMI meters are complex receivers specially designed to accurately measure a wide range of unknown signal types. 'T'hey are expensive and several will FREQUENCY 1 kHz
10 kHz 100 kHz 1 MHz 10 MHz 100 MHz 1 GHz 10 GHz 100 GHz
\1M 7
NMJ/27
I
I
NM 37 /57 NM67 EATON ESH3
ESJp
I ROHDE & SCHWARZ
8.3.1 Introduction Spectrum analysers are somewhat complementary toEMI receivers and many EMC laboratories will use both equipments during development and conformance EMC testing. The spectrum analyser is an extremely versatile piece of equipment which enables the EMC engineer to quickly gain an overview of the nature of a signal or complete emission spectrum. In contrast to EMI receivers most scanning superheterodyne spectrum analysers have wide-bandwid th front ends with a preamplifier or mixer as the first stage and with only minimal lowpass filtering. This can make them more prone to signal overload in the presence of broad band impulse noise signals than the more protected EMI receiver front ends. The noise figure for a superheterodyne spectrun1 analyser at around 20-30 dB is usually higher than that for an EMI receiver which may be in the region of 6-12 dB. However, the advantages offered by the much wider frequency span (and lower cost) of the spectrum analyser must be set against this lower sensitivity. Unlike EMI receivers, spectrum analysers are used in a wide variety of applications in electronic and communications engineering: sales specifically for EMC work form only a small fraction of the total market. As interest in EMC has grown and the need for cost-effective multipurpose RF measurement instrumentation has increased, manufacturers have become more aware of the need to supply spectrum analysers with preselectors and a range of detector functions to enable these upgraded equipments to be used for EMC conformance testing to some regulations.
There are three types of spectrum analyser:
EMC30 EMC60
Figure 8.8
8.3 SpectrulTI analysers
8.3.2 Analyser types
EMC 11
ELECTROMETRICS
be req uired to cover the frequency range 10kHz to 18 or 40 GHz. Many can be swept internally to produce a spectrum display and almost all have a standard digital interface bus through which they can be remotely operated. They are however most useful, not as a spectrum analyser, but for making accurate and reliable measurements of specific signals within a complex spectrum. This leads to a level of measurement confidence which is necessary to meet the EMC test standards laid down by procurement or licensing authorities.
IFE~
Examples qffrequency coverage by commercial EMI receivers
(i) (ii) (iii)
Frequency scannIng superheterodyne receIvers Con tiguous band pass filter banks Digital signal capture and software Fourier transform systems
RECEIVERS, ANALYSERS AND MEASUREMENT EQUIPMENT
The frequency-scanning receiver is by far the most widely used spectrum analyser. One of the first commercial equipments was produced by HewlettPackard in the early 1960s followed by the more compact and capable 141 T series which came into wide use after its introduction in the late 1960s. Since that time the spectrum analyser has benefited from the revolution in measurement instrumentation brought about by the introduction of microprocessors during the 1980s. There are a wide range of excellent processor based synthesised LO analysers available covering frequency ranges from 10kHz to 2 GHz and beyond with bandwidths as low as 10Hz. Such performance is available from a single equipment and at a reasonable cost. Any scanning receiver cannot detect more than one frequency (bandwidth) at any instant and this can result in problems when measuring pulsed signals. I t is sometimes difficult to resolve the difference between a multiple line spectrum and a low repetition rate broadband impulsive signal without extensive manipulation of scan rate and bandwidth. In certain circumstances it may be necessary to produce a spectrum display from a single pulse (e.g~ NEMP) and this cannot be achieved using a scanning-type analyser. For crude single pulse spectrum analysis a series of contiguous bandpass filters can be used which produce a histogram-type spectrum display. With the advent of very fast analogue-to-digital converters and fast Fourier transform software it has now become possible to capture a single pulsed waveform and then compute its spectrum. There are a number of commercial systems available with time resolutions of about 1 ns [6]. Such analysers are not commonly used in EMC testing at present but with the increasing interest in interference
135
produced by switching and other low repetition rate pulsed interference events these software based analysers will find increasing use.
8.3.3 Analyser operation A simplified schematic diagram of a typical scanning heterodyne spectrum analyser is given in Figure 8.9. An example of an analyser input and first mixer sections which largely determine the noise figure, dynamic range, intermodulation and spurious responses is shown in Figure 8.10. In general, spectrum analysers do not have such a good noise figure as a purpose-designed EMI receiver, with typical values being 20-30 dB for HF jVHF equipments and higher than this for microwave analysers. It can be seen in Figure 8.10 that with the input attenuator set at 0 dB for greatest sensitivity, the first mixer is divorced from the input port only by a small fixed attenuator and a lowpass filter. When a broadband coherent impulse signal is fed to the analyser input it is possible to overload the mixer with only modest signal amplitude levels. White [3J suggests that in the limiting case, when the input signal is produced by a very wideband fast-edged impulse generator, a spectrum analyser may have little usable dynamic range without a preselector. In my experience, spectrum analysers when used with care, noting their broadband dynamic range limitations, are a most useful piece of equipment. They are the real-time eyes of the EMC test engineer, rapidly conveying the full spectrum of information about the unknown signal being measured. By tuning in to a single frequency of interest and putting the analyser into a video signal display mode it is possible to see the modulation
B lock diagram of multiple-stage superheterodyne spectrum analyser
Reproduced by permissiolJ of" Hewlett Packard
INPUT ATTENUATOR
LOW PASS 3dB FILTER AITENUATOR
FIRST MIXER
LOW PASS FILTER
SECOND MIXER
t
RF
r
.L
..1.
INPUT
1 tn 1 i
1sfLO.
'"
/
BAND PASS FILTER
2ndL.O.
NOTCH FILTER
n
2ndlF
Figure 8.10 Spectrum analyser wideband front end
OUTPUT
W~
Reproduced by permission of Hewlett Packard
136
HANDBOOK FOR EMC TESTING AND MEASUREMENT 1Hz 10Hz 100Hz
1k
10k
1001<
1M
1G
10M 100M
3585 B
100 100G
~590 ~ I 8592 B 8566 B
Hewlett Packard TR 4172
I
Advan1est
M~
611A
MS620 MS68 MS 2802 A MS710 Anritsu FSA
I
FSB
Rohde & Schwarz
Figure 8.11
Examples qffrequency coverage by commercial spectrum analysers
the display screen as an display. This often enables the test and customer's technical representative to the source of an interference signal and the culprit circuit. This leads to spectrum used during development testing. of spectrum analysers lies with their range, functional displays and compact nature. The cost per octave covered is much less than that of an EMI receiver and a good spectrum component of a Modern well systems are controlled for ease of and data recording. Many are tailor-made EMC test work with the inclusion of bandpass or ......,v....... YYr·""'c.'plpr .... nrc and a range of IF bandwidths and detector functions MIL STD 461, FCC and CISPR. The covered by hTcp'rc are shown Figure 8.11. has been in this chapter to an overview of the properties of spectrum and EMI meters of interest to those with EMC The design of receivers is an extensive subject in itself and is dealt with in a number of texts, for example References 7 and 8, which El\1C test personnel may find useful. Some manufacturers offer a of notes relating to receivers which are also valuable in EMI measurements [9-12J. IJ_LLJllJ'J'.LLll-
'---'!J"v.L ....., ...... '-/.L.L
l- ...
JL ...... .:;;..
A bank of fixed bandpass filters can be switched manually or automatically into the signal path prior to the first preamplifier or mixer circuit. A more advanced automatic tracking bandpass filters based preselector is available with some spectrum analysers, turning them into wide band EMI receivers over the frequency range of 20 Hz2 GHz, which includes the frequency bands of interest for FCC and CISPRjEN specifications. One such system is offered by Hewlett Packard [13J with the model 85685A. It contains both tracking bandpass filters and preamplifiers covering the frequency range 20 Hz to 2 GHz and endows the spectrum analyser with 30 dB improvement in amplitude measuring range [14]. Belding [15J discusses the theoretical basis for spectrum analyser preselection and explores the measurernent situations where its use is advantageous with particular reference to FCC type open range testing. Preselectors are also available for microwave measurement receivers such as the EMC-60 with an FE-60 [ 16J frequency extender allowing coverage froIYl 18 to 40 GHz. The improved dynamic range and reduced spurious responses obtainable with the preselector are accompanied however by an increase in the receiver noise figure, owing to additional front-end components and some increased signal attenuation in the passband. Typical noise figures with and without preselection are shown in Table 8.1.
Table 8.1 Microwave receiver noise figure Direct input
With preselector
Frequency range
dB 14 14
dB 20 23
GHz 18-26.5 26.5-40
'An 'A
' J ' t - ' l - ....lAA ... U .......
F-,
Preselectors and filters
Modern spectrum can be equipped with filters to reduce the problems wide bandwidth out-of-band
8.4.2 Bandlimiting filters Individual filter units are used in EMC testing for a variety of purposes. They are often built to be easily connected into the coaxial line from a sensor or antenna to a receiver or RF voltmeter and may be lowpass, highpass, bandpass or bandstop. These filters can be used to bandlimit spectra containing small signals of interest in the presence of large signals, to achieve the best receiver sensitivity and linearity. Tunable narrowband rejection filters [17J are sometimes used to reduce the dynamic range of signals containing high-level carriers or other narrowband emissions so that lower level signals can be investigated. Lowpass filters are used for example in test configurations for l11ethods CS-Ol, CS-03, CS-04, CS-05, CS-08 and RS-04 of MIL STD 462. Commercially available filters of this type [18J can have stopband
RECEIVERS, ANALYSERS AND MEASUREMENT EQUIPMENT
floor levels of better than 100 dB and can be obtained with typical cut-off frequencies and -100 dB frequencies as shown in Table 8.2.
Table 8.2. EMI measurement filters (lowpass) Cut-offfrequencyf
-100dB frequency 3f
MHz MHz 0.1 0.3 0.2 0.6 0.5 1.5 1.0 3.0 2.0 6.0 sequence repeats up to a cut-offfrequency of 50 MHz Highpass filters are needed for methods CE-O 1 and CE-02 of MIL STD 462 to eliminate the fundamental power line frequencies and harmonics which otherwise would saturate or destroy EMI receiver front ends. Typical filter response curves are given in Figure 8.12 where it can be seen that the two lowest frequency filters provide 100 dB of attenuation to 50/60 Hz and 400 Hz mains power frequencies, respectively. All filters used for bandlimiting signals before the input to an EMI receiver are matched 50 ohm devices and usually have coaxial connectors. Bandpass filters are often used for isolating a known frequency of interest from a spectrum with high-level adjacent noise bands so that the receiver or spectrum analyser can be adjusted for maximum sensitivity, linearity and suppression of spurious responses. It is important for the EMC engineer to have access to a range of high q uali ty filters for the purposes described to avoid the pitfalls of making measurements on signals that cause the measurement instrumentation to display false amplitude data about the spectrum being
CD
-20
"0
0
-40
--I
Z
0
t=
-60
a:
w -80 Cf)
8.5 Itnpulse generators
8.5.1 Description An impulse generator is a device which produces a series of very short su bnanosecond-d ura tion pulses with a variable pulse repetition frequency of typically 50 Hz-10kHz to yield a flat wide band pulsed RF spectrum of known spectral density measured in dBIlV/MHz. Impulse generators are used in EMC testing for a variety of purposes [19]: Broadband calibration of EMI receivers or spectrum analysers Shielded structures attenuation measurement 'Transient testing of devices such as information technology equipment Determination of filter frequency responses Circuit coupling or crosstalk measurement. The main use of impulse generators with a calibrated output is as a secondary calibration standard for EMI receivers. The frequency coverage of an impulse generator can extend from a few hundred Hz to 1 GHz with a spectral flatness of ± 1 dB. The output level is usually adjustable in 1 dB steps up to a value of above
100 dBIlV/MHz.
8.5.2 Design
Rc
~
-100
100
Figure 8.12
investigated. Instrumentation can be overloaded, or operate in a nonlinear manner when confronted with an unknown and complex spectrum containing a mixture of high and lowlevel CW and impulse signals. The ability to insert bandlimiting filters and adjust the receiver input attenuation provides an effective way of ensuring that these errors are minimised.
The generator is normally designed to produce a short pulse at a high-voltage level by alternately charging and discharging a short length of coaxial transmission line. Figure 8.13 shows a coaxial line of about 10 cm length being charged from a stable high-voltage source through a high-value resistor, where
0
Cf) Cf)
137
1k 10k FREQUENCY Hz
100k
Example of the insertion loss of commercial highpass filters (50 n coaxial) for use in EMC testing. Solar type -7205
Reproduced by permission or Solar Electronics
» <:'0
with R c as the charging resistor and <:'0 as the line impedance. The capacitance of the short length of line and the charging resistor determine the charging time constan t and the line charges to a value Vc in that time scale. When the closing switch, which may be a mercury-wetted reed relay, is operated, the stored energy in t~1e transmission line is allowed to escape into the load resistor (50 ohm) and a short voltage pulse is
138
A HANDBOOK FOR EMC TESTING AND MEASUREMENT
Figure 8.13 rast discharge coaxial line impulse generator (a) Impulse generator diagram ( b) Waviform and spectrum from impulse generator
50n
STABLE HIGH VpLTAGE SUPPLY ~
100V OUTPUT SPECTRAL DENSITY UP TO 100 dB Jl V / MHz
"FLATil PORTION OF OUTPUT SPECTRUM
>..... U5
z w o
100 V , . . - - - - - · - -....
PRF ~ 100 Hz
frl~
0..-'
..... ~ 0>
FOURIER TRANSFORM
::::>0
ov""'"-----.......;;::::::11111..... TIME -t=:::.5 ns---
formed at the ou tpu t terminals. The pulse wid th is determined largely by the length of the coaxial line. White [3J gives the voltage spectral density of the ou tpu t pulse as
where S(])
~
Vc t
~
f
~
~
2 V:. t sin (n] t)
nit
V/Hz
8.5
signal spectral density charging voltage in volts pulse wid th in seconds frequency
The first null value in the spectrum occurs at ]~
N
.... J:
..... w 0.«
~
--T
..J
« Cl:
::::>0
S (] )
_ _11dB
IltHz.
If the pulsewidth is typically 0.5 ns long the first zero is at 2 GHz. When] « lit the (sin n ]t) In]t expression tends to 1 and 8.6
1-'his shows that at freq uencies well below the first null the signal spectral density is flat with an amplitude of 2 V:. t. Thus these impulse generators are useful as calibration devices as the spectral output can be accurately determined from simple measurable quantities. It is possible to improve the useful upper frequency of the flat part of the spectrum by designing a mismatch peaking circuit which increases the higher frequencies in the
en>
w::t C)
«
~
~
4
•
FREQUENCY
Impulse generator used to calibrate receivers over this frequency range.
LINE SPACING AT PRF
pulse. Using this technique it is possible to produce reliable and cheap instruments with spectral flatness of ± 1dB to above 1 GHz.
8.5.3 Use of impulse generators Commercially produced impulse generators are available as stand-alone equipments [20J or are built-in to some EMI receivers [21 J. As men tioned earlier the au tpu t swi tch from the coaxial line maybe a relay. I t can be triggered to close either linked to the power line freq uency, or to a variable frequency source. If pulse-repetition frequencies above a few hundred Hz are required the switch is usually replaced by a solid-state device such as a fast FET. A good reliable impulse generator is essential for the EM C test la bora tory. I t can be used to calibrate and check EMI meters and spectrum analysers for amplitude accuracy, spurious signal generation and dynamic range com pression; or it can be used to give a direct reading of equivalent broadband signal strength by adjusting the output to give the same reading on the EMI meter as the unknown interference signal being measured. l'his is generally known as measurement by substitution and is a very accurate but slow method of measurement.
RECEIVERS, ANALYSERS AND MEASUREMENT EQUIPMENT
8.6 Digital storage oscilloscopes 8.6.1 Advantages of digital oscilloscopes Some of the more comprehensive EMC immunity tests such as those in MIL STD 461/2 (CS06, CS10/ 11, RS05) and IEC801/BS6667 call for continuous and transient stimuli signals to be imposed on the equipment under test and monitored with a fast oscilloscope using a high-impedance probe or other sensor. There are also conducted emission tests such as MIL STD 461/2 CE05, CE07-1, SP-P-90203 [22J and EN550 14/BS800 in which transient or very low repetition rate fast burst type signals must be measured. For most of these tests details of the peak amplitude, waveshape and repetition frequency must be measured. Traditionally, oscilloscopes have been used to measure the signal ofinterest which is usually superimposed on a power line waveform which may itself have an amplitude of ± a few hundred' volts. Triggering a conventional oscilloscope on the wanted interference signal can present problems especially ifits amplitude and position with respect to the mains frequency are variable. Setting up an oscilloscope to record a single transient event is also usually difficult and requires considerable experimentation and takes up valuable time in the EMC facility. It is difficult to produce a record with the transient in the centre of the scan even with delayed trigger circuits on standard oscilloscopes. Permanent records of transients have normally been made by photographing the screen, either with . an open shutter and a single scan, or by writing the trace on to a storage screen and then using a timed exposure. Many of these problems are eliminated when digital storage oscilloscopes are used and the measurement and recording of transient waveforms can be achieved quickly and accurately.
139
conductor switching and regulating circuits in mains power supplies. In such cases the noise bursts may be locked to the power line waveform; in other applications the noise bursts may not be synchronised or may even occur randomly. The detailed vvaveform within a burst is often complex and may be rather variable but if examined closely it usually has some distinguishing feature such as an obvious oscillatory frequency generated by unintentional inductance/capacitance, which may indicate the source of the interference. Using conventional oscilloscopes it can sometimes be difficult to examine individual time-expanded bursts in detail owing to triggering and screen writing speed limitations. Some conducted emission tests require the exported spikes from an EUT to be measured when the device is turned on or off or functioned in some other way. Figure 8.15 shows a typical measurement setup where a single transient must be measured on top of the power line voltage. In some cases, it is possible to use lowpass filters to reduce the amplitude of the power line waveform and obtain a better trace on a conventional oscilloscope. A digitising transient capture instrument or digital oscilloscope is ideal to record this single interference event. Figure 8.16 shows the product EXPORTED TRANSIENT NOISE BURST
/ DIGITAL TRANSIENT RECORDER OR OSCILLOSCOPE
EUT
producing noise burst when switched
VOLTAGE PROBE
on or off
INDUCTOR
8.6.2 Typical waveforms to be measured
Figure 8.15
Figure 8.14 shows typical recurrent noise bursts which may be produced by poorly designed semi-
Measuring exported voltage spikes with digital transient recorder
600----r----------.....----------TRANSIENT NOISE BURST
( with fine structure )
> 200 w
w
~
~
o
~
o
..J
o
III-I~"",,,,"'--#---~-""~--I"TIME ms
>
w
> -200
~
0..
z
::J
0
-400
-600
1O~s
Figure 8.14
Transient capture oj noise burst on mains pouJer Line
100ms
1mS
10mS
100mS
DURATION OF TRANSIENT ENVELOPE
MAINS POWER WAVEFORM
Figure 8.16
Example of exported spike limits from SP-P-90203
140
A HANDBOOK FOR EMC TESTING AND MEASUREMENT 200 ms----..., I
1+---0.9
I I I
w C)
~
....
zw
J
...J
I
0
>
.... z
I
z
,
w U5
« a:
:::J ()
0.1
I
l-
I
> 200 ms
}'igure 8.17
~ 0.5+--+-+---------....
time
I I
...1
I
Example of exported transients defined in BS800. Individual impulses shorter than 200 ms spaced closer than 200 ms continuing for more than 200 ms
Figure 8.19
Typical output waveform oj ESD generator (BS6667 pt2)
Reproduced by permission of BSI
Reprod uced by permission of BS I
specific SP-P-90203 specification limits for transients emitted from an avionics EDT in the form of amplitude-time envelopes for AC and DC supplies. The exported transients must lie within these envelopes. Confirmation of this requires that the worst-case transient can be recorded over a time scale of microseconds to many hundreds of milliseconds. Other examples of the type of lo"v repetition-rate noise burst signals of interest in EN550 1/BS800 as emitted from household electrical appliances can be seen in Figure 8.17. This specification permits the use of a disturbance analyser [23] which can measure and record transient amplitude and occurrence data for statistical analysis. Such instruments are complementary to detailed measurements of individual events with digital oscilloscopes.
8.6.3 Recording injected pulses for immunity testing Fast-transient recorders are also used to monitor the amplitude and waveshape of transients or spikes being injected on to the power lines connected to an
EDT. An example of the signal which must be injected in accordance with MIL STD 461B CS06 is given in Figure 8.18. The risetime of this spike is around 2/ls which is relatively slow. Oscilloscopes and digital transient capture instruments need to be capable of recording risetimes much faster than this. For example the risetime of the simulated NEMP waveform used in MIL STD 461 C RS05 is IOns and that for the ESD pulse specified in IEC80 1/BS6667 is 5 ns. As an example, the ESD wave shape is given in Figure 8.19.
8.6.4 Digital transient recorder architecture Digital transient recorders can be extremely useful in EMC measurement. The advent of fast, reliable ADC integrated circuits and inexpensive memories has led to a number of instruments which for the first time make the capturing of low repetition or single-shot fast transients a simple procedure. There are three types of waveform digi tiser: (i) (ii) (iii)
transien t digi tiser with circular addressing random interleaved sampling RIS digitisers and sampling digitisers.
400 CJ)
I-
w~
300
t::e O:X
200
«g>
100 I-
~~ a:::Q)
0
g~
....J
0
> w
0
:2Q)
;:)
w=
CJ)~
::i a.. :2
«
w ~
-100 a:::
CJ)
5
10
15
20
25
TIME lIs
Figure 8.18
Example of injected spike waveform MIL STn 461B CS06. Use left or right ordinate whichever is less in particular application
When armed the first type continually digitises an incoming waveform and constantly overwrites a block of memory from start to finish and back to the start, in a circular fashion. When a trigger pulse is produced the digi tisa tion con tin ues un til a user-specified post trigger time has elapsed and then the digi tisa tion ceases and the memory is locked. The captured waveform complete with pre- trigger information is read ou t from the memory and displayed on a screen. Instruments with a digital output bus can download the data to a PC or direct to a plotter. A typical instrument will have a sampling rate of around 100-200 megasamples per second, a
RECEIVERS, ANALYSERS AND MEASUREMENT EQUIPMENT
repetitive signal effective sample rate of 4 GS/s, a single-shot bandwidth of 100-300 MHz, an 8 to 12 bit am pli tude resolution and two or more input channels. l'ypical instruments are LeCroy 9410, HP 54112D and Tektronix RTD710A. Among the fastest single-shot digitisers currently available is the LeCroy 7200 series with four channels operating at 1 GS/s and an analogue bandwidth of 400 MHz. The RIS digitiser can yield improved waveform resol u tion in some cases by sampling anum ber of identical input transients at slightly different stages in the waveform and then interleaving all the data. A sampling digitiser or sampling scope can only work with a long train of identical pulses or a continuous waveform. Under these conditions a digitised waveform can be produced with an effective bandwidth of beyond 30 GHz [24]. In addition to the types of instruments already discussed there are some transient capture equipments using special very fast waveform storage techniques which can also provide a digital output. The Tektronix 7250 can capture and display a single transient waveform with a SOps risetime and the Tektronix model SCD1000/ 5000 has a time resolution of 5 psjpoint. Some digitisers have built-in signal processing capability and can quickly calculate the Fourier transform of a captured waveform and display them together on a single screen. Instruments such as HP 5180T/U, Tektronix TD2301 and LeCroy 7200 series have this ability to convert from time to frequency domain. These powerful instruments have a great deal to offer the practising EMC engineer or equipment designer who needs to develop electronic products which can meet EMC standards, as they can give a rapid insight into the full nature of a wide range of interference signals. An excellen t technical tu torial concerning the fundamentals of digital transient capture instruments IS produced by LeCroy [25 J. Inspection will demonstrate the capabilities available and need for careful specification of the requirements for instruments of this type before purchase by EMC engineers, if they are to obtain full advan tage from this technology.
8.7 AFjRF volttneters When performing low-frequency conducted susceptibility tests such as MIL STD 461/2 CSO 1 and CS02 it is necessary to measure the injected signal voltage on the lines to the E UT wi thou t loading them with a low impedance (e.g. 50 ohms input of an EMI meter). I t is also inappropriate and unnecessary to use expensive and sensitive
141
EMI meters or spectrum analysers fitted with high-impedance probes to measure these signals which often have an amplitude of a few volts RMS. The preferred economical method is to use a high impedance RF voltmeter which can measure the required signal at a fraction of the cost of using an EMI meter. There are several factors which determine which type of AC voltmeter is appropriate: For conducted susceptibility tests on DC lines a broadband AF /RF voltmeter can be used. There are a number of designs using diode bridges, crystal detectors and sometimes preamplifiers to provide measurement from a few kHz to above 2 GHz and amplitudes over the range ill V to 10 volts. AF /RF voltmeters with both balanced and unbalanced inputs are available. A useful description of AF /RF voltmeter design techniques, including explanation of average, RMS and peak reading meters, crest factor and form factor has been produced by Rohde and Schwarz [26J. For measuring signals injected into AC power lines a tuned AC voltmeter is req uired which can filter out the power frequency and its harmonics producing a measurement of only the injected signal amplitude. These equipments often use a combination of fixed frequency highpass filters and a tunable bandpass filter with bandwidths as low as 10Hz in the case of AF voltmeters. The frequency range of _the test, e.g. audio frequencies up to 50 kHz for MIL STD 461/2 CS01 and RF frequencies up to 400 MHz for CS02, will determine the req uiremen t for differen t AF and RF selective voltmeters. The use of a high input impedance (> 100 k ohm/3-5 pF) frequency selective RF voltmeter is shown in Figure 8.20 in a MIL STD 461/2 CS02 test setup. When making measurements on AC lines, care must be taken to ensure that adequate power-frequency rejection is embodied in the AF /RF voltmeter to enable the correct amplitude of the injected susceptibility signal to be measured. A range of selective AF /RF voltmeters is commercially available from a number of electronics instrument manufacturers.
8.8 RF power tneters RF power meters fall into two types: ou tpu t power meters and directional power meters. Output power meters measure the RF power delivered to the sensing head and display it either on an analogue meter or on a digi tal display or via a digital interface bus. The power meter sensor head should have a very low SWR « 1.3: 1 [27J) over a wide frequency range (up to 26.5 GHz) if measurement uncertainties owing to reflected power are to be minimised.
142
A HANDBOOK FOR EMC TESTING AND MEASUREMENT
Figure 8.20 Use of highimpedance RF wideband voltmeter in conducted susceptibility testing. Susceptibility limits are in terms of both voltage and current
measures susceptibillity current
CURRENT PROBE
ACorD POWER INPUT
MIL STD 461 CS 02 TEST
Most RF power meter sensor heads are constructed using one of two basic mechanisms [28J: conversion of RF power to thermal energy and measurement by thermocouple, and diode rectification and measurement by a sensitive voltmeter. In general, the first is used for moderate to high power meters and the second for detecting powers as low as 100 PW (- 70 dBm) . In addition to their general use as measurement meters, for checking the output power of RF amplifiers for instance, these instruments are very useful in performing semiautomatic EMC RF and microwave radiated susceptibility measurements. In such a test configuration the radiated field strength at the EDT has been previously measured and defined in terms of the forward RF power delivered to the radiating antenna, see Figure 8.2l. A power meter and a suitable directional coupler, or a directional power meter [29J can be used to measure the power to the antenna and convey the level to a computer controller which compares the measured value with the value required to produce the nominated field strength at that frequency. The controller then adjusts the ou tpu t of the signal source to reach the required power to be delivered to the antenna. SOFTWARE & DATA
EUT
measures susceptibility-voltage
HIGH INPUT IMPEDANCE RF VOLTMETER - wideband/ tunable
An automatic power measurement loop such as described, can rapidly generate the correct field strengths on the EDT as the frequency in the band is being swept, and so increase the throughput and productivity of the test house. Software can be devised to ensure that at no time during the power setting process does the resultant E-field on the EDT exceed the required value at each frequency. Users of EMC test facilities should look for the ability to perform semiautomatic tests of this type when choosing where to have their equipments tested, as they are likely to obtain faster tests and better value for' money.
8.9 Frequency m.eters These instruments are not now used as widely as they once were following the introduction of lowcost, reliable and accurate synthesised signal sources that can be used in EMC testing. Such sources are accurate to a few Hz in hundreds of MHz. A frequency meter is however useful for carrying out spot calibrations on signal sources together with a spectrum analyser to check harmonic purity. A modern spectrum analyser with a synthesised LO can itself be used to measure frequency to within a few Hz in a GHz and can therefore be substituted for a frequency meter.
INSTRUMENTATION FOR SUSCEPTIBILITY TESTING
8.10 Signal sources
8. 10. 1 Signal synthesisers
Figure 8.21
Automatic control offield strength in radiated susceptibility test using RF power meter
In the past, EMC testing has required a wide range of signal sources to meet the frequency, amplitude and waveshapes needed for susceptibility testing. Frequencies covered extended from a few Hz to 40 GHz and a large number of sources were required. Most early signal
RECEIVERS, ANALYSERS AND MEASUREMENT EQUIPMENT
generators did not benefit from synthesiser technology and thus were subject to time and temperature drifts and were also manually driven. White [30] summarises the situation with regard to nonsynthesised sweepers and power oscillators. Modern synthesised signal sources can ~over very wide frequency ranges and only a few instruments are needed in a modern well equipped EMC test facility. These highly capable equipments tend to be expensive, and relying on so few sources to cover the required frequency range can present a significant problem when they are not available owing to periodic calibration or malfunction, as the ability to conduct a whole range of tests may then be affected. In specifying the requirement for suitable synthesised signal sources for use in EMC testing it is necessary to consider Frequency range Frequency resolution Frequency stability Maximum output power and attenuator range Absolute level accuracy Spectral purity Settling time Modulation capability AM, FM, pulse and possibly phase modulation Ease of programming and type of data bus Case shielding against RFI Size, weight, reliability and cost. It is possible to cover the frequency range up to 1 GHz specified for immunity testing of commercial electronics equipment with only two signal sources. For example; •
R&S APN 1 Hz-260 kHz, resolution 0.1 Hz
,[ < 20 kHz and 1 Hz f > 20 kHz, output •
50 f.1V to 20 V HP 8657A 100 kHz-I040 MHz, 1 Hz, output -143 to 13 dBm.
resolution
Coverage from 1 to 18 GHz can also be obtained in a single equipment such as the HP 8672S (100kHz-18GHz), resolution 1 Hz and 3kHz, output 120 to +13 dBm (2 dBmf > 2 GHz). For a full understanding of the variety of synthesised signal sources available and their suitability for EMC testing the reader should consult a range of specialist instrument manufacturers. Figure 8.22 is a guide to the range of typical synthesised signal sources which are available.
8.10.2 Signal sweepers Sweepers do not usually have the frequency accuracy of synthesisers bu t can produce a levelled output (within about 1 dB) as the
143
10Hz 100Hz 1 kHz 10 kHz 100kHz 1 MHz 10MH~ 100MHz 1GHz 10GHz 100GHz 1 MHz
AFG/U 260 APN
20
)3
SPN SMG
1
SMGU/SMHU
2.16 4.32
SWM 02/05 Rohde & Schwarz MG 440 E
20 0.5 MG 545 E MG 649 A
.1
18
2
Anntsu 8904 A 332 58
600 11
8656 B 8644 A 86600
0.99 2.06 2.6
~ 2
Hewlett Packard
Figure 8.22
8360 series
26
50 110
Examples of synthesised signal generators
frequency is rapidly swept between the ends of a given band. The oscillators are usually voltage tuned (varicap diode or YIG) depending on freq uency, and the fast ou tpu t sweeps can be used for spectrum/network analysis with couplers, detectors and a suitable display. When manually operated or used with long scan times (10-100 s) , sweepers can be used as the signal source for susceptibility testing. Most are programmable and they are usually less expensive than syn thesisers.
8.10.3 Tracking generators These devices are variable frequency oscillators with maximum output powers in the range of 0 to 10 dBm which can be locked to the centre frequency of the measurement bandwidth of a spectrum analyser and track its scan accurately over the band of interest. This ability results in a cheap scalar network analyser which can be used by EMC engineers and equipment designers to measure filter responses, cable crosstalk, shielding efficiency, cable losses and antenna passbands and correction factors. When used with two current probes as shown in Figure 8.23 this simple network analyser can yield a great deal of information about the RF coupling behaviour of a complex systenl of interconnected equipments, such as a distributed mainframe computer installation, and can help uncover frequencies at which radiated emissions or susceptibilities are likely to occur. I t has been shown that the RF common-mode cable response in such a system is often closely correlated with the radiated emission from them [31], as in Figure 8.24. I t also indicates frequencies of probable susceptibility.
144
A HANDBOOK FOR EMC TESTING AND MEASUREMENT
Figure 8.23 Tracking generator and spectrum analyser used to measure cable transfer functions in multibox E UT
cable # 1
cable # 2 ----CABLE UNDER TEST
R X PROBE FREQUENCY
SIGNAL LOOP TRACKING GENERATOR SIGNAL SOURCE CABLE UNDER TEST TRANSFER FUNCTION (raw data)
W Q
:::> f-
::J 0..
TRACKING GENERATOR SIGNAL PASS BAND OF SPECTRUM ANALYSER Both scan together
IlJ ~t.===========f=2~ ::E
JJL.
SPECTRUM ANALYSER
«
SIGNAL TRACKS WITH ANALYSER • to measure probe I probe coupling
O..-------r------.-----r------.-----__, ·10
FUNDAMENTAL LINE SELF RESONANCE
_ ·20 CO
"0
CURRENT TRANSFER CHARACTERISTIC as measured in' \ 8.23
W
...J
«
u w
\
(/)
Cl
::> :J
I0~
«
RADIATED SIGNAL STRENGTH 1m from the cable -70
-8001-------1------1-----'-----...1.----"' 80 40 60 20 100 o FREQUENCY MHz
Figure 8.24
Current transfer characteristic and measured radiated signal strength from simple test transmission line
Reproduced by permission or BAe Dynamics Ltd.
Many spectrum analysers, including those with synthesised local oscillators have builtin tracking generators and this extends the capability of the spectrum analyser in carrying out diagnostic EMC measurements as described.
8.11 RF power all1.plifiers 8.11.1 Introduction Power amplifiers are required to raise the signal power ou tpu t from a synthesiser or sweeper signal source to a level required by the ou tpu t
transducer to perform the EMC test in question. Thus amplifiers will usually have a stable input signal from a well defined source impedance but must drive current probes, capacitive coupling blocks and a wide variety of antennas which are often situated in reflective screened rooms. Such ou tpu t loads will not in general present a good VSWR to the amplifier output. In fact, EMC testing is one of the most demanding uses to which high power broadband RF amplifiers can be put in terms of output VSWR and final stage amplifier protection. The frequency range of EMC susceptibility tests is large (up to 18 or 40 GHz) and a number of amplifiers are required to cover this range in up to ten contiguous bands. As they also need to provide at least 100 W output power in bands below 1 G Hz and 10-100 W above this frequency they tend to be one of the most expensive capital assets in the test laboratory. I t is therefore important that EMC test engineers and managers in companies developing products which must meet EMC standards, and who plan to invest in EMC test equipment, should understand the performance limi ta tions and design compromises pertinent to RF amplifiers. One of the most difficult requirements to satisfy is the production of an amplifier with broadband coverage and high power in the same package. Part of the problem lies in the bandwidth limitation of the power devices themselves where the internal capacitance is the limiting factor. If this capacitance is reduced by making smaller semiconductor devices, their power handling is also reduced. The
RECEIVERS, ANALYSERS AND MEASUREMENT EQUIPMENT amplifier designer faces limits in device gainbandwidth product and power handling which can only be minimised by careful circuit design. Unsophisticated broadband high-gain (1 MHz1 GHz, 30 dB) amplifiers have a tendency to break into oscillation and much design effort is directed towards ensuring stable operation, particularly when driving poorly matched loads which is the norm in EMC testing.
10,000
10 kW ( 10kHz - 100 MHz)
I
Specifying RF amplifiers for a range of EMC tests is a demanding task and mistakes can lead to gaps in the testing offered, an unacceptable level of amplifier malfunction or an increase in test times, all of which can have an adverse economic impact on the facility operation. The following list suggests the performance parameters which should be addressed when evaluating the suitability of an amplifier for use in EMC testing.
8.11.2.1 Frequency range This should be as broad as possible consistent with cost. The fewer band switches or amplifier changes the faster the test can be conducted. Typical practical amplifier frequency ranges are Solid-state amplifiers (100 W-1 kW) 10Hz -100 kHz 10 kHz -200 MHz 200 MHz-400 MHz (for power output of > 100 W) 200 MHz-1 G Hz (for ou tpu t powers < 100 W) TWT amplifiers (10W-100W) 1 GHz - 2GHz 2GHz 4GHz 4GHz - 8GHz 8 GHz -12.4 GHz 12.4 GHz-18 GHz TWT amplifiers (1 W-I0W) 18 GHz -26.5 GHz 26.5 GHz-40 GHz.
I
2.5 kW ( 10k - 220 MHz) I
1000
1 kW (220 - 400 MHz)
I
1kW (10 k -220 MHz) 750 W ( 10k - 220 MHz)
750 W ( 400 MHz - 1 GHz) I
I-
~
250W (10 kHz220 MHz) 100
I
i
100 W (100 -1000 MHz)
:::>
o
ex:
50
w
~
a.
I
300 W ( 400 MHz - 1 GHz) 200 W ( 220 - 400 MH~ )
I-
8.11.2 Specifying an amplifier
145
(1 -1000 MHZ)
I
10
10 W ( 1 - 1000 MHz)
I
I
5 W ( 500 kHz - 1 GHz) 1 .01
1
100
2CfO
400 FREQU~NCY
Figure~ 8.25
I
600
I
800
1000
MHz
Examples oj RF amplifier power output and bandwidth. Data Jor examples from amplifier research
Reproduced by permission of Amplifier Research
with the widest bandwidth and the highest power possible within the budget constraints. EMC immunity levels are likely to increase in the coming years and more RF power will be req uired to carry out the tests. A factor of three increase in field strength in a radiated susceptibility specification will req uire a factor 10 increase in power amplifier capability. Amplifier cost increases very rapidly with output power and it is therefore preferable to purchase expansion potential at the start rather than pay for increasingly capable new equipment. A chart of high-performance amplifier capability [32J (power against frequency) is given in Figure 8.25. The trend represented in this figure is a movement fron1 very broadband at low power (1 GHz at 5 W) to a smaller bandwidth at very high power (100 MHz at 10kW).
8.11.2.2 Power output Typical power output required for 1-3 m EMC susceptibility testing at up to 10 V jm is 10 to 100 W. The amplifier manufacturers do not always make it clear whether they are specifying maximum output power, mean power (according to some definition) or minimum output power. Whenever possible, amplifiers for use in EMC testing should be specified against minimum power output. This will enSlJre that the field strengths or induced current levels can be produced and will give in some cases up to 3 dB headroom in performance [32]. Experience suggests that the best purchase strategy is to specify units
8.11.2.3 Amplifier gain The amplifier gain required will depend on the ou tpu t level of the signal source being used and the ou tpu t power calculated as being required to produce the required field strength or induced current specified in the tests. Usually the signal source will have an ou tpu t in the range 0+ 13 dBm and a gain of 40-50 dB will be required to reach full ou tpu t in excess of 100 W. The maximum output power of an amplifier will vary across its frequency range typically by ± 1.5 dB, and the rated output should be specified at the
146
A HANDBOOK FOR EMC TESTING AND MEASUREMENT 20,000 r----r:':~r:--__,-~--.--.......---r-......-~....-......----.- ......-.--.......-
Figure 8.26 Amplifier output power exceeds rated power through frequency band for ~ high quality equipment. w ( Selection from Amplifier ~ Research products) ~
10,000
_
.........± 1.5dB
_
]± 1.5 dB
5000 2000
___~~
!!,i~::.~
:::J
0.... :::J
250L
o
o w
200
~ ffi
100
a.
20
a:
10
o~
min gain = 54 dB _ _ _ _ _ _ _ _ _- - - - - 5 0 L
----- --- --- --- - - ---
LL
5
( 1.5 dB =
lower bound of this range to ensure the specified output power is always available. See Figure 8.26.
8.11.2.4 Gain compression An amplifier will respond nonlinearly when the input signal is sufficient to drive the output near to saturation, when no more power can be generated. As the output approaches saturation the amplifier gain is compressed and starts to fall off as illustrated in Figure 8.27. Knowing the output power at which the gain compression is 1 dB for example, is important if the equipment is to be used for amplifying signals with superimposed AM. If AM signals are passed through a nonlinear amplifier, spurious distortion signals will be generated. This is not so important if FM or pulsed signals are being amplified, and amplifiers are often run at the saturated level for these signals. GAIN THEORETICAL COMPRESSION LINEAR I ( 1.0 dB ) OPERATION ~~#
T7-----~-71
"0
0: W
~
o
r
~
/
OUTPUT POWER at 1.0 dB gain compression
~
I
I
I
0I-
ACTUAL AMPLIFIER OUTPUT
I I
::> 0-
I-
::>
o
(G = slope)
Maximum drive level for linear operation
I I
-I
I
INPUT POWER dBm
Figure 8.27
Gain and output power behaviour near amplifier maximum output
Reproduced by permission of' Amplifier Research
5 10 20
50 100 200 500
FREQUENCY MHz
Reproduced by permission of' Amplifier Research
cc
x 1.4) _
Saturated output power, CW 2
~
--..... . . =& 1.5dB
50
8.11.2.5 Harmonic distortion This is caused by nonlinearities In the amplifier response and creates spurious signals at harmonics of the fund amen tal at the expense of power in the fundamental signal frequency. The choice of amplifier design is importan there; Class A operation results in the greatest linearity and the lowest distortion figures. Unfortunately this mode of operation is not suitable for high power systems owing to energy dissipation. Typical worst-case harmonic distortion components are 15-20 dB down on the fundamental signal at the rated power output of a commercial amplifier. The control ofharmonic distortion is important in EMC susceptibility testing as it is possible to ascribe the failure of an EDT to the susceptibility signal at the fundamental frequency when it may have been caused by a harmonic signal. In the worst case where the transfer function of the antenna or probe is such that it preferentially selects the harmonic at the expense of the fundamental, a ghost susceptibility can be produced. Where possible, amplifiers should be operated well below gain compression to reduce the harmonic distortion to a minimum. This is a further reason for purchasing amplifiers with a greater power capability than is strictly necessary for a particular test.
8.11.2.6 Intermodulation distortion This effect is generated when two or more signals are fed into the amplifier and nonlinearities act to mix the signals together producing sum and difference frequencies. This effect is not as significant for EMC testing as harmonic distortion
RECEIVERS, ANALYSERS AND MEASUREMENT EQUIPMENT
because usually only one signal is amplified at a time and injected into or radiated onto the EUT.
8.11.2.7 Output protection Traditionally, high-power RF amplifiers have used directional couplers on the ou tpu t to detect a poor match with the load. The signal from the reverse power port of the directional coupler is used to trip the power supply to the final stages of the amplifier and thus protect them against damage caused by the absorption of the reflected power. This protection feature is necessary but can cause problems in EMC susceptibility testing where serious mismatches occur frequently as the signal is swept through the frequency band and the amplifier repeatedly trips out, extending the test time considerably. Modern high-power semiconductor RF amplifiers [31 J can be designed conservatively so that the reflected power from an open or short circuit can be absorbed in the output stages without causing damage. This results in the ability to conduct the EMC susceptibility test in a single sweep without repeated stops to reset the amplifier.
8.11.2.8 TWT microwave amplifiers Travelling wave tube amplifiers are currently used to provide high output powers of 100 W or more up to around 18 GHz. They are based on a thermionic valve or tube the general construction of which [33J is shown in Figure 8.28. Typical amplifiers have an octave bandwidth and gains in the region of 35--40 dB. Sometimes TWTs are noisy amplifiers and are often combined with solid-state microwave preamplifiers in a single equipment to improve the system noise figure. l'he protection of travelling wave tube highpower microwave amplifiers [34J relies on VSWR r--------------- -----, I I
CONTROL ANODE
I
\
!H~ 1
I~~~~ I:.: I I
I
ELECTRON BEAM FOCUSING MAGNET
I ATIENUATOR
/
TUBE BODY /
>;:::==::tl1fJWm~ tmm~~~
CATHODE
a
HELIX
/l
COLLECTOR
I
T6
ELECTRON BEAM
I I
J
TRAVELLING WAVE TUBE
Figure 8.28
High-power microwave travelling wave tube and power supplies
Reproduced by permission or Macmillan
147
detection, tube element current monitors, and power supply shut-down techniques. Making EMC susceptibility measurements at microwave frequencies with TWT amplifiers is much more time consuming than at below 1 G Hz using solidstate equipment. The amplifiers only have an octave bandwidth, the waveguides and antennas need to be 're-plumbed' with almost every change of band and the TWT amplifier is designed to trip ou t in the presence of high levels of reflected power due to mismatches.
8.11.3 RF amplifiers -
conclusions
'The selection, purchase and use of high-power broadband amplifiers for use in EMC susceptibility testing calls for careful consideration if the best outcome is to be obtained within a limited budget. Only large test houses and aerospace! military and automotive companies will have a need to purchase the largest I-10kW amplifiers for large system evaluation. More modest am plifiers in the region of 100 W ou tpu tare sufficien t for testing most military and commercial electronic equipments and small units. Amplifiers with outputs of a few watts are extremely useful in susceptibility development testing of small units or circuit boards when used in conjunction with bounded-wave devices such as GTEM cells.
8.12 Signal modulators 8.12.1 Modulation requirements The requirements for the modulation characteristics of EMC susceptibility test signals vary widely with the specification. Often a minimum set of conditions is laid down with the proviso that other modulation types, levels and frequencies should be added if the EUT is designed to operate in an environment that contains these specific signals. For example, an equipment that is to be sited near a radar transmi tter may have to pass a susceptibility test with the specific modulation characteristics of that radar. Generally, the specification of modulation signals is more detailed for tests on military equipment than for those on commercial electronics. A typical set of modulation rules for tests on military equipment is given in MIL STD 462 (N3, p117) and is listed in Appendix 2 Table A2.1. A more comprehensive list of modulation rules for susceptibility tests on military equipment is to be found in DEF STAN 59-41 (part 3 page 12); and is given in Appendix 2 Table A2.2.
8.12.2 Built-in modulators From inspection of these tables it can be seen that the modulation types required for EMC tests fall
148
A HANDBOOK FOR EMC TESTING AND MEASUREMENT
into four basic types: CW, AM, FM and pulse. It is possible that other more sophisticated types of modulation such as phase modulation, may occasionally be required, but in most cases modulation sources which can deliver the four basic types are adequate for EMC testing. Modern s)'nthesised signal sources usually have built-in modulators with a reasonable capability for AM, FM and pulse operation. Typical performance figures are given in Table 8.3.
Table 8.3 AM FM
Pulse -
~ypical
built-in modulator performance
Modulation frequency to 50 or 100 kHz Modulation depth to 99 or 100 % Modulation rate typically 50 or 100 kHz but up to 10 MHz Deviation typically 100 kHz-20 MHz Risetime from 5-400 ns PRF typically 0.1-1 MHz but up to 10 MHz
Not all synthesised signal sources will have the full range of modulators and care must be exercised in matching the performance of the modulator with the modulation requirements for the range of tests which must be performed. A reasonable costconscious compromise is to use internal modulators for AM and FM and slow (> 0.5 f.1s) jlow repetition rate « 1 MHz) pulse modulation, together with an external fast pulse modulator [35 J. External fast « 100 ns) pulse modulators may be based on PIN diodes and are usually only needed for simulating radar modulation at microwave frequencies. The requirement is likely to be specified in the test plan or contractual documentation in addition to the more general range of modulation parameters set out in the relevant EMC standards. A wide range of pulse generators is available from a number of electronics instrument manufacturers which are suitable signal sources to drive modulators. Most have 5 V rrTL compatible 50 ohm outputs [36J, and some are capable of higher ou tpu t voltages [3 7J .
8.12.3 Arbitrary waveform generators A synthesised signal generator or sweeper with a suitably capable internal modulator section will require an input from a mod ula tion signal source. This can be a simple analogue function generator [38J or a sophisticated synthesised function [39J or arbitrary waveform generator [40]. Typical function generators will produce sine, square, triangle, trapezoidal AM and pulsed waveforms with a variable modulation depth at frequencies up to about 20 MHz. They may also include FM and
sweep-frequency capability. An arbitrary function generator can produce any programmed waveform required with frequencies up to about 300 MHz. Some arbitrary waveform generators can take direct digital information from a transient digitiser used to record a signal and then reproduce the exact waveform which has been captured. LeCroy [41 J produced a useful tu torial reference concerning arbitrary waveform or arbitrary function generators. The ability to synthesise any type of complex waveform is invaluable in conducting development research into RF system susceptibility. Measured real-world interference currents on system cables can be reproduced accurately at a range of increasing power levels to determine the susceptibility threshold for the system. Arbitrary function generators are also extremely useful in prod ucing specific waveforms, such as those for NEMP or ESD tests, as the complex waveform characteristics can usually be programmed from simple mathematical functions and can be changed at will. This obviates the need for a range of waveform specific signal generators and although arbitrary waveform generators are expensive, they may prove to be a cost-effective choice for a modulation signal source.
8.13 Directional couplers, circulators and isolators
8.13.1 Amplifier protection devices High-power EMC susceptibility testing calls for the use of high-power amplifiers covering an extended frequency range including microwave bands. The widely varying load VSWR forces the need for protective devices to be installed on the amplifier output. Directional couplers, power circula tors and waveguide isola tors are all used in this application. The choice of which device to use will depend on the specific amplifier output circuit, the load and whether the output line is waveguide or coaxial cable.
8.13.2 Directional couplers A directional coupler is an RF transmission line device which bleeds off a known portion of power flowing in the forward or reverse direction in the waveguide or coaxial cable. This power can be measured and the value of the main power flow can be calculated. The device can therefore be used to sample the forward power flow, to measure the reverse power and perhaps use it to shut down an amplifier which is driving a bad mismatch. With appropriate detectors and meters the reflection coefficient or VSWR can be continuously displayed.
RECEIVERS, ANALYSERS AND MEASUREMENT EQUIPMENT
FORWARD POWER MEASUREMENT PORT
8.13.2.1 Waveguide couplers Consider the two parallel waveguides shown in Figure 8.29. Liao [42J gives an explanation of the functioning of this type of directional coupler. If a small hole is made between the two waveguides, a fraction of the power flowing in the primary guide will propagate into the secondary guide through the slot antenna and then flow outwards in both directions. If two identical holes are made Ag /4 apart, or more strictly at a distance L
8.7
apart where n is any positive integer and Ag is the wavelength in the guide, the forward power flowing from port 1 through the two holes and in the direction of port 4, will add and produce an output. The backward waves in the secondary guide propagating toward port 3 are out of phase with each other and cancel to produce no output at that port. Thus the coupler acts in a directional manner.
L=(2n+1>\
I.
4"
POWER IN . \.. Primary waveguide.
PO~ i
- - ----""' - I
--1Iloo--'- -
.1
-"' -
I
POWER OUT / -
-
-
-
_1 PORT 2 I
------Ii' Irl ; =~-= ~-.: ~ =---=:= =added PORT 4
Figure 8.29
-----------C 1 2 3 D--....
-
POWER INPUT
w
C>
--- -- -- --A
B ----lIoo-
---~-----~--
~'A. 9 / 4-1-'A. 9 /4-1
\ POWER OUTPUT PRIMARY WAVEGUIDE
0.8---~---...-----------.
« ~
o
>
~!:(
~~
5!z u.. w f2 ~ wo:: C>::> «(J)
0.4
~~ >:iE
~t;c
0::
w > w 0::
1.5
1.0 ACTUAL FREQUENCY DESIGN FREQUENCY
}-'igure 8.30
Improvement in useful bandwidth with three-hole waveguide coupler ( a) Two-hole coupler ( b) Three-hole coupler
Reproduced by permission or HMSO
PORT 3 cancelled=:=' =.(
Secondary waveguide
149
i Output used to measure forward power in primary waveguide
Operation of simple two-hole waveguide directional coupler. Ag == Wavelength in the waveguide
The useful bandwidth of the coupler is clearly limited by the fixed hole separation and phase cancellation which can be achieved. The bandwidth can be increased by moving to a three-hole coupler as shown in Figure 8.30 where the amplitude of the leakage wave through the middle hole 2 is twice that of the other two holes 1 and 3 [43]. The reverse voltage divided by the forward voltage in the secondary guide is a measure of the useful performance of the directional coupler. I t is plotted against frequency divided by design frequency in Figure 8.30 and the increase in useful bandwidth of the three-hole coupIer can be seen in Figure 8. 30b. There are two main criteria which define the performance of a directional coupler:
Coupling factor: With matched loads on ports 2 and 3 in Figure 8.29 the coupling factor is defined as 8.8
Typical values for coupling factor given by Dunlop and Smith [44J are -3, -10, -12 and -20dB.
Directivity: The existence of the coupling holes themselves will produce reflections in both the primary and secondary guides and there will not be total isolation between ports 3 and 4. The directivity is a measure of the ratio of power at these ports for a matched load at port 2: directivity
==
1010g (P4/ P3)
8.9
For a good coupler design this val ue is in the region of30 to 40 dB. Most waveguide couplers are limited to octave bands and a commercial example of a high performance coupler is the HP 752A series [45]. It is possible to put two waveguide couplers together as shown in Figure 8.31 and install diode Diode detector produces DC voltage proportional to reverse power "C ~
MATCHED LOAD
4
FORWARD POWER-TO-LOAD
Vr
_ _ _~I
~
I
:~:::;-~I_~B ~ ./'1~ --1 'A.g
"4
MATCHED LOAD
Figure 8.31
Vf
REFLECTED POWER FROM LOAD
0 Diode detector produces DC voltage proportional to forward power
Dual directional coupler
Reproduced by permission or HMSO
150
A HANDBOOK FOR EMC TESTING AND MEASUREMENT
detectors in the forward- and reverse-coupled ports to enable the forward and reverse power to be deduced from the voltages Vi and Vr • Then the reflection coefficient p is
DC signal is proportional to reflected power
DIODE DETECTOR
8.10 and
VSWR == 1 + P
8.11
1-p
8.13.2.2 Coaxial couplers The need for ease of connection and maximum bandwidth in transmission lines and components for use in susceptibility testing, drives the EMC engineer to the widespread use of high-grade lowloss coaxial cable where possible. Thus directional couplers based on coaxial rather than waveguide designs are useful. Kraus [46] gives an explanation of the functioning of a coaxial directional coupler which uses small lengths of wire parallel to the centre conductor within the coaxial system and terminated at opposite ends by a resistance equal to the characteristic impedance of the wire. The other ends of .the wires are brought through the outer sleeve as shown in Figure 8.32 to form the forward and reverse voltage ports. A commercial example of a broadband 2-18 GHz directional coupIer with precision type N coaxial connectors is the HP 7725 [47] which has a coupling factor of 20 dB and a directivity of around 30 dB with a power handling capacity of 50 W. Such a coupler is ideal for use with broadband network analysers or as a component in levelling broadband power amplifiers for EMC susceptibility measurements as shown in Figure 8.33.
Forward power to hom is measured and fed back to signal source as a levelling loop.
Figure 8.33
Dual directional coupler used to level forward power and provide reflected power amplifier shut down signal when conducting radiated susceptibility test SHUNT COAXIAL HYBRID RING
I2Zc
1------11
E - PLANE WAVEGUIDE HYBRID RING 3A.g 4
3
t 1
E
OUTPUT FOR WAVE TO THE LEFT ( REFLECTED POWER) INNER CONDUCTOR
3
---~-
OUTPUT CONNECTOR
fo LOAD
Za = impedance of waveguide =wavelength in wavegu ide
Ag AUXILIARY CONDUCTORS
\ OUTPUT CONNECTOR
Figure 8.34
Coaxial and waveguide hybrid rings
Reproduced by permision of Prentice Hall Computer Publishing AUXILIARY CONDUCTORS
Figure 8.32
OUTPUT FOR WAVE TO THE RIGHT ( FORWARD POWER)
Coaxial directional coupler
Reproduced by permission of McGraw-Hill
8.13.3 Hybrid rings, circulators and isolators 8.13.3.1 Hybrid ring A four-port hybrid ring can be constructed In ei ther waveguide or coaxial transmission lines as
in Figure 8.34. I t has the property that there is no direct coupling between arms 1 and 4 or between 2 and 3 [48]. Power flows from port 1 to port 4 only by virtue of reflections generated by mismatches at ports 2 and 3. This device can be used as automatic amplifier output protection as a high-power load can be placed at port 4 and the amplifier power entering port 1 will be automatically diverted safely to this load should a mismatch occur at the normal load port.
RECEIVERS, ANALYSERS AND MEASUREMENT EQUIPMENT
8.13.3.2 Circulator A circulator is a nonreciprocal transmission device that uses the property of Faraday rotation of the wavefield in a ferrite material [42]. I t is a multiport waveguide junction in which the power can flow only from the nth port to (n + 1) th port in one direction, see Figure 8.35. The design uses a combination of side-hole directional couplers and nonreciprocal phase ferrite shifters. This device can also be used to protect the ou tpu t of expensive microwave amplifiers when conducting EMC radiated susceptibility tests. PORT 4
PORT 3
PORT 1
PORT 2
Figure 8.35
Four-port circulator
microwave amplifier tubes from dalnaging the input signal generator. The isolation is gained however at a price, namely the slight forward power insertion loss, which might typically be up to 1 dB at frequencies below 26 GHz. Isolators are a useful component in constructing robust high-power microwave susceptibility test systems which can withstand the daily 'abuse' from mismatches caused by wavefield reflections inside shielded rooms, and the occasional cable fa ul t or broken waveguide flange.
8.13.4 Protection devices -
Waveguide isolators also use Faraday rotation and incorporate a 45° waveguide twist to isolate the forward and reverse power in the guide [41]. Dunlop and Smith [44] explain the principle of operation and the usual waveguide configuration is shown in Figure 8.36. The resistive vane is oriented in the plane of the reflected wave E-field acting as the built-in reflected power load. These isolators are often used to prevent the reverse power flow from the inputs of high-power
REFLECTED POWER 0
45 rotated clockwise E fields of reversed power absorbed in resistive vane
All the devices described above can be used to protect the expensive investment of high power broadband amplifiers and signal sources which are used in EMC testing. They are inexpensive by comparison, and their specification should be considered carefully together with the amplifiers and antennas which make up the test system. A working knowledge of their function and capabilities is essential for the EMC engineer who is entrusted with carrying out susceptibility testing while protecting the amplifiers and keeping the repair time for this equipment to an absolute mInImum.
8.14.1 Introduction When the number of tests being performed in a facility to a single or restricted range of specifications increases, EM C engineers consider the optio.n of automating the process. A company whose products are all tested to EN 55022 might find it economically worthwhile investing in the microprocessor- based measuremen t eq uipmen t and desktop or minicomputer controllers required. They could then purchase or write the software needed to carry ou t these particular tests and ensure tha t all tests were carried ou t identically. In addition, automation of EMC testing has a number of potential advantages. • •
,.>
"'I
h:,. -,. l'
IN
~ 45
E,."- ~ 1~
Figure 8.36
0
rotated clockwise
~ 450 rotated
•
anti-clockwise
E-field rotation in waveguide isolator
Reproduced by permission
conclusion
8.14 Automatic EMC testing
8.13.3.3 Isolators
POWER
151
or Van Nostrand
Reinhold
I t increases the work through pu t by speeding up measurement. I t can be used to deskill the testing job, although this is probably a shortsighted economy because EMC testing is best undertaken by staff who will be alert to unusual phenomena and be capable of their correct interpretation. It allows reports to be produced quickly and cheaply to a standard format.
In most automated EMC test facilities the testing cost can be kept low and the job satisfaction for the
152
A HANDBOOK FOR EMC TESTING AND MEASUREMENT
test engineer improved as the equipment takes care of the routine aspects of measurement, leaving the engineer free to concentrate on quality issues and experimental or diagnostic procedures which need the flexibility of the human mind.
8.14.2 Automated emission testing Fully automatic, blind EMC testing is not possible, nor perhaps desirable. Compu ter assisted or semiautomatic testing which cuts down test time and drudgery, most certainly is. The development of suitable software that can cope with all the decisions that need to be made during an emission test is not trivial. Consideration must be given to Frequency stepping increment Scan speed and the assembly of multiple scans to fully cover the bandwidth of a pulsed signal spectrum Selection of bandwidth according to test specification, measurement frequency, or signal conditions Alteration of detector function and classification of signal types Real-time manipulation of input and IF attenuation depending on signal level Determination or recognition of background ambient signals and appropriate marking (when making open-range measurements) Swi tching antennas or sensors with frequency, or location on or around an EUT Adjusting the height of an antenna to find maximum signal strength from an EDT on an open site Driving the turntable in steps during an opensite test Periodic calibration routines Data storage and manipulation Storage and retrieval of dozens of sensor transfer functions Storage, retrieval and comparison of specification limits with measured levels Calculation of out of limits signals, data presentation and storage Plotting rou tines and prin ting of test instrumen t parameters to meet QA requirements Filing of archive material in the form of removable discs, tapes or cassettes. Many of these factors are considered in a paper by Sikora [49J dealing with the production of EMC test automation algorithms and methods of performing computer controlled EMI compliance tests are examined by Derewiany et at. [50J. A brief description of some hazards to be avoided with automated EMI testing IS given by Archambeault [4J.
Most manufacturers of EMI receivers and spectrum analysers for use in E~1C emission testing supply computer assisted test suites with software tailored to military and commercial equipment test standards. The verification and documentation of these software packages are most important as they form part of the traceable quality chain which underpins statements of measurement accuracy and repea tability.
8.14.3 Automated susceptibility testing I t is more difficult to automate radiated and conducted susceptibility testing because the control computer must be able to monitor the performance of the EDT directly, to stop frequency scans and carry out investigative procedures at susceptible points. In addition, the number of connections between signal sources, modulation sources, power amplifiers and antennas or injection probes mean that the testing will inevitably be time consuming. While compu ters can be useful in setting up test configurations and in controlling some signal sources and feedback levelling loops, susceptibility testing is labour intensive as it is of a primarily investigative nature.
8.14.4 In the future? There can be no doubt that computer technology will become increasingly powerful, cheap and available. Software will also improve and exciting prospects lie ahead in the field of EMC design, modelling and testing. The bringing together of large complex design models and responsive EMC test algorithms within the framework of a knowledge-based architecture may result in much more careful and discriminating tests being carried out on particular types of equipment, or the execution of specific tests designed to reveal the true EMI characteristics of individual items being tested.
8.15 References CONNEY, M. and ERICKSON, S.A.: 'Considerations in the design of a multi-bandwidth, sensitive, wide dynamic range, wide frequency range EMI receiver'. Proceedings of IEEE symposium on EMC, 1990, pp. 634-637 2 EMI meter NM17-27 instruction manual. Carnel Labs. Corp, 21434 Osborne Street, Canoga Park. CA 91304, USA, p. 5.15 3 WHITE, DJ.: 'A handbook series on electromagnetic interference and compatibility, volume 2 test and measurement procedures'. Don White Consultants, Germantown, Maryland, USA, pp. 3.66-3.86
RECEIVERS, ANALYSERS AND MEASUREMENT EQUIPMENT 4 ARCHAMBEAULT, B.R.: 'Hazards to avoid with automated EMI testing'. Interference technology engineer's master 1985, pp. 118-122 5 DUFF, W.G.: 'Fundamentals of electromagnetic compatibility'. Interference Control Technologies Inc, Gainsville, Virginia, USA, pp. 319-320 6 'TD2301/TD1301 High performan~e digitizer systems with Fourier analysis capability'. Tektronix catalogue, 1990, pp. 162-163 7 STURLEY, K.R.: 'Radio receIver design'. (Chapman & Hall, London, 1949) 8 LANGFORD-SMITH, F.: 'Radio designers handbook' (Iliffe Books, London, UK) 9 'AN 246-1/5952-8815 Optimising the dynamic range of the HP 3585A spectrum analyser'. HP, Winnersh, Wokingham, Berks, UK 10 'AN 150/5952-1213 Spectrum analyser basics'. HP, Winnersh, Wokingham, Berks, UK 11 'AN 150-1/5954-9130 Spectrum analysis AM and FM'. HP, Winnersh, Wokingham, Berks, UK 12 'Signal strength & interference measurements'. News special 1, Rohde & Schwarz, Ancells Business Park, Fleet, Hampshire, GU13 8UZ, UK 13 CISPR EMI receIvers 8573A/8574A. HewlettPackard 1990 product catalogue, p. 125 14 85685A preselector, Hewlett-Packard 1990 product catalogue, p. 126 15 BELDING, R.L.: 'RF preselection requirements for spectrum analysers'. Proceedings of IEEE symposium on EMC, 1985, pp. 94-97 16 Microwave EMI receiver EMC-60/FE-60. ElectroMetrics Ltd, I vel Rd, Shefford, Beds, SG 17 5JU, UK 17 Band rejection filters TRF11-TRF15. Electro-Metrics Ltd, Ivel Road, Shefford, Beds, SG17 5JU, UK 18 'High and low-pass RFI filters, instruments, components and accessones for the RFI/EMC engineer'. Data sheet 6625A, Solar Electronics Co, 901 North Highland Ave., Hollywood, CA 90038, USA 19 KEISER, B.: 'Principles of electromagnetic compatibility' (Artech House, Norwood, MA, 3rd edn.) p. 347 20 Carnel Labs. Corporation (formally Eaton) 91263-1 stand-alone 1 GHz impulse generator 21 EMI meter NM17-27 with built-in impulse generator instruction manual. Carnel Labs. Corp, 21434 Osborne Street, Canoga Park, CA 91304, USA, p. 2.1 22 SP-P-90203 Product-specific EMC standard for the Tornado military aircraft, Panavia, Europe 23 Disturbance analyser Series 606, Dranetz company, USA 24 Digitising oscilloscopes HP 54120 series, Hewlett Packard 1990 test and measurement catalogue. HP, VVinnersh, Wokingham, Berks, RG11 5AR, UK, p. 60 25 'Fundamentals of digital oscilloscopes and waveform digitising, A technical tutorial'. 1990 product catalogue, section IV-I. LeCroy, 700 Chestnut Ridge Road, Chestnut Ridge, NY 10977-6499, USA 26 1990/1 measurement equipment catalogue. Rohde and Schwarz UK Ltd, Ancells Business Park, Fleet, Hampshire GU13 8BR, UK, pp. 384-387 27 'Power sensors' in Test and measurement catalogue
28
29
30
31 32
33 34 35 36 37 38 39 40
4J
42
43
44
45 46 47 48 49
50
153
1990. HP Winnersh, Wokingham, Berks, RG 11 5AR, UK, p. 204 1990/1 measurement equipment catalogue. Rohde and Schwarz UK Ltd, Ancells Business Park, Fleet, Hampshire GU13 8BR, UK, p. 428 Directional power meter RFM 100 in Rohde and Schwarz measuring equipment catalogue 1990/91, p.429 WHITE, DJ.: 'A handbook series on electromagnetic interference and compatibility, Volume 2 test and measurement procedures'. Don White Consultants, Germanton, Maryland, USA, p. 3.92-3.103 PITT, A.D.: EMC guidelines, ST23357. British Aerospace Dynamics Ltd, Filton, Bristol, UK, 1979 'Guide to broadband power amplifiers'. Amplifier Research, Rev. 0190, 160 School Hse Rd, Souderton, PA 18964-9990, USA SCHEVCHIK, V.N.: 'Fundamentals of microwave electronics'. (Macmillan, NY, USA, 1963) pp. 186-203 TWT amplifiers. Keltec Florida Ltd, PO Box 862, Shalimar, FL 32579, USA Fast pulse modulator, HPl1720A. HP 1990 catalogue, p. 392 Pulse generator, PM 5771, 1 Hz-100 MHz. Philips Instruments UK Pulse generator PG 73N, 0.ls-10ns. Lyons Instruments, Hoddesdon, Herts, UK Analogue function generator HP 3212A. HP 1990 catalogue. p. 428 Function generator AFG. Rohde and Schwarz measuring equipment catalogue 1990/91, p. 90 Arbitrary function generator 9100, 1990 product catalogue. LeCroy, 700 Chestnut Ridge Road, Chestnut Ridge, NY 10977-6499, USA 'The hows and whys of arbitrary function generators, A technical tutorial'. 1990 product catalogue, section IV-48, LeCroy, 700 Chestnut Ridge Road, Chestnut Ridge, NY 10977-6499, USA LIAO, S.Y.: 'Microwave devices and circuits'. (Prentice Hall, Englewood Cliffs, NJ, USA) pp. 158-169 GLAZIER, E.V.D. and LAMONT, H.R.L.: 'The services textbook of radio, volume 5, transmission and propagation'. (HMSO, London, UK, 1958) p. 225 DUNLOP, J. and SMITH, D.G.: 'Telecommunications engineering'. (Van Nostrand Reinhold, UK) p. 288 'Waveguide directional couplers HP752 series. HP 1990 catalogue, p. 345 KRAUS, J.D.: 'Electromagnetics'. (McGraw-Hill, 3rd edn.) p. 420 'Coaxial directional coupler' HP 772D, HP 1990 catalogue, p. 344 'Hybrid ring, reference data for radio engineers' in 'ITT handbook' (Howard W. Sams) pp. 25/18 and 25/19 SIKORA, P.A.: 'Writing a useful EMI test automation algorithm'. Proceedings of IEEE symposium on EMC, 1988, pp. 377-382 DEREWIANY, C.F. and KOWALCZYK, K.R.: 'Methods of performing computer controlled EMI compliance tests'. Submarine electromagnetic systems dept. Naval Underwater Systems Center, New London, CT 06320, USA
Chapter 9
EMC test regitrles and facilities 9.1 Introduction
possible to cover all these interesting topics, but information is widely available in the proceedings of relevant specialist groups sponsored by the lEE, IEEE and others. This chapter concentrates on the testing regimes and test facilities most generally in use for electromagnetic compatibility testing.
9.1.1 Main test regimes This chapter examines the three principal test regimes and facilities in which these devices and equipments are used to conduct EMC tests: testing in screened chambers, open-range testing, and 'low-level swept' and bulk current injection testing. The majority of standard EMC test work carried ou t on commercial and military electronic equipment falls into one of these three regimes.
9.2 EMC testing in screened cham.bers
9.2.1 Enclosed test chambers In conducting EMC tests to measure both equipment RF emissions and susceptibilities, it is desirable to isolate the test space from the outside electromagnetic environment. If arbitrarily varying ambient RF signals (up to field strengths of 10 V /m in industrial areas) are allowed to mix with the signals of interest ofa mV/m or less from the EUT, it will be time consuming and almost impossible to separate out the signals which need to be measured to the levels required by the specifications. Equally, it is undesirable (and illegal) to radiate high field strengths across whole bands of frequencies when conducting radiated susceptibility testing. rrhus the use of screened chambers became widespread as a result of military procurement EMC requirements which came to the fore during the late 1960s and early 1970s. Much of the custom and practice with regard to the use of screened chambers has been built up on the basis of standards imposed on the aerospace/military equipment industries such as MIL STD 461/2/3. Many of the tests currently being carried out are either the same, or directly related to those in early specifications such as this. Of course, increased knowledge and test experience gathered over 20 years have been incorporated into modifications to test methods, or to new tests such as those relating to BCI (bulk current injection) contained in the UK DEF STAN 59-41 DCS02. The advent of widespread governmental or self regulation of EMI in the early 1980s connected with commercial electronic products such as digital computers, brought about the widespread use of EMC tests based in some cases on earlier CISPR type methods. These new tests were largely concerned with the control of radiated and conducted emissions (FCC part 15, VCCI, etc) and made use of open sites rather than screened rooms as suggested for testing military equipment.
9.1.2 Special testing There are of course other specialised types of electromagnetic testing which are either related to the type of electromagnetic threat employed or signature being measured, or to the scale of the testing on large systems. The special testing techniques related to threat or signature type are •
•
•
Special electromagnetic threats HIRF (high intensity RF) tests Ligh tning strike tests NEMP (nuclear electromagnetic pulse) testing EMP (electromagnetic pulse) and HPM (high power microwave) testing Special signature testing Tempest (emission security) tests Spacecraft EM 'cleanliness' (from DC magnetic fields to millimetric waves) Special test techniques related to the scale of a test include whole-ship testing whole-aircraft clearance telephone switching centres large distributed computer facilities large communication centres transportation signalling centres.
In some cases, it is possible that these large systems need to be tested for EMC, NEMP/EMP, lightning and Tempest. The range of testing requiren1ents that facilities must sometimes meet leads to increasing facility cost, and the procurement or licensing authorities may not impose the full range of specifications for all these electromagnetic effects on a given system unless absolutely necessary. In a book such as this devoted to generalised EMC testing it is not 154
EMC TEST REGIMES AND FACILITIES There are therefore almost two parallel streams of EMC testing with each general approach having its own advantages and penalties optimised for the type and cost of testing which needs to be carried out in the civil and military fields. I twill be interesting to see if the sharing of testing information between these two communities leads to a consensus oh the best and most cost-effective test methods for particular types of equipment. The practice of testing in screened rooms has then largely been established by the need to test equipment to military specifications where the EUT performance is critical, the unit cost is high and the project budgets are large. In these circumstances it is possible to build elaborate high cost (> £5 M) test facilities on the basis of large, sometimes very large (1 OOx 80 x 20m), shielded enclosures in which to carry out the work. White [1] gives examples of large facilities constructed for ballistic missile and aircraft testing. The only other product areas which have invested in such large facilities are the automotive and space industries and to a lesser degree the civil aircraft sector. Screened test chambers used for EMC testing can be divided into four types: standard shielded chambers, shielded and anechoic chambers, mode-stirred chambers, and novel facilities. Screened chambers made from metal sheets are usually constructed in the form of a rectangular box with parallel sides, but other configurations such as cylinders and tapers have been built for special req uiremen ts of items to be tested or to minimise reflectivity. Screened chan1bers are an expensive item of equipment and costs typically range from £20,000 for an 8 x 6 x 4 m room to more than £20 M for one in which a large system could be tested.
9.2.2 Standard shielded enclosures Standard screened rooms are mostly constructed from thin galvanised sheet steel and wood sandwich modular panels that are clamped together with special pressed metal strips to produce an RF-tight joint. This permits the building of structures of variable size which can be dismantled and re-sited if required. Other types of construction include self supporting allwelded sheet steel, copper sheet, metal foil, and 'chicken wire', perforated sheet or expanded steel mesh. Each construction technique provides a certain degree of shielding for a given enclosed volume at a particular cost. Careful analysis of the shielding requirement to carry out a given EMC test must be undertaken if the correct performance of room is to be achieved for the minimum cost.
155
Farsi [2] gives a good introduction to shielding theory and shows that for plane waves the shielding effectiveness is S
==
R
+A +B
(all in dB)
where R == reflection loss A == absorption loss B == internal loss The internal loss factor B is usually neglected if the absorption loss is significan t [2]. The reflection loss IS
R where
==
168
+ 10logb/fll (dB)
9.1
f == frequency
in MHz b == material conductivity Il == material permeability
The reflection loss for a plane wave is a function of the material conductivity and is high at low frequencies and reduces at higher frequencies. Expressions for the reflection losses of the electric and magnetic field components of a wave are given in Reference 2 which allow the loss to be calculated for nonplane waves such as occur in the near field of a source. The absorption loss is dependent on the type of rna terial and its thickness and is
A
==
3.34tjfbll (dB)
9.2
where t == material thickness in mils (,thou') f == frequency in MHz b == material conductivity Il == material permeability White [3J, quoted by Farsi [2J, gives a table for the absorption loss per thousandth of an inch thickness for materials commonly used to construct shielded rooms, see Table 9.1. The configuration of a typical small screened room complex used for EMC testing is shown in Figure 9.1. I t has the EMI receivers, power amplifier 'transmitters' and the EUT shielded each from the other, and all from the outside environment. I t is possible to have a shielded room only for the EUT in a single cell facility, but the isolation scheme shown in Figure 9.1 is useful for multicell operation in a large test facility complex where emission and susceptibility testing on different EUTs may be running simultaneously. The practical performance of screened rooms is unlikely to be determined by the plane wave propagation through the shield material but rather by leakage through panel seams, corner joints, penetration panels and access door surrounds. Typical shielding effectiveness of a moderate sized modular room of 12 x 7 x 5 m with large 4 x 4 m doors, air vents and multiple penetration panels is shown in Figure 9.2.
156
A HANDBOOK FOR EMC TESTING AND MEASUREMENT
Table 9.1
Electromagnetic characteristics of metals and absorption loss with thickness
Metal
Silver Copper Gold Aluminium Zinc Brass Nickel Bronze Tin Iron Steel (SAE 1045) Stainless steel
Absorption loss, dB per 0.001 in 1 MHz 10 kHz 100 Hz
Relative conductivity
Relative permeability 100 kHz
0.34 0.03 0.28 0.26 0.17 0.17 0.15 0.14 0.13 4.36 3.32 1.47
1.05 1.00 0.70 0.61 0.29 0.26 0.20 0.18 0.15 0.17 0.10 0.02
1 1 1 1 1 1 1 1 1 1000 1000 1000
0.03 0.03 0.03 0.03 0.02 0.02 0.01 0.01 0.01 0.44 0.33 0.15
EMI RECEIVER SHIELDED ROOM
EUT POWER LINE FILTERS
\ POWER AMPS & SIGNAL ____ SOURCES I
I
I
I
L:N:AJ__--\.\---.. .l AIR VENTS
Figure 9.1
~POWER
\
POWER LINE FILTERS
AMPLIFIER SHIELDED ROOM
TEST AREA SHIELDED ROOM
Compact multiscreened room EMC test cell
3.40 3.00 2.78 2.60 1.70 1.70 1.49 1.42 1.29 43.6 33.2 14.7
together. Figure 9.3 shows the attenuation afforded by a 6 mm-thick mild steel all-welded construction with two moderate sized doors (2.5 and 3.5 m sq.) and the usual complement of air ven ts and penetration panels [4 J. The shielding performance of screened rooms to be used for EMC testing is almost always measured according to the method specified in MIL STD 285 [5]. The original standard was introduced in June 1956 and is still largely unchanged. Cardenas [6J has conducted experiments to define the most practical measurement methods based on MIL STD 285, which requires attenuation measurements to be made using a range of identical antenna sets similar to those shown in Figure 9.4. 140
Improved low-frequency shielding can be obtained by constructing a chamber from large heavy gauge steel plates which are all welded
100
co
"'0
z
o ~
120
co
120
"C
Z
0
80
zw
60
~ 100
~ ::J
~~
80
~.
c
60
~
40 i--_"""--_..J-_--'-_.....&_----I......._""'-_~ _
w
o oa: w Z w
o
«
Figure 9.2
40
-
6 mm welded steel plate - typical modular screened room
20 ___'
1kHz 10kHz 100kHz 1MHz 10MHz 100MHz 1GHz 100Hz 1000Hz
00
-
0
1
FREQUENCY
Typical attenuation oj modular steel-screened room. Dimensions: 12 X 7 X 5 m; Access: Doors 4 X 4 m and 1 x 2 m,' Attenuation: measured at apertures (doors, attenuvents, etc.)
Reprod uced by permission of BAe Dynamics
Figure 9.3
10
100
1k
10k 100k 1M 10M 100M 10 FREQUENCY Hz
100
Good low-Jrequency performance oj welded steel room. Welded room type: cylindrical 14m dia X 12m high. - - 6rnm welded steel plate; - - - typical modular screened room
Reprod uced by permission of BAe Dynamics
EMC TEST REGIMES AND FACILITIES
157
Shielded enclosure
Shielded enclosure
72" where practicable
(b)
(a) Electro-metrics , RVR -25M
Shielded enclosure
Shielded enclosure I I
i!
min. 2" Receiver
I
Figure 9.4
100
co
"0
Z 80 0 i= « 60 :::>
Planewave specification
Electric Field sPecification
z
~
40
20 0.001 0.01
0.1
1.0
10.0
100 1,000 10,000
FREQUENCY MHz
Figure 9.5
72" where practicable
~ ~
·s..
1 ( 1 .
~A.
12-8-2
li-~
( d) (c) Typical equipment corifigurations Jor measurement oj screened room attenuation. (a) LF magnetic Jield test equipment, (b) Planewave « 1 GHz) test equipment ( c) Electric Jield test equipment (d) Planewave (10 GH;~) test equipment
Strictly, the MIL STD 285 requires the Tx antenna to be outside the room in all cases to minimise the swamping effect of ambients, but at VHF and above, any signal which has leaked into the room sets up standing waves which makes it difficult to locate the source of a leak. At these frequencies the Rx antennas have some discrimination against ambients and thus it may be better to put the receive antenna outside the shield [6J. The National Security Agency in the USA issued a specification NSA65-6 for the performance of screened chambers constructed from metal foil which is shown in Figure 9.5. Although the cheaper foil-shielded rooms covered by this specification were not intended for EMC testing they are in fact quite adeq ua te for this purpose with attenuation of 100 dB from 1 MHz to 10 GHz. Well-constructed shielded rooms are very efficient
w
!
Example oj screened room performance requirement. Specifications NSA 65-6 100 dB
at isolating the test volume from the outside electromagnetic world. Both radiated susceptibility and emission EMC tests can be conducted without causing RF jamming problems to outside communications or being confused by the penetration of ambient electromagnetic noise. There is, however, a major drawback in the use of shielded enclosures. Because the walls, floor and ceiling are highly conductive and usually orthogonal or parallel, the room becomes a high-Q, multifrequency resonator where con1plex standing wave patterns can be set up along all three principal axes. The exact standing-wave patterns will depend on the shape of the box, the positions of the source antenna, the measurement antenna (or EUT) and the frequency. Typical of the variation in field strength which occurs inside these rooms is that shown in Figure 9.6 [7]. The test configuration is that specified by MIL STD 462 RS03 and the lower panel of the diagram shows that the variability in the field strength at the E UT is at least a factor of ± 1O. This order of uncertainty will be present in all radiated susceptibility and emission ITleaSUrements which are made inside a simple undamped shielded enclosure. A study of the effect of standing waves at frequencies below 100 MHz in a shielded enclosure against equivalent open-range measurements has been made by Stuckey et al. [8J. Other investigators [9J have made confirmatory measurements of predictions generated by a 2D computer model of reflection proce,sses in chambers. This model is based on amplitude and phase reconstructions from distributed Hugens
158
I
A HANDBOOK FOR EMC TESTING AND MEASUREMENT
GROUND PLANE
I
EUT
I
I
1 metre
1 FIELD GENERATING
~ / ANTENNA
a..------
\'''-'''l~
;. 1 metre
;.
1 metre
1 metre
t
FIELD MEASURING ANTENNA
~~
SHIELDED ENCLOSURE
WA\
T~ CONTROL FIELD
~ 1 fetre
=10 VIm
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_
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c
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sources on the chamber walls. A visual impression of the complexity of a typical standing-wave pattern at a single frequency can be gauged by studying the model graphical output given in Figure 9.7. This shows the calculated standing wave pattern at 180 MHz with a centrally positioned source in a 5 X 5 m (floor area) reflective shielded chamber. I t is possible to exploit the standing wave properties of a simple shielded room to generate high field strengths for limited input power [1 OJ by using a special commercially available susceptibility antenna, known as the Cavitenna [11 J. The device, when fixed to the inside wall of the screened room, excites the entire room using the structure as a ground plane to increase the radiator's effective electrical size. Thus a 1.2 mlong antenna can produce equivalent field strengths to a 5 m log periodic at a frequency of 30 MHz. The antenna is set up above the ground plane bench as shown in Figure 9.8a; with the aid of a levelling loop amplifier it can efficiently produce the field strength from 30 MHz to 1 GHz shown in Figure 9.8b with an average value of 25 V 1m for an input power of between 1 and 7 W. Other techniques have been tried which aim to
-I
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Variation offield strength at EUTfor constant 10 Vim control field. Location of aerials for radiated susceptibility testing shown for MIL STD 462 test method RS03
PLAN VIEW
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Figure 9.7
G'Ialculated E-jield standing-wave pattern in square cross section screened room. 5 m-square chamber with central source at 180 MHz
Reproduced by permission or NPL/HMSO
(b) Figure 9.8
~]Jicient field strength generation in undamped screened room using wall-mounted ,cavitenna)
EMC TEST REGIMES AND FACILITIES mInImise the standing-wave problem in screened rooms without the introduction of large volumes of costly radio absorbent material. It has been suggested [12J that asymmetrical shaped chambers w here few of the reflecting surfaces are parallel can be helpful in controlling the allowable chamber modes, resulting in a reduction in typical field variability in a rectangular chamber from +20 to -40 dB to less than ± 10 dB. The specification and purchase of a screened room suitable for EMC testing is an important task [13J and involves considerations which lie beyond the immediate issues of size, shielding performance, delivery and cost. I t is important to consider the through-life operating costs of the enclosure as daily use of this expensive hardware will inevitably result in damage and maintenance problems. These could be costly if the room and particularly the doors and door seals are badly designed or of poor q uali ty. Salati [14 J details cost-effective maintenance for shielded enclosures and suggests appropriate repair procedures.
9.2.3 RF anechoic screened chambers 9.2.3.1 Partial solutions The most obvious way of suppressing the standing waves inside a screened chamber is to cover all or most of the reflecting surfaces with radio absorbent material which can significantly attenuate the reflected waves and prevent modal patterns from forming. Unfortunately this solution has two clear disadvantages: (i) (ii)
The cost is extremely high largely owing to the material cost. The working space in the room can be substantially reduced by the absorber and thus a larger and more expensive chamber is required.
The full RAM-lined large chamber solutions are only used in industries where the investment can be justified, such as the aerospace or automotive sectors. Small to medium sized chambers which are more widely used in the electronics ind ustry are often not equipped with RAM on cost grounds unless mandated by appropriate test standards. This has led to a number of low-cost schemes being proposed [9, 15-17J which use some RAM in special configurations to mi tiga te the worst effects of standing waves on measurement accuracy. One concept [15 J is that of placing absorber only on the back wall of the room, and building an absorbing hood around the EUT as shown in Figure 9.9. Such a configuration reduces the sharp peaks and troughs infield strength
159
REFLECTED WAVES
SCREENED ROOM
Figure 9.9
ABSORBER WALL
Hooded test area configuration in screened chamber
produced by the standing waves to manageable proportions. When used in conjunction with an automatic field levelling loop, field variability of around ±2 dB is claimed from 50 to 100 MHz and better than ± 1 dB up to 550 MHz in a 4 X 4 X 2.5 m chamber. This test configuration is useful for testing an E UT up to a half-rack size. Another approach [16J is to damp out the resonances by extracting energy with RAM at standing wave sites of maximum field strength within the room. The undamped frequency response is shown in Figure 9.10 for a 2.5 X 2.5 x 5 m room with a first resonance at 75 MHz. Figure 9.11 shows how the first resonance is reduced as the loading provided by the absorber is increased with increasing conductivity. Finally, this brief survey of low-cost techniq ues to· reduce standing wave problems in screened rooms by using only small amounts of RAM, includes a suggestion made in 1981 [9J. This involves a combination of the asymmetrical room concept and that of strategically sited RAM in an elliptical
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Frequency response of shielded room excited with small magnetic loop
160
A HANDBOOK FOR EMC TESTING AND MEASUREMENT
9.2.3.2 Full RAM solutions Screened chambers with nonparallel walls, barnes and angled planes are all a ttempts to either minimise the amount of RAM required, or in the past to compensate for poor attenuation properties of RAM available at that time. With the development of high-quality broadband pyramidal RAM made by impregnating a carbon carrying latex into a flexible polyurethane foam plastic [18J it is possible to produce simple rectangular chambers with good reflection attenuation at an acceptable cost. Anechoic chambers can be divided into two kinds:
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Figure 9.11
Suppression of screened room resonance with increasing RAM loading at the room centre
chamber. The scheme uses a RAM column sited at the secondary focus. If the radiating antenna is placed at one focus 'it can be seen in Figure 9.12a that the calculated field strength peaks at the other owing to the range/phase invariance properties of the bounding ellipse. I t should then be possible to remove energy efficiently from the system with relatively small amounts of RAM placed at this importan t location as indicated in Figure 9.12 b.
\
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* Location of RAM
Figure 9.12
RAM column centre at second focus of ellipse
Novel elliptical chamber design combining asymmetric chamber and strategically sited RAM (aj Focal source at 30 MHz - no RAM (b j Small amounts of RAM placed as shown
Reproduced by permission of NPL/HMSO
(ii)
Full anechoic facilities with quiet zones of up to 1-3 m across and reflectivities as low as -35 dB. These are specialist chambers used mainly for measuring polar diagrams of an tennas moun ted on vehicles, aircraft and spacecraft. Semianechoic chambers that reduce standing waves to manageable proportions such that reliable EMC measurements may be made. These chambers sometimes have a reflecting ground plane floor and simula te an openfield site for making CISPR, FCC type measurements [19 J.
In the following text only chambers of the second kind which are sufficiently anechoic to conduct successful EMC tests will be addressed. The performance of an EMC semianechoic chamber (SAC) can be gauged by reference to a typical example devised and built by IBM [20]. The screened chamber dimensions are 10.5 x 6.5 x 4 m with HPY-24 pyramidal absorber [21] on three walls and the ceiling. Special thin ferrite absorber NZ-31 [22J was placed on the far wall and door which was the closest to the EDT location. Standard EMC antennas such as biconics and tuned dipoles were used to investigate the SAC performance. Figure 9.13 shows the improvement in chamber resonances after the room was equipped with absorber. All the high-Q, peaks and troughs are suppressed leaving a chamber which is within ± 1.5 dB of the equivalent open-range performance at frequencies up to 230 MHz and within ±3 dB up to 950 MHz. Kuester et al. [23J have produced a theoretical model for calculating the reflectivity of absorber lined metal walls and this has been improved on by Gavenda [24]. He used the simplifying method of electrical images to allow calculation of the field strengths after multiple bounce reflections in a semianechoic chamber with partially reflecting surfaces. The model still has considerable limitations and cannot be used to completely design the optimum SAC. However the model is helpful at
EMC TEST REGIMES AND FACILITIES
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Antenna coupling plots made in IBM semianechoic chamber showing how absorber material suppresses screened-room resonances (a) 30-80MHz (b) 80-130MHz (c) 130-180 MHz (d) 180-230MHz ( i) Before RF absorber (ii) After RF absorber
Reproduced by permission or IBM
low frequencies where the absorber reflectivity is of the order of only -1 to -3 dB per bounce. l'he performance of RAM is defined as the level of reflected energy from a RAM covered surface as compared with that reflected from the same area of metal surface. The general construction of a block of pyramidal broadband absorber is shown in Figure 9.14. The height of the pyramids varies from a few cm to 4 m depending on its desired attenuation at low frequencies. Typical normal incidence reflected attenuation as a function of pyramidal absorber electrical thickness [25] is shown in Figure 9.15a with the off-normal reflectivi ty shown in Figure 9 .15b. Figures for the reflectivity of a typical high-performance pyramidal RA~1 as a function of frequency and pyramid height are given in Table 9.2.
FOAM PYRAMIDS
/
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Figure 9.14
Construction ofpyramidal foam RF absorber
Reproduced by permission of' Emerson and Cuming
161
162
A HANDBOOK FOR EMC TESTIN"C AND MEASUREMENT 50.......-----r-----T---~--..
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tivity of -20 dB is required at the lowest frequency of interest. By reading off the electrical length of the absorber for -20 dB reflection from Figure 9.15a and selecting the 150-in-high absorber in Table 9.2 one can calculate that an anechoic chamber fitttJd with this material would fulfil the ± 1.5 dB measurement uncertainty criterion at frequencies as low as 16 MHz. Semianechoic rectangular chamber ground reflection EMC test ranges with reflections which are sufficiently controlled to enable testing to FCC, CISPR and VDE requirements at 3 m are commercially available [19]. Typical calibration curves relating chamber performance to theoretical opensite range attenuation can be seen in Figure 9.17.
(b) Performance graphs for pyramidal foam absorber (a) Generalised reflectivity of eccosorb VHP at normal incidence ( b) 0fJ-normal reflectivity of eccosorb VHP. Eccosorb VHP is manufactured by Emerson and Cuming Ltd
Figure 9.15
Reprod uced by permission of Emerson and Cuming
Lawrence [18J derives a graph which relates field measurement uncertainty in an anechoic chamber to absorber reflectivity. This is reproduced in Figure 9.16 and indicates that for measurement uncertainties of 1.5 dB, absorber which has a reflecTable 9.2
Measurement uncertainty as function of chamber reflectivity
Example of the reflectivity ofpyramidal RAM as function of thickness and frequency
Guaranteed maximum reflectivity in dB Material
120 MHz
VHP-2 VHP-4 VHP-8 VHP-12 VHP-18 VHP-26 VHP-45 VHP-70
-30
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-35 -40
200 MHz
300 MHz
500 MHz
1 GHz
3 GHz
5 GHz
10 GHz
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Eccosorb VHP is manufactured by Emerson and Cuming Ltd. Suffix number is pyramid height in inches.
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Typical calibration curve ofactual site attenuation against theoretical site attenuation (Performance ojsemianechoic chamber designed to simulate open-area test site)
Reproduced by permission or Rayproor
SPECULAR REFLECTION POINT FOR REFLECTED RAY
DIRECT AND REFLECTED WAVEFRONTS IN PHASE
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Figure 9.18
Narrow angle tapered chamber ensures direct and reflected rays stay almost in phase
9.2.3.3 Tapered anechoic chambers This design of chamber usually permits lower frequency operation at a given reflectivity level than for a similar sized rectangular design. The source antenna is mounted at the apex of the taper section and for taper angles of 30 degrees or less the specularly reflected waves from the sides of the taper are at grazing incidence to walls resulting in little path difference between the direct and reflected radiation [18J . Under these conditions an almost unperturbed wavefront propagates down the taper to the cubical working volume as shown in Figure 9.18. This style of chamber is not particularly suitable for EMC measurements and is not widely used. I t is more appropriate to antenna and other measurements where a low reflectivity quiet zone rather than a semicontrolled large working space is preferable.
9.2.3.4 Conclusion -
chamber in which to carry out EMC testing, some assis tance is at hand. l'he commercial manufacturers have wide experience of actual installations and the obtainable performance; they can advise and are often able to offer turnkey packages which will minimise the risks to the customer. A number of review papers have been written dealing with chamber design criteria [26, 27J and chamberj anechoic material selection [28J.
1000
FREQUENCY MHz
Figure 9.17
163
9.2.4 Mode-stirred chambers The problem of controlling, or at least minimising, the measurement uncertainty caused by multiple standing waves inside a reflective screened roonl has been tackled in another way which is complementary to the 'absorption' approach. This alternative technique deliberately maximises the number of possible reflection modes which can be sustained within a highly reflective chamber in an effort to expose theEUT to all possible field strength values at each frequency of interest in the band being investigated. This concept is known as mode stirring or tuning and in the mid 1970s seemed to be a promising lowcost alternative to semianechoic chambers for EMC testing. Much has been written with regard to this concept [29-32J and a full understanding of its technical basis is not a simple matter. The method offers some potential advantages for EMC susceptibility testing in addition to lower cost than for a semianechoic chamber. The main advantage results from the wave polarisation being randomly varied in the isotropic homogeneous field which exists in the mode-stirred chamber [33J. With the E UT immersed in such a field there is no need to reorient it and repeat the test up to three times to cover all polarisations. The reverberating chamber, as it is sometimes called, is also capable of providing efficient conversion of source RF power to high field strength for performing tests on large objects or complete systems. There is then the clear potential
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For managers and EMC engineers who may be faced with the responsibility of specifying and selecting a screened anechoic or semianechoic
Figure 9.19
Example of mode-tuned enclosure systemJor EMC measurements (NBS-US.fl)
Reproduced by permission or NBS
164
A HANDBOOK FOR EMC TESTING AND MEASUREMENT
to reduce test time and cost as secondary benefits of using a mode-stirred chamber. A typical chamber configuration is shown in Figure 9.19 with the mode stirrer constructed in the form of multiple irregular shaped and sized paddle wheels which are fixed to the walls or ceiling and driven by stepper motors to ensure the maximum number of possible chamber modes are activated. The validity of this method depends on the maximum number of chamber eigellmodes being excited with a known mode density as a function of frequency. The maximum number of modes are generated when the chamber is large compared with the frequency and this technique is valid for frequencies of 200 MHz or above in chambers of the order of 5 x 5 x 3 m. The optimum design criteria for reverberating charnbers are to make the volume as large as possible and the ra tio of the squares of linear chamber dimensions as non rational as possible [33J. Reverberating chambers can be operated in two ways:
Mode tuned where the paddle wheel is stepped slowly through many positions within one complete rotation at a given test frequency. The time for which the paddle blades are stationary is determined by the time it takes the EDT to respond to the imposition of a new stimulus. Mode stirred where the paddle wheel is moved continuously during the test at a given frequency. In this case the rotation rate should be slow enough for the EUT to respond to the changing wavefield conditions.
(i)
(ii)
Typical field strength variability with frequency for the NBS (US National Bureau of Standards) chamber is shown as a function of frequency in Figure 9.20 as measured by two different sensors. 45r---------------,.-----------,
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Figure 9.20
Maximum and average electric field strengths generated inside the NBS chamber (empty) mode tuned)
Reproduced by permisssion
or NBS
The curves relate to the maximum and mean field strength recorded at each frequency when the chamber is mode tuned. It has been suggested that mode-stirred or mode-tuned chambers can be used for radiated emission testing [34J, but the practice has not been taken up widely in the EMC test community. Indeed, mode-stirred chambers have not been as widely used for susceptibility testing as predicted, probably owing to the difficulty in relating the measurements made in this way to the results obtained from more conventional test methods. Some measurements have been made in an attempt to correlate susceptibility results obtained using mode stirred and anechoic chambers, which showed that the EUT response was less in the reverberating chamber than in the traditional anechoic chamber test [35]. There are however, examples where these chambers have been used successfully, such as testing shielding effectiveness of cables and connectors to MIL STD l344A method 3008 as reported by Crawford et al. [36J at NBS.
9.2.5 Novel fatilities When the systems to be tested are very large, such as equipment for communication centres, computer or telephone switching installations, it may not be possible to provide economically conventional electromagnetically enclosed testing facilities such as those previously described. Depending on the amount of screening attenuation and minimum internal reflectivity required to carry out the particular tests, a number of unconventional chamber solutions may be considered which include large 'chicken wire' cages, metallised airinflated structures and underground caverns. The chick-wire cage can be built from wood and a metal wire mesh to provide large enclosed volumes at low cost. The penalties include achieving only moderate attenuation of around 40 dB at VHF and no suppression of internal standing waves. Metallised air-inflated structures are capable of providing moderate to large volumes with floor areas up to the size of a couple of tennis courts. They are expensive to purchase but can be moved to new locations and re-used many times. Shielding is relatively poor 20-40 dB but this approach is an option for testing large systems. To my knowledge underground caverns have been used for a number of EMC tests which include spacecraft and commercial computer systems. This approach can afford good screening of better than 90-100 dB at most frequencies, and with the addition of a small amount of RAM the cavern can closely simulate the EM performance of an open test site. The working volume is
EMC TEST REGIMES AND FACILITIES
165
usually inexpensive to hire or purchase as it already exists as a byproduct of mineral extraction. This makes the use of large underground sites, which can have floor areas the size of a football pitch, very cost-effective for testing large distributed systems which may have many interconnecting cables. The main drawbacks to using a facility like this are the usually limited shaft or roadway access, the possibly high humidity environment and the safety and evacuation considerations. An underground facility may be an ideal solution when a project is subject to costly multiple EM requirements such as EMC, NEMP and Tempest.
a standard test under the ambient conditions at each test site as this may be reflected in test costs. In such conditions measurements may need to be repeated and taken close in to the EDT (at 3 m). This can lead to difficulties with extrapolation to measurements made at other distances on another site. Because of this difficulty, a problem could arise if the measurements have to be checked by a laboratory working on behalf of the regulatory authority which is blessed with low ambients and capable of measuring at 10 or 30 m. I t is therefore preferable to use a test facility where the ambients are low and measurements can be made directly at 10 or 30 m.
9.3 Open-range testing
9.3.3 Testing procedure
9.3.1 Introduction
To make more consistent and reliable measurements, the ground at an open-air test range is covered with a large metal plate or mesh to provide a known constan t cond uctivi ty and dielectric constant resulting in predictable RF reflection properties. A typical mesh would extend for 60 X 30 m to give a lower working frequency of 20 MHz [37]. The mesh size d is determined by the requirement d < AI10 where A is the wavelength at the highest frequency at which the range will be used. This is typically 1 GHz and so the mesh size should be less than 3 em. It is important not ·to obstruct the ground plane with buildings or test equipment which can cause RF reflections, and for this reason the receivers and antenna positioning equipment are often located close to the receiving antenna but under the ground plane. A similar provision can be made for the mains supply and support equipment for the EDT. An example of a useful site configuration is shown in Figure 9.21. The details of a suitable measurement site are specified in the various regulations which apply to the equipment being tested, and these must be c'onsidered carefully by the site operators. In the
Open-site RF measurement is the traditional and straightforward approach to making measurements of the EMI characteristics of equipment. The technique has been used for testing both civil and military electronic equipment. Open range testing became the norm for testing commercial equipment with the advent of. the VDE and FCC regulations for example, concerned "with the measurement of radiated emission levels from household appliances, office equipment and information technology devices such as personal computers. The test specifications and regulations governing the construction and calibration of open test ranges are dealt with fully in Chapter 2. The aim of this section is to explore the construction, certification procedure and operation of these facilities.
9.3.2 Test site The ideal test site consists of an obstruction-free ground area and hemisphere above it. The site should be located well away from sources of electromagnetic noise such as power lines or air conditioning plant con taining electric motors, thermostats etc. Ideally, the site should be located in the countryside well away from all industrial EM noise and broadcast transmitters. Some screening of TV and VHF radio transmitters could be afforded by a ring of hills covered with trees which would surround the ideal test site. It is not always economically viable however for companies to set up a test facility at some distance from their city based customers to take advantage of the low EM ambients. The alternative is to attempt to make sensitive radiated emission measuremen ts in the presence of high level and randomly changing ambient signals, and this can be extremely difficult and time consuming. Potential customers of such facilities should enquire about the average tim.e taken to complete
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RECEIVING ANTENNA \
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Example oj open-range test facility
166
A HANDBOOK FOR EMC TESTING AND MEASUREMENT diameter = 2d
----to-!
RECEIVING ANTENNA
Area within the ellipse must be free of all RF reflecting objects
Figure 9.22
Requirements for open-area test site. EUT to antenna distance d can be 3, 10 or 30 m
context of this text on EMC testing consider as an example the requirements specified by CISPR 22 (testing of IT equipment) which call for adherence to CISPR 16 (specification of radio interference measuring apparatus and measurement methods). (Details contained in section 3 of the CISPR 16 Document.) The size of obstruction-free area is determined by the measurement distance (3, 10 or 30 m) with a shape specified in Figure 9.22. The ground plane often extends to cover the full obstructionfree area to give the best results. For test sites where compliance is not possible, a minimum set of req uiremen ts for free area and ground plane are given and can be seen in Figure 9.23.
----------........ /"'/\ ~m \ II ~3m~~ 3m · EUT II \ TEST AE~AL I II
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SURROUNDING BOUNDARY
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Minimum requirements for alternative test site (a j Minimum alternative measurement site (b j Minimum size of metal ground plane. D d + 2 m, where d is the maximum test unit dimension; W == a + 1 m, where a is the maximum aerial dimension; L == 3, 10, or 30 m
Reproduced by permission of BSI
rrhe purpose behind the detailed E UT configuration/layout and the height scanning of the receiving antenna required by many test standards is to measure the maximum interference generated by the equipment at each frequency of interest. The EU1' shown in Figure 9.21 is placed either in contact with the ground plane or at a distance of 0.8 ill above on an insulating table if it is a freestanding item. All cables should be of the type and length specified for the EDT being tested. If the cables are very long they can be bundled into 30-40 cm-Iong bunches at the centre of the cable. During testing the EDT is exercised in one of its operational modes, the receiver is tuned to a frequency within the band of interest and the antenna is moved through a range of heights usually from 1 to 4 m above the ground and the maximum signal level is recorded. This is necessary to measure the signal strength from the EU1' at that frequency for the minimum site attenuation. (This is considered later when discussing site calibration.) The EUT must be rotated on a turntable, or the receiving antenna can be repositioned at various compass points, to detect the maximum emission as a function of azim u th angle if the E urf cannot be rotated. The structure of the radiation field emanating from an EDT is usually complex with many peaks or deep nulls at various azimuth and eleva tion angles for each frequency being radiated. The ability of open-field measurement methods to measure the maximum fields at each frequency of interest has been studied in some detail [38, 39J. I t is difficult to see how the accuracy of open-site tests can be improved without more frequent sampling of the emitted radiation pattern from the EUT and thus further increasing the protracted testing time. The polarisation of the receiving antenna is changed at each frequency and location to measure the worst-case emission field strength. All signal strengths should be measured using both an average and quasipeak detector function on the EMI receiver to fully cornply with CISPR 22. The frequency is then changed and the test repeated until the band from 30 to 1000 MHz has been fully explored and the maximum field strengths recorded. The operating mode of the EDT is then changed and the entire test sequence is repeated. If the recommended tuned dipoles are used as the receiving antenna, their length must be physically adjusted at each frequency above 80 MHz (for CISPR 22 and frequencies below 80 MHz a fixed length equivalent to the 80 MHz tuned length is used down to 30 MHz). Other specifications such as FCC part 15 require the dipole to be tuned over the whole frequency band. Clearly, the test time required for the multiplica-
EMC TEST REGIMES AND FACILITIES
tion of these measurement parameters is enormous and simplifying procedures have to be adopted in practical tests. The height positioning of the antenna and the rotation of the EUT can be automated with motor positioners. 'The frequency bands can be rapidly scanned using automatic receivers and the data quickly accumulated and logged using a control computer. Only those frequencies where the measured signal strength approaches the specification level need attract the full rigour of the test procedure to ensure that the maximum field strength is measured. All these measurements must be made in the presence of ambient signals which ideally must be at least 6 dB below the specification limit. At most test sites this is rarely the case and computer aided signal identification is often used to recognise, mark and ignore some of the more stable ambient signals from broadcast transmitters. Broadband antennas such as biconics and log periodics are permitted under most testing regula tions and their use, together with scanning receivers, can lead to a much more reasonable testing time.
9.3.4 Site calibration Site attenuation is defined as the ratio of the input power to the transmitting antenna to the measured power at the output of an identical receiving antenna operating with a given polarisation when both are located at known positions on the site as in Figure 9.24. By measuring this attenuation as a function of frequency it is possible to validate the site for testing by comparing it with a reference site which is usually operated by the government metrology agency such as NBS in the USA or NPL in the UK. It can be seen from Figure 9.24 that the power measured at the receiving dipole is a vector combination of the contributions from the direct path and the reflected path from the ground. The
167
amplitude and phase changes introduced into the reflected path by the increased path length and the addi tional phase change on reflection will produce a systematic variation of received power as the height of the receive antenna is changed. The site attenuation at a given frequency j, range d, transmi tter antenna height H t and polarisation is the minimum value measured at a given H r where the contributions from the direct and reflected paths reinforce each other. Examples of calculated site attenuation as a function of receiving antenna height, at a number of frequencies for both horizon tal and vertical polarisation, have been made by Ma and Kanda [37J and are shown in Figure 9.25. By selecting the minimum attenuation from a height scan at each frequency for horizontal and vertical polarisations at the measurement distances of 3, 10 and 30 m, one can plot a suite of site attenuation curves shown in Figure 9.26 [40]. It can be seen that the attenuation values (for the NBS open range) vary from 20 dB at 30 MHz to above 40 dB at 1 G Hz for 10m separation, and that the relationship between attenuation and separation at any frequency is approximately 20 dB per decade. The calculated and measured values of attenuation agree well at most frequencies. 5 .....- ........- .......---.~-...-- __- ......- -__- ...
J: lI <=>
iIi I
97MHz 311 MHz 1GHz
430MHz
3
W
...J
0
c- 2
o
1 10
20
25
30
35
40
45
50
RELATIVE INSERTION LOSS dB
(a) 5....--........- .......----,~- .....-...,..----r--....--.... :I:
143MHz
44MHz
4
459MHz
1GHz
lI
o W3
I
W
....J
~ 2 Ci 1 .....10
......15
......20
.......~-.......~..............~-- .....- ..... 25 30 35 40 45 50
RELATIVE INSERTION LOSS dB
Figure 9.24
Open-area test site calibration setup (horizontal polarisation). Site attenuation, A == 10 log PI.N / POUT (max) for fixed values of d == 3, 10 and 30 m and with POUT ( max) selected from measurements with receive antenna height H r == 1 - 4 m
Figure 9.25
(b) Calculated open-area site attenuation with frequency, height and polarisation ( a) Horizontal polarisation, (b) Vertical polarisation. Source: M a and Kanda, NBS-USA
Reprod Llced by permission or NBS
168
A HANDBOOK FOR EMC TESTING AND MEASUREMENT to 60
Cable and connector losses and VSWR Position of EUT cables and peripherals Nonuniform field strength/range relationship .
"'0 CJ)
Source height: 2m. Vertical polarisation
~ 50 ...J
•
5 40 ~ 0:: 30
UJ
30m
CJ)
~ 20
10m 3m
~
~
10
Z ~
0 101
102
(a)
103
FREQUENCY MHz
~ 60 CJ)
Source height: 2m. Horizontal polarisation
~- 50 ...J
5 40 ~
ffi
30m
30
CJ)
~ 20 ~
10m
:::>
~ 10
(b)
0 101
Figure 9.26
9.3.5.1 Reflections from objects
3m
z
~
In August 1985 a study (CISPR/B/WG/2) analysed data gathered from a number of regulatory authority test sites on a single item of computing equipmen t measured during the previous year. Dash [43J suggests that, as expected by some EMC practitioners, the correlation between measurements from the various test sites was. poor, with a standard deviation of 8.5 dB. This means that the emission field strengths at anyone site can only be predicted to an accuracy of around 30 dB (to a 95% certainty) based on the figures from any other site. I t is therefore clear that although the site attenuation may be measured quite accurately, this does not mean that the EM C tests that are carried out on the sites are equally accurate.
102 FREQUENCY MHz
103
Calibration of open-area test site (site attenuation against frequency) (a) Vertical polarisation) (b ) Horizontal polarisation) • Measured data, Source height 2 m
Calculated data.
The performance of well constructed open-range test sites has been widely reported, for example the IBM Endicott facility [41 J, and general design studies have been carried out by DeMarinis [42J.
9.3.5 Measurement repeatability The estimated worst-case uncertainty in defining the range attenuation as calculated by the NBS [37J is ± 1.2 dB. The worst case difference between measured and calculated site attenuation is given as 1 dB for horizontal and 1.9 dB for vertical polarisation. These figures may be true for a carefully controlled government establishment range charged with setting standards for industry, but published evidence suggests that the everyday problems encountered at industrial test sites make this accuracy difficult to achieve. 1-'he following factors affect the measurement accuracy on an open site: Reflections from objects Nonconductive weather cover structures Antenna impedance and VSWR changes with height An tenna size field averaging Balun VSWR and losses Differences in commercial antennas
An indication of the effects of reflections at the boundary of a 10 m measurement site can be gauged by calculating the modifications to the normal site attenuation curve when a large metal pIa te with its long axis parallel to the measurement axis is introduced on the site boundary. The errors are reported [42J to be ± 1.5 dB with the plate at 15 m and ±0.5 dB when the plate is 60 m away from the measurement axis. Measurements have also been made with trees 15 m from the edge of the range area which introduced errors of 1-2 dB at certain frequencies around 80 and 180 MHz [42J. From these data it would seem that reflections from objects outside the stipulated flat object-free area can contribute between 1 and 2 dB to measurement uncertainty.
9.3.5.2 Weatherproof covers Some open test ranges provide a weatherproof shelter over the test area in which the EUT is mounted so that testing is not compromised by bad weather. The construction of these shelters has been investigated as a possible source of measurement error due to reflections from their walls and roof. The use of various materials in shelter construction has been investigated [42J and crude measurements of reflection coefficients have been made and are given in Table 9.3. This shows that pinewood is a poor rna terial to use while plastic and glass fibre sheets have low reflection coefficients. However, Dash and Straus [44J show that dielectric constant and thickness, not conductivi ty, maybe the key factors in determining the suitability of materials for EMC test site shelters. Wood has a relatively low dielectric constant of
EMC TEST REGIMES AND FACILITIES
Table 9.3
Examples oj RF reflection coefJicients oj number oj materials commonly used in EUT covers on open-area test range
Material Yellow pine plywood Fir plywood Waferboard Drywall or sheetrock Acoustic ceiling material Composite wall w/fir plywood, sheetrock, fibreglass insulation, and cedar clapboard siding Composite wall as above, but with asbestos roof shingles Composite technology plastic and glass fibre wall with foam insulation
Reflection coefficient 0.45 0.27 0.25 0.23 0.09 0.3
169
HEIGHT OF THE CENTRE OF VERTICAL HALF-WAVE AERIAL ABOVE THE GROUND (in wavelengths) 0.25 0.3 0.4 0.5 0.6 0.7 0.75 100
a
90
ui 80 0 Z
« ten
70
Ci5
w 60 a::
z
0 50 i=
« CS « a::
40 30
0.27
o
0.03
0.1
Figure 9.27 1.3-2.5 [45J but plastics have values from 1 to 5 and must be used with care and only in thin sheets. Site attenuation measurements in the presence of an EDT shelter made from radome plastic rna terials [46] has shown some evidence of a change in the maxima and minima of the height scan pattern (Figure 9.25). This effect is not noticed when these data are processed into the range attenuation plots, as only the minimum attenuation figure is used. The presence of the plastic shelter has altered the physical position of the minimum attenuation path on the range. An overall estimate of the errors introduced by a well designed weather cover are of the order of ± 1 dB.
9.3.5.3 Antenna impedance changes with height The change of antenna impedance with height can be thought of as being due to the interaction with its image created by' the ground. The closer the antenna is to the ground the greater is the mutual impedance effect. The resistive component of the impedance of a vertical and horizon tal halfwave dipole antenna above the ground is shown in Figure 9.27 [4 7J. The vertical dipole is well behaved at a height greater than half a wavelength, but the resistance of the horizontal dipole is still changing from 60 to 80 ohms when it is a full wavelength above the ground. If the antenna impedance changes there will be a mismatch with its balun, connecting cable and the receiver input. Thus the voltage across the receiver input port will be unknown and this will introduce an error into the measurement. For example, the measurement uncertainty due to antenna mutual impedance/VSWR changes in a horizontally polarised biconic antenna 1 to 4 m
0.2 0.3 0.4 0.5 0.6
0.7 0.8 0.9 1.0
HEIGHT OF A HORIZONTAL HALF-WAVE AERIAL ABOVE THE GROUND (in wavelengths)
Variation oj radiation resistance with height above ground Jor vertical and horizontal polarised ha1jwave dipole
Reproduced by permission of RSGB
above the ground over the frequency range 30-300 MHz has been shown be ±2 dB or more [48].
9.3.5.4 Antenna size
field averaging
This has been discussed in Chapter 6 in relation to the difference in field strength measured on an open test site with a point source at 3 m, when using a short CISPR 80 MHz dipole/or biconic as compared with a 30 MHz FCC tuned dipole. Figure 9.28 shows the antenna sizes in relation to 5m W
>
°c~o-~ ....Z
:;
....Z
-e
0
Size of 30 MHz tuned dipole ____
4m
°o.."C -
W
(J)
(I)
E
~W E 0::Cl') :::>-
3m
Size of CISPR 80 MHz tuned dipole or bicon ic antenna
2m
~
C/)W
~Z
We:(
~...J
80MH~ A
LLo..
0°
.... z :c:::>
0° _0::
1m
B
~o
5
10 15 20 25 30 dB (arbitrary scale) E FIELD MEASURED AT A POINT (source is at a distance of 3m.)
Figure 9.28
Field averaging with large dipoles. Field exposed to: A == CISPR/bicon, B == 30 MHz dipole
170
A HANDBOOK FOR EMC TESTING AND MEASUREMENT
the field profile as a function of height [49J. The larger 30 MHz dipole averages the 17 dB field variation along its length and indicates a field strength which is 8.5 dB lower than the actual field strength at a height of 1 m. The shorter CISPR 80 MHz dipole and the biconic only have a 3 dB field variation with length and make a reasonably accurate measurement of the real field strength.
9.3.5.5 Balun VSWR and losses Chapter 6 concluded that the typical VSWR for a commercial biconic antenna was between 1.5 and 4: lover the frequency range 30 to 300 MHz compared with a Roberts tuned dipole of less than 1.5: 1. Clearly the higher VSWR of the commercial an tenna will result in greater measurement uncertainty as the antenna impedance varies with height. The loss in a typical commercial balun for a biconic antenna is in the range 0.5--2 dB (30300 MHz) vvhich reduces the minimum detectable signal in the band compared with that obtainable with the Roberts tuned dipole balun which has a loss of only 0.1-1 dB (30 MHz-l GHz).
9.3.5.6 Differences in commercial antennas Work carried out at Hewlett Packard open test ranges [50] has shown that range calibrations can vary by up to ±6 dB when carried out using various combinations of log-periodic antenna pairs. The antennas are all manufactured to the same design by a single company and differ only in serial number. The errors are usually confined to a few frequencies between 300 MHz and 1 GHz where sharp response variations are observed over a hundred MHz or so at up to ±6 dB and may be the manifestation of antenna/cable matching problems. l'hese effects are not observed when using tuned dipoles for range calibration, but these are slow to use and a wide band antenna is preferred. One solu tion is to fit 3 or 6 dB pads at the an tenna output when using complex wideband antennas such as the log-periodic or biconic to help control VSWR problems. The effect of a simple fix such as this is a reduction in measurement sensitivity: the preferred solution is to produce wideband antennas with high-quality baluns with close manufacturing tolerances. Investigative work such as this on the properties of antennas, indicates that the EMC test engineer can take nothing for granted when attempting to make accurate radiated emission measurements and should make every effort to carry out measurement checks.
9.3.5.7 Cable and connector losses/VSWR Investigations carried out by workers at Digital Equipment Corporation in the USA [51] have
shown that significant errors of up to 10 dB can be introduced by the change in position of the antenna cable during a test. This effect has been attributed to two causes. T'he first is the cable acting as a parasitic element close to the antenna which modifies its properties, and the second more serious cause is due to leakage between the cable and antenna elements due to imperfect current balance and leaky connectors. Broadband dipoles have shown relatively poor balun isolation resulting in 6-10 dB errors. Again, large differences in balun performance were discovered in similar biconic antennas with different serial n urn bers [51] . Some of these effects can be reduced by placing a common mode ferrite choke over the outer of the antenna coaxial cable to suppress unbalanced current flow.
9.3.5.8 EDT cables and peripherals Most of the 30-40 dB possible uncertainty between radiated emission measurements on the same EUT at different open sites [43] cannot be accounted for by a combination of the 1- 1-0 dB errors resulting from the issues discussed. The ANSI C63 study group was set up in 1985 to examine the problem of variable cable layouts in FCC MP-4 and CISPR 22 tests on IT equipment. The wording in these standards regarding placement of cables and peripherals calls for their placement to find the worst-case emission levels. This allows each test site to place the EUT components in different positions in search of the worst case emission configuration. This will result in different radiation patterns and different test results. The working group therefore considered that certain combinations of definite fixed cable placements and peripherals should be prescribed. This approach sacrifices the ability to find the absolute worst-case emission levels but increases the test repeatability. The details of suggested cable and peripheral placement are inappropriate for listing here but are given by Dash [43].
9.3.5.9 Nonuniform field strength/range relationship Within the CISPR 22 standard, para 9.4 relates to measurements in the presence of high ambient signals. It permits measurements to be made at reduced distances from the EUT to increase the measured signal level in an attempt to overcome the masking ambient signals from broadcast transmitters and the like. The regulation permits the scaling of the specification limit in a linear manner with measurement distance. This procedure is highly suspect when conducted in an EM radiation field of complex
EMC TES1' REGIMES AND FACILITIES configuration where the far-field boundary distances for components at different frequencies are not well known. The site attenuation curves as measured with two similar dipole antennas do show a simple field strength/range scaling law. However, work carried out in Japan by the Matsushita Company [52J to derive the site attenuation curves using EDTs with realistic radiation patterns and wavefield impedances has shown that the 20 dB/decade scaling law is inadequate. The test set up is shown in Figure 9.29a and the range a ttenua tion curves in Figure 9.29b. The correction curves from 3 to 10 and 30 m are shown in Figure 9.30 and differ from the assumed 10 and 20 dB by up to 8 dB at 80 MHz.
\ITEMI EJ 1
METER
hr
(a) 60 ht = 1m.
50 CD
"C
z
hr = 1 to 4m. (0 = 3m., D = 10m.) hr = 2 to 6m. ( 0 = 30m.) D=30m
40
0
~ :::>
Government regulatory agencies such as the NBS can construct open-range test sites for making radiated emission measurements at 3, 10 and 30 m with site-attenuation performance close to the theoretical values. The open-range radiated emission test methods as defined by regulations such as FCC, ClSPR and VCCl are time consuming to carry out fully, and the presence of EM ambient signals is an additional problem. The foregoing discussion relating to open-range emission testing shows that the practising EMC test engineer and customers need to be aware of the sources of uncertainty that can undermine the most strictly regulated test regime. These problems all stem from the attempt to make extremely detailed and difficult electromagnetic measurements of complex unknown wavefields by using simple test equipment and quick procedures which result in affordable testing. EMC testing provides an indication of emission levels from an EDT when measurements are carried out to a standard proced ure. I t is not an attempt to make full and accurate measurements, with a close estimate of associated uncertainty, irrespective of the time and cost involved. 'The overarching aim of con trolling the increase in EM emissions from electronic equipment probably justifies these less than rigorous but economically acceptable test methods.
9.4 Low-level swept coupling and bulk current injection testing
20
~
10 0
20
9.4.1 Introduction 1000
100
FREQUENOY MHz
(b) Figure 9.29
Typical open-area test site calibration oj' range attenuation (a) Site attenuation parameters (b) Site attenuation for 3) 10 and 30 m (horizontally polarised) . ht == 1 m) hr == 1 to 4 m (D == 3 m) D == 10m)) hr == 2 to 6 m (D == 30 m)
CD 30t---~-----~--------------
Measured correction factor
"C
Z
20
z 10 w Or--20
/ ---.L
Measured correction factor
100
~
«J=
9.3.6 Comments on open-site testing
30
z w
o ~ :::>
171
1000
FREQUENCY MHz
Figure 9.30
Measured conversion factors to normalise measurement distances of 30 m and 10 m to 3m
These two techniques are distinct but complementary. They have evolved together over the last 10 years in the UK, primarily in conjunction with immunity clearance of aircraft [53J, military hardware and automobiles. The two techniques have become popular throughout the EMC community and are being taken up in the USA and elsewhere. Several bulk current injection (BCl) tests have been incorporated into the UK DEF STAN 59-41 EMC requirements for triservice military equipment. The low-level swept coupling tests are normally carried out on complete systems to establish the bulk or common mode induced current spectrum on a cable loom for a given external EM field as it is swept in frequency from about 5 to 400 MHz. This induced current can then be scaled with external field strength to derive the expected current in the cable loom which would be induced by the specified field to which the system must be immune. This current can then be injected into the loom in the BCl test at levels equal to or above those corresponding to the specified immunity level. By directly
172
A HANDBOOK FOR EMC TESTING AND MEASUREMENT
injecting current into the cables it is possible to simulate to some extent high-level free-field radiated susceptibility tests but using only a fraction of the RF power required to drive large antennas. This means that it becomes possible to carry out technically meaningful immunity tests on large systems at an economic cost.
9.4.2 Low-level swept coupling Consider an electromagnetic wave incident on a complex system as shown in Figure 9.31. The diagram shows the possible coupling routes by which EM energy can interact with and propagate through the structure and its apertures to the semiclosed metal equipment box which contains circuit boards populated with susceptible components. The size of box apertures, circuit boards and components dictates that they will be electrically short at wavelengths much longer than
--
1
WAVE FIELD+SYSTEM STRUCTURE & CABLE COUPLlNG+SUSCEPTIBLE I BOX or BOXES ~ MAIN COUPLING PATH UP TO ~ 400 MHz I I
I
..
I ~F wove Incident I
their physical dimensions and exhibit poor coupling efficiency. These direct wave to component coupling paths are therefore secondary at all but the highest frequencies beyond about 0.5 GHz. The main coupling path is provided by the conducting system structure and the extended cable runs within it. This route predominates for all frequencies up to about 400-500 MHz for systenls which are between 2 and 10 m long. The simplified coupling scheme showing only the structural and cable coupling route is given in Figure 9.32 where the unknown individual transfer processes within the structure and cables are represented by a series of complex frequency dependent impedances. At present no real credence can be given to E- and H-field measurements made in the restricted spaces inside cOlTIplex systems such as aircraft [54] and automobiles using triaxial sensors or other probes. This situation, together with the limitations in
field on system structure
i RF curr?nt 1\ pattern In ,\
s!ructu~e diffraction fields
Internal mutual impedance
Multi-eonductor transmission
coupling to wiring loom
line response to coupled energy
I
curren~
delivered
I I
via sy.stem cabling. (Dominates susceptibility up to 400MHz)
I
RF
t~ suscephble tJ.ox
SECONDARY COUPLING /' PATHS "" Direct pick-up by exposed wiring loom due to apertures in structu re.
Figure 9.31
Schematic ofpotential RF coupling paths into system
WAVEFIELD PARAMETERS E IH
TEST POINT uTP1 u
=VIm ~AVEFIELD
1
TO BULK CABLE CURRE T TRANSFER FUNCTION ( mAl VIm)
=AIm Z=o 2 I pO=W/m !
II
STRUCTURES & CABLES 2 - 10m long
..
-current in rnA
I
RF CURRENT FLOW
I
Za / INCIDENT WAVEFIELD
CIRCUIT BOARDS
BULK RF CURRENT ( located .inside the IMPEDANCE NETWORK PROBE susceptible box) representing each step in coupling path
Figure 9.32
Main structure and cable dependent coupling path (up to
~400
MHz)
EMC TEST REGIMES AND FACILITIES
computing multiconductor transmISSIon line responses, leads to severe problems in tracing the RF energy through the coupling chain to the susceptible box. I t is possible to measure the incident E- and Hwavefield cOlnponents and to determip.e wave impedance, polarisation and angle of incidence on the structure. I t is also easy to make an undisturbed measurement of the bulk or common-mode cable currentat the test point 1'P l, close to the connector on the box of interest which contains the susceptible circuits. See Figure 9.33. If the incident field strength is kept constant while the frequency is changed we are able to measure a swept frequency coupling response of the system under test. For example, the induced current in an aircraft cable harness for a 10 V 1m external field is presented in Figure 9.34 [54] and shows maximum coupling at a frequency corresponding to the resonant electrical length of the structurel cable harness. Coupling factors for various cables connected to a personal compu ter are shown in Figure 9.35 and are typical of a wide variety of equipments, having peak values of a few mA/V 1m.
173
The test set up for n1easuring low-level swept coupIing to a system under test is shown in Figure 9.36. The first step in the test procedure is to generate a levelled field at the test location [55]. This is done by placing the fibre-optically coupled field sensor at the location where the E UT will be and then generate in the control computer a set of amplitude values for the tracking generator attenuator for which a constant field strength is produced. The same values will be used when radiating the swept signal to ensure that the EUT is subjected to the calibrated levelled field. The distance from the broadband antenna to the EUT should be sufficient to ensure that the EUT is in the far field at all frequencies of interest. A current probe which is connected to a fibre optic transmitter is placed around the required cable loom close to the connector on the C/P ON VIDEO CABLE
C/P ON COMPUTER MAINS LEAD
EE
~ --
~
2
E
~
z
a
i= ()
OI:.:L-. 0
Z
CURRENT PROBE measuring bulk cable current
\
crex: fi
EQUIPMENT UNDER TEST
""""""" 200
~
100
100 C/P ON DISC BUS CABLE
C/P ON PRINTER BUS CABLE
~
200
FREQUENCY MHz
FREQUENCY MHz 4f-'
«
ex:
l-
t?
6 ~~2~ E
()
o
FIBRE OPTIC TX CONTROL FIBRE TO ~ ---DATA FIBRE F.O. TRANSMITTER carrying bulk current signal to fibre optic receiver
Figure 9.33
-
o
100
200
FREQUENCY MHz
Figure 9.35
Examples of coupling transfer functions for various cables on personal computer system. ( CIP == current probe)
Measuring bulk cable current with isolated current probe and fibre-optic transmitter
CURRENT PROBE AROUND FIBRE OPTIC WIDEBAND CABLE OF INTEREST ~ELEMETRY SENDER
40---------r----.....----------~
STANDARD FIELD MEASURING SENSOR
0. 0. c:(
E
Used to "pre-calibrate"
the field at the test ~ object location. (Derive a power table and store in the control computer.)
30
==
IZ ill
a: a: :::::> ()
20
..J
() Q
"/ "'""-
BROADBA~ND ANTENNA .....
CD
c:(
w
S~C~~~~ _D~;T~CE
FIBRE OPTIC LINK
ill
"'" . . . . . .
~ ~ ~ ....... ~
10
() :::::> Q
If
~
5
Figure 9.34
FIBRE OPTIC RECEIVER
L----+-A
U ~j
25
Typical bulk current induced into cables in aircraft. Incident field strength 10 Vim horizontal polarisation
FIBRE OPTIC
~
10 - 100 W
0
",""
TRACKING GENERATOR
Figure 9.36
CONTROL COMPUTER
PLOTIER
Whole vehicle low-level swept-jrequency coupling test
174
A HANDBOOK FOR EMC TESTING AND MEASUREMENT
-10
FRONT OF VEHICLE
FRONT OF VEHICLE
«
co -20
"0
UJ
co -30
...J
« () ~
-40
I-
z
UJ
0:: 0::
::> ()
-50 -60
Q UJ
()
::> -70 Q
~
FREQUENCY = 60 MHz
-80
20
40
60
80
100
120 140
160 180 200
Figure 9.38
FREQUENCY MHz
Figure 9.37
Typical induced cable current in complete vehicle
susceptible electronics box and the control compu ter rep rod uces the levelled field as the frequency is scanned from about 5 to 400 MHz. The received fibre optic signal is converted back to an analogue electrical form and fed to the spectrum analyser which is driving the tracking generator providing the stimulus signal. The spectrum analyser raw data are corrected for the current probe transfer impedance and the true cable induced current is displayed as a function of frequency on the computer monitor. Typical ind uced currents in the looms of a vehicle are shown in Figure 9.37 for an incident levelled field strength of 20 Vim. Note that the maximum coupling value is 1.5 mAIVlm at 50 MHz (-30 dBA at 20 V 1m) for this vehicle. The production of a field to curren t coupling curve enables the EMC engineer and product designer to have an overview of the electromagnetic coupling properties of the EDT. I t is common for coupling curves to be of the form shown in Figure 9.37 where the coupling increases from a low level at low frequencies (where the EDT is electrically short) up to a maximum at near structural resonant frequencies and then to show a number of higher order coupling responses in the region where the EDT is electrically long. I t is interesting to investigate and understand the physical properties of the EDT which influence the coupling. 1'his can be done by selecting a frequency of interest, such as 60 MHz in the case of the coupling curve shown in Figure 9.37 (which is in the frequency range of maximum coupling) and rotating the EDT while recording the induced current. In this way, a 'pick-up' polar diagram can be produced, the in terpreta tion of which can often reveal those aspects of the E UT structure which are determining the system coupling. Figure 9.38 shows these polar diagrams for a saloon car at
FREQUENCY = 160 MHz
Wavefield coupling polar diagrams with current probe monitoring pickup in engine management cable harness - - V polarisation - - H polarisation
60 MHz (1st resonance) and 160 MHz (well above 1st resonance) for both vertical and horizontal polarisation. Notice how at 160 MHz the maximum coupling to a cable which is being measured (located under the bonnet), occurs when the field is incident on the front of the vehicle. 1'his is not seen at 60 MHz where the w hole vehicle is resonant. The use of spot-frequency coupling polar diagrams can be well illustrated with an example of a less complex structure than a saloon car which has many different sized door and window apertures. Figure 9.39 shows the coupling response from external field to a cable loom in a miss.ile. The physical structure of the missile can be represented by a short fat electrical dipole with the centre load termination being the impedance of the fore body to motor body mechanical joint. Generating the polar diagram at the frequency of maximum coupling produces the response shown 0
« co -10
"0
I-
z -20
UJ
0:: 0::
::> -30 () UJ
...J
co -40
« () 0
UJ
-50
()
::> 0 ~
-60 -70 ' - - - I I - - - - - - l l - - - - - L _ . . . . - L . _ - - ' - _ - - - ' - _......._ .......... ............ 40 50 60 70 80 90 100 110 120 130 FREQUENCY (MHz)
Figure 9.39
Example of RF current induced in system main harness as function offrequency. Swept frequency field strength == 20 Vim
EMC TEST REGIMES AND FACILITIES TAIL
175
PICK UP PROBE
\
\
EUT MALFUNCTION LINE (automatic indication)
Figure 9.41 HEAD ON
Figure 9.40
Coupling polar diagram oj system at primary resonance Jrequency shows dipolelike response
In Figure 9.40 which is almost perfectly dipolar. This shows that the missile has minimum coupling when the field approaches head or tail on and has a maximum repose when it is broadside on. This is indeed the response of a dipole and so this experiment confirms that the main coupling feature and electrical structural model of the missile is that of a simple dipole. Such information can be used to suggest coupling red uction schemes and is very useful in assessing the electromagnetic compatibility of systems in their expected operational environment.
9.4.3 Bulk current injection When the indilced bulk cable current coupled from an external wavefield has been determined using the LLSC technique, the scaling factor from the LLSC test level to the specified immunity level can be applied to the measured induced current. This level can then be injected into the loom under test via a high power current injection probe and the EDT observed for signs of malfunction. Much has been written about the advantages and technical justification of this technique [56-59J and it is not possible to review all the work here. Space permits only a brief explanation of the key points involved in BCI measurements. The test setup for an automatic or semiautomatic bench test on an EDT is shown in Figure 9.4l. The control computer sets the frequency and adjusts the power level to the injection current probe. The forward power is monitored and recorded for reference. The injected current is increased until the EDT malfunctions and the
Operator intervention via keyboard when EUT malfunction is observed. (If no automatic link is possible.)
Example oj automatic bulk current injection test corifiguration
control compu ter records the current flowing in the loom as measured by the pickup current probe via the spectrum analyser. The current' readings are then corrected for pickup current probe transfer impedance. The injected current is reduced and the current at which the EDT reverts to normal operation (if it does) is recorded automatically by the computer. The process is repeated as each new frequency is tested until the whole band of interest has been covered. An example of the output of an automated BCI test on a microprocessor based electronics unit is shown in Figure 9.42 where failures occur for bulk cable currents of around 10 rnA flowing in the rnulticond uctor unscreened cable. Experiments [60J have shown that when the injection and pickup curren t probes are positioned as shown in Figure 9.41, and as specified in DEF STAN 59-41, the measured cable current is very nearly equal to the current -10 J:-
U«
-20
~:::
-30
:fen
!;(cn ..... 0::
z::::>
O -40 w 0::0
0:: 0 ::JZ
uo
-50
~~ Ur!
-60
~p=
o...J we:(
u:2
-70 - -
::JI-
0::::> ~w
-80 -90
20
Onset of failure - Release of failure
40
60
80
100
120
140
160
180 200
FREQUENCY MHz
Figure 9.42
Example oj injected current level as Junction ojJrequency at which microprocessor vehicle engine management system is disrupted
176
A HANDBOOK FOR EMC TESTING AND MEASUREMENT
in the box under test for bulk current injection, and that this is also true for direct illumination by a wavefield. This gives confidence that the results obtained via BCI tests can be related to operational exposure of the equipment to EM fields. Lever [61] has shown good correlation between BCI system-induced failures and those experienced during 'normal' radiated susceptibility measurements on automobiles. The BCI technique has also been adapted for use in development testing and assessment of a wide range of electronic systems including IT equipment [62]. In this case the injection probe is constructed from two inexpensive ferrite C-cores and the stimulus is provided by a pseudorandom sequence generator which provides a broad spectrum ou tpu t. Having described them separately, the LLSC and BCI measurement techniques can now be brough t together to show how cost-effective immunity testing can be carried out on large complex systems such as aircraft, automobiles and commercial vehicles for which standard radiated susceptibility free-field testing may be prohibitively expensive. The LLSC coupling data are scaled by the ratio of the test field ( 1-10 v 1M) to the specified immunity field (say 200 Vim) and the scaled induced current is plotted as the lower curve in Figure 9.43. The results of the BCI test, which are a set of cable currents as a function of frequency at which the EDT is compromised, are plotted on the same graph as the upper curve in Figure 9.43. The margin of safety between the available current in the looms for the specified field level and the required current for a failure is a direct measure of the degree to which the immunity
specification is exceeded. This conveys far more information than a simple traditional radiated susceptibility no fail test at the specified field level. The BCI and LLSC tests can permit the careful optimisation of EMC safety margins above the specified limit for production items, by gathering information on the statistics of variability of system coupling from the LLSC test and on the failure current as measured by the convenient BCI test.
9.5 References
2
3
4
5
6
7
8 50 Bulk cable current required for unit malfunction
40
E 30
t-
9
\
Induced bulk cable current for "X" VIm (coupling measurements)
« Z
W
a:
10
0:
:::> 20 0
10
11 0
0
50
100
200
FREQUENCY MHz
Figure 9.43
Deriving susceptibility safety margin from bulk current injection and bulk current coupling measurements
12
WHIl'E, D.R.J.: 'A handbook series on electromagnetic interference and compatibility, vol. 2, Test methods and procedures'. Don White Consultants, Germantown, Maryland, USA, pp. 3.7-3.23 FARSI, M.: 'EMIjRFI shielding: theory and technique'. Interference Technology Engineer's Master, 1988, pp. 228---238 WHITE, D.R.J.: 'A handbook series on electrornagnetic interference and compatibility, vol. 3'. Don White Consultants, Germantown, Maryland, USA, 1973, p. 10.75 Low-frequency cylindrical screened chamber 14 m dia. x 12 m high, constructed for GEOS spacecraft programme 1973-6. BAe Dynamics Ltd, Filton, Bristol, UK MIL STD 285: Attenuation measurements for enclosures, electromagnetic shielding for electronic test purposes, Method of, 25 june 1956 CARDENAS, A.L.: 'The examination of standards for testing RF shielded enclosures', EMC Technol., jan-Feb 1988, pp. 26-38 CARTER, N.J. and THOMPSON, j.M.: 's.usceptibility testing of airborne equipment The way ahead'. 2nd symposium and technical exhibition on Electromagnetic compatibility) Montreux, 1977, pp. 83-88 STUCKEY, C.W., FREE, W.R. and ROBERTSON, D.W.: 'Preliminary interpretation of near field effects on measurement accuracy in shielded enclosures'. Proceedings of IEEE symposium on EMC, 1969, pp. 119-127 BENHAM, G., COLEMAN, C and MORGAN, D.: 'Development of an elliptical anechoic chamber'. Final report BT11868 of contract NPL196j0060, 1981, BAe Dynamics, Filton, Bristol, UK SHEPHERD, D.R. and GOLDBLUM, R.D.: 'Exploiting shielded room characteristics to provide a low cost, semiautomated test system for EMS and EMV testing'. Mikrowellen Mag., 1982, 8, (6), pp. 714-718 Cavitenna AT2000, Accessories for RF testing. Amplifier Research, 160 School House Road, Souderton, PA 18964-9990, USA EDWARDS, D.J. and BURBIDGE, R.F.: 'An experimental investigation into the performance of asymmetric screened rooms for electromagnetic measurements'. Proceedings of IEEE symposium on EMC, 1990, pp. 114-118
EMC TEST REGIMES AND FACILITIES 13 LAHITA, M.J.: 'RF shielded enclosure purchasing considerations'. Interference Technology Engineer's Master, 1988, pp. 37 and 386 14 SALATI, B.D. and CHAPMAN, C.J.: 'Maintenance of aged modular shielded enclosures'. Interference Technology Engineer's Master, 1988, pp. 38-388 15 BUCELLA, T. and SANCHEZ, G.: 'A low-cost RFI susceptibility testing system'. Proceedings of IEEE symposium on EMC, 1981, pp. 31-35 16 DAWSON, L. and MARVIN, A.C.: 'Alternative methods of damping resonances in a screened room in the frequency range 30 to 200 MHz'. 17 BAKKER, B.H. and PUES, H.F.: 'A novel technique for damping site attenuation resonances in shielded semianechoic rooms'. Proceedings of IEEE symposium on EMC , 1990, pp. 119-124 18 LAWRENCE, B.F.: 'RF anechoic chamber test facilities'. EMC Technol., April-June 1983, pp. 32-38 19 Indoor ground reflection EMC test range, System 86/3. Rayproof division of Keene Corp, Norwalk, Connecticut 06856, USA 20 NICHOLSON, J.R.: 'Perform 'open field' measurements in a shielded enclosure'. 2nd symposium and technical exhibition on EMC, Montreux, June 1977, pp. 413-418 21 'Ultra high performance flexible pyramidal absorber'. Technical bulletin 8-2-2, 1-74, Emerson & Cuming Inc. 22 'Thin-film absorber for 50 MHz-1 GHz'. Technical bulletin 8-2-17, rev 6/75, Emerson & Cuming Inc. 23 KUESTER, E.F. and HOLLOWAY, C.L.: 'Improved low-frequency performance of pyramid cone absorbers for application in semianechoic chambers'. Proceedings of IEEE symposium on EMC, 1989, pp. 394-399 24 GAVENDA, J.D.: 'Semianechoic chamber site attenuation calculations'. Proceedings of IEEE symposium on EMC, 1990, pp. 109-112 25 Eccosorb VHP RAM. Grace electronic materials catalogue, p. 1-100. Emerson & Cuming (UK) Ltd, 838 Uxbridge Rd, Hayes, Middlesex, UB4 ORP, UK 26 MISHRA, S.R. and PAVLASEK, Z.T.J.F.: 'Design criteria for cost-effective broadband absorber-lined chambers for EMS measurements'. IEEE Trans., 1982, EMC-24, (1), pp. 12-19 27 NICHOLS, F.J. and HEMMING, L.H.: 'Recommendations & design guides for the selection and use of RF shielded anechoic chambers in the 30-1000 MHz frequency range'. Proceedings of IEEE symposium on EMC, 1981, pp. 457-464 28 TSALIIOVICH, A.: 'RF absorber qualification criteria and measurement techniques'. Proceedings of IEEE symposium on EMC, 1990, pp. 361-366 29 MENDEZ, H.A.: 'A new approach to electromagnetic field strength measurements in shielded enclosures'. Wescon record, Los Angeles, CA, Aug. 1986 30 LIU, B.H., CHANG, D.C. and MA, M.T.: 'Eigenmodes and the composite quality factor of a reverberating chamber'. NBS tech. note 1066, Aug. 1983 31 CORONA, P.: 'Electromagnetic reverberating enclosures: behaviour and applications', Alta Freq., 1980,49, pp. 54-158 32 BEAN, J.L. and HALL, R.A.: 'Electromagnetic
33
34
35
36
37
38
39
40 41
42
43
44
45 46
47 48
49
50
177
susceptibility measurements using a mode-stirred chamber'. Presented at IEEE symposium on EMC, 1978 MA, M.T. and KANDA, M.: 'Electromagnetic compatibility and interference metrology'. NBS technical note 1099 section 8 ROE, M.J.: 'An improved technological basis for radiated susceptibility and emission specifications'. Presented at IEEE symposium on EMC, 1978 CRAWFORD, M.L. and KOEPKE, G.H.: 'Comparing EM susceptibility measurement results between reverberation and anechoic chambers'. Proceedings of IEEE symposium on EMC, 1985, pp. 152--160 CRAWFORD, M.L. and LADBURY, M.J.: 'Mode-stirred chambers for measuring shielding effectiveness of cables and connectors: An assessment of MIL STD 1344A method 3008'. Proceedings of IEEE symposium on EMC, 1988, pp. 30-36 MA, M.T. and KANDA, M.: 'Electromagnetic compatibility and interference metrology'. NBS technical note 1099 section 2.22 DVORAK, T.J.: 'Fields at a radiation measuring site'. Proceedings of IEEE symposium on EMC, 1988, pp. 87-93 MISHRA, S.R. and KASHYAP, S.: 'Structure of EM field at an open-field site'. Proceedings of IEEE symposium on EMC, 1988, pp. 94-98 FIZGERRELL, R.G.: 'Site attenuation'. Proceedings of IEEE symposium on EMC, 1985, pp. 612-617 MADZY, T.M. and NORDBY, K.S.: 'IBM Endicott EMI range'. Proceedings of IEEE symposium on EMC, 1981, pp. 17-21 DeMARINIS, J.: 'Studies relating to the design of open-field EMI test sites'. Proceedings of IEEE symposium on EMC, 1987, pp. 115-126 DASH, G.: 'Computing equipment standards - An update on cable and peripheral placement'. Proceedings of IEEE symposium on EMC, 1987, pp. 332-337 DASH, G. and STRAUS,!.: 'Studies on the use of wood in open area test sites. Proceedings of IEEE symposium on EMC, 1988, pp. 295-303 'Reference data for radio engineers'. (Howard W. Sams, 1975, 6th edn.) pp. 4.28-4.31 MAEDA, A.: 'Note on EMI measurement at openfield test site (6): Effect of EU1-' shelter'. Proceedings of IEEE symposium on EMC, 1990, pp. 241-246 'Radio communication handbook' (RSGB, London, 5th edn.) p. 12.56 BENNETT, W.S.: 'Antenna to ground plane mutual coupling measurements on open-field test sites. Proceedings of IEEE symposium on EMC, 1988, pp. 277-283 BRENCH, C.E.: 'Antenna differences and their influence on radiated emission measurements'. Proceedings of IEEE symposium on EMC, 1990, pp. 440-443 BENNETT, W.S.: 'Radiated EMI measurement reproducibility'. Proceedings of IEEE symposium onEMC, 1987, pp. 90-93
178
A HANDBOOK FOR EMC TESTING AND MEASUREMENT
51 DeMARINIS, J.: 'The antenna cable as a source of error in E1\1I measurements'. Proceedings of IEEE symposium on EMC, 1988, pp. 9-14 52 T'KEYA, S. and MAEDA, A.: 'Note on EMI measurement at open-field test site'. Proceedings of IEEE symposium on EMC, 1985, pp. 593-599 53 CARTER, N.J.: 'The application of low-level swept RF illumination as a technique to aid aircraft EMC clearance'. Interference Technology Engineers Master, 1990, pp. 236-252 54 PANKHURST, R.V.: 'The coupling of electromagnetic interference into aircraft systems'. Proceedings of IERE conference on EMC, 1980, pp. 117-128 55 CARTER, N.J., REDMAN, M. and WILLIS, P.: 'Validation of new aircraft clearance procedures'. Proceedings of IEEE symposium on EMC, 1988, pp.117-124 56 AUDONE, B., FERRERO, G., GIORCELLI, L. and VECCHI, G.: 'Critical examination of bulk current injection techniques: Theoretical aspects'. Proceedings of IEEE symposium on EMC, 1988, pp. 109-115 57 OBERTO, G., BOLLA, L., ROSTAGNO, G. and
58
59
60
61
62
BARARDO, R.: 'Critical examination of bulk current injection techniques: Experimental comparison'. Proceedings of IEEE sympoSiUll1 on EMC, 1988, pp. 101-107 KERSHAW, D.P. and WEBSTER, M.J.: 'Evaluation of bulk current injection technique'. Presented at IEEE symposium on EMC, 1990, (late submission) NEWTON, P.M.: 'Aircraft testing in the electromagnetic environment'. Proceedings of ERA seminar on DEF STAN 59-41, December 1990, ERA Technology, Leatherhead, Surrey, UK BURBIDGE, R.F., EDWARDS, D.J., RAILTON, C.J. and WILLIAMS, D.J.: 'Aspects of bulk current immunity test'. Proceedings of IEEE symposium on EMC, 1990, pp. 162-168 LEVER, P.H.: 'Development of a system level bench test for the automotive industry'. Proceedings of IEEE symposium on EMC, 1990, pp. 270-275 MARSHALL, P.C.: 'Convenient current injection immunity testing'. Proceedings of IEEE symposium on EMC, 1990, pp. 173-176
Chapter 10
ElectroDlagnetic transient testing 10.1 Introduction
The first two have been covered as part of the continuous interference tests, as they have developed alongside traditional EMC testing and are considered together with continuous interference, often in the same standards. The remaining three areas are dealt with in this chapter.
10.1.1 Transient types Over the last 20 years traditional EMC testing has concen tra ted on radiated and conducted emission and susceptibility testing. This is carried out by examining or generating signals as a function of frequency, and this activity is sometimes referred to as operating in the frequency domain. Some discontinuous signal tests have also been carried out as part of EMC work with specifications such as MIL STD 461. These spike tests simulate switching transients which may be induced onto power lines when a neighbouring unit is operated. Other additional power-line surge and drop-out tests have also grown in importance over the years. Now that EMC design is becoming a sophisticated and integrated part of electronic design, one sees that the effects of both continuous interference and transient effects are having to be designed out at the same time, and usually by the same design team, as solutions for one type of interference may compromise those for another. Cost-effective protection across the whole field of RF interference demands that a coherent set of design techniques are employed to combat both continuous and transient interference with the minimum component count and cost. In some cases, the solutions to transient problems using nonlinear voltage clipping devices, such as zener diodes or spark gaps, are not used at all in controlling continuous interference. However, to be effective their location in the circuit and their position on the circuit board can be crucial, but this may compromise the optimum design for reducing continuous interference. The designer must be able to produce a design which is the best cost-effective compromise solution for all the con tin uous and transient req uiremen ts called up in the range of standards which the product must meet. For transient signals these standards can include consideration of:
10.1.2 Continuous and transient signals Before discussing these topics in detail it is relevant to point out some key differences between the conventional continuous interference EMC testing and that required to demonstrate compliance with 'transient' specifications. There are two aspects which .are different: The instrumentation is different The conceptual models needed to best understand the types of signals are different. The instrumentation used for continuous and transient measurement is different because it is required to capture and present signal data in different ways. Spectrum analysers or scanning EMI receivers are needed to look at the continuous interference as a function of frequency [1]. The limits for conducted and radiated interference (and susceptibility) are all drawn on frequency plots. Therefore all the instruments are designed to produce data in that form. These instruments generally do not preserve and present phase data on the signals being measured, only amplitude is recorded after being measured with a given IF jvideo bandwidth and filter shape. Measurement of fast transients cannot be made sensibly using scanning receivers. Fast oscilloscopes or digital transient recorders must be used to capture the waveform [2]. Certain aspects of the waveform such as amplitude, risetime, PRF, etc., are then con1pared with the limits set out in the applicable specification. Most transient tests are susceptibility tests and the instrumentation is primarily used to confirm the waveform being injected into the EDT while it is monitored for any malfunction. Test engineers and equipment designers will find that when dealing with both continuous and transient phenomena it is vitally important to be able to hold two conceptual models in their minds and to be able to switch easily between them.
Power-line spikes Power-line surges and drop ou ts ESD -- electrostatic discharge NEMP - nuclear electromagnetic pulse Ligh tning strike 179
180
A HANDBOOK FOR EMC TESTING AND MEASUREMENT
These are the frequency domain concept and the time domain concept. The frequency domain is useful for representing the spectra of continuous signals. The form of the spectrum is fixed (within the observation time) and does not change or evolve with time. A spectrum can be considered to have two components, amplitude and phase, see Figure 10.1. All the details of the signal can be recorded if both the amplitude and phase data are measured, but it is usual in EMC testing, when using a spectrum analyser or EMI receiver, to measure only the am pli tude of the signal as a function of freq uency. This results in some limita tions with regard to spectrum manipulation (as will be seen) and it is not usually possible to unambiguously reconstruct
AMPLITUDE PLOT
~
The magnitude trace is the only one produced by a scanning spectrum analyser. All phase data is lost using such a device
30
w o
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8
Z
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20
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10
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0 -10 ............._ ......._ .......-...._......_ .............- ... o 10 20 30 40 50 60 70 80 FREQUENCY MHz PHASE PLOT
+1t W
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o Z « w
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0
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o Figure 10.1
10
20 30 40 50 60 FREQUENCY MHz
70 80
Example of signal spectrum showing both magnitude and phase components
the signal waveform with only the am pli tude of the spectrum. Thus conversion from the frequency domain (spectrum) to the time domain (waveform) is unfortunately normally barred. The time domain contains only a record of signal amplitude as a function of time with respect to a time zero, as would be seen on an oscilloscope screen. It displays no frequency data about the signal directly but a time-domain record can yield this information by the application of the Fourier transform [3-5J.
10.2 Fourier transforIlls 10.2.1 Introduction The Fourier transform technique was developed by].B.]. Fourier (1768-1830) to assist in solving problems in heat conduction [4 J, but is so powerful that it has become very widely used by physicists and engineers and can be applied to a whole range of problems. In the present sphere of interest it is the means by which one can transpose data between the frequency and time domains. This will allow EMC designers and test engineers to view information contained in a signal from the point of view of both domains and thus achieve maximum understanding of it. The examination of prominent ampli tude peaks in a spectrum and their comparison with dominant waveform features can lead to an insight into the type of circuits which are generating the interference signal. This is particularly useful when carrying ou t design or diagnostic EM C testing.
10.2.2 The transform Fourier series, Fourier transforms, Laplace transforms and convolution theory are all linked mathematical ideas which shed light on the relationship between the frequency and time domains. The subject is wide and detailed, with whole texts such as that by Bracewell [3J devoted to its explanation. Clearly, in this chapter there is no space to do other than give some of the concepts involved and ·to state the formulas which apply. Understanding Fourier transforms is so important to being able to understand both frequency and time-domain data that the reader is urged to undertake additional study by recourse to the References. The central idea behind the Fourier series! transform is that within linear systems a complex periodic signal or response can be constructed by the superposition of a number of simple harmonic functions. This is illustrated in Figure 10.2 where a complex signal is presented in the time domain
ELECTROMAGNETIC TRANSIENT TESTING AMPLITUDE OF WAVEFORM COMPONENTS
181
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r1'-l'
FREQUENCY
I
o
"SPECTRUM" OF INDIVIDUAL FREQUENCY ~COMPONENTS
WAVEFORM BEING ANALYSED
~DOMAIN ~
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11ME
OBSERVATION
Figure 10.2
Relationship between time and frequency domains
(1), all the simple harmonic components required to synthesise the complex shape are laid out in freq uency order (2) and the spectrum is then simply the view along the frequency axis (3). Consider the example of a complex apparently discontinuous signal such as a square wave. It can be constructed by the linear superposition of a DC term and a large number of harmonically related cosine/sine waves as shown in Figure 10.3 together with the general expression for the Fourier series. A pictorial representation of the Fourier series for a pulsed waveform is shown in Figure 10.4 with each line representing a component in the series. The first few lines of the pulsed waveform spectrum can be seen represen ted in the frequency domain (3) in Figure 10.5. The individual cosine waves which make up the complex time-domain waveform are shown in (2). They are depicted as all having the same phase with regard to the starting point on the time axis. I t is therefore dictated that
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Reproduced by permission
or Hewlett-Packard
INDIVIDUAL COSINE WAVES
"
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where f (t) represents the waveform in (1) and An are the amplitude coefficients, n the harmonic number, and w the angular frequency.
Frequency domain view (Spectrum analyser)
Figure 10.5
1(0) ". f(f) =
00
+ :I: an cos n 0) f + :I: bn sin nO)f n=1
n=1
~ 1
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n=O
Sum of Fundamental, Fundamental 2nd Harmonic Average value and average value of wave 2nd Harmonic /
3 "
00
f(t)
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FREQUENCYf Rectangular pulse and Fourier series spectrum
Prequency and time domains (cosine waves only)
10.2.3 Introducing phase Unfortunately, this expression does not allow us to compute the time domain waveform from a knowledge of the amplitude and frequency of the lines in the Fourier spectrum as shown in Figure 10.5(3), because it is assumed that all the simple harmonic components are cosines with the same phases. In general, the phases will not be the same and so an extra term is introd uced: 00
Figure 10.3
The Fourier series: building up a pulse
Reproduced by permission or Hewlett Packard
J(t)
LAn cos (nwt+cPn) n=O
10.2
182
A HANDBOOK FOR EMC TESTING AND MEASUREMENT
where c/Jn is the phase of the nth component. But cos (OJt
+ c/J) == cos c/J . cos OJt -
sin c/J . sin OJt
in which cos c/J and sin c/J are constants that can be represented by a and b: cos (n OJ t + c/J n)
== an
Incorrect - arbitrary waveform. (No phase data)
ARBITRARY
7
w
The Fourier transform pair is expressed usually in one of three ways [3J and is given here as stated in Reference 8:
F(w)
=
J~ooj(t) e-iWldt
(direct transform)
cos n w t - bn sin n w t
and this result leads directly to the expression for the Fourier series given in Figure 10.3. If only the amplitude of the cosine and sine terms are known (as measured by a spectrum analyser) this is equivalent to losing the phase information which is required to synthesise the time-domain function f(t) and it cannot be determined unambiguously. By assigning different val ues of phase to the spectral lines in view (3) of Figure 10.5 a range of arbitrary waveforms can be produced in addition to the correct waveform, but all of which have the same amplitude spectrum, as illustrated in Figure 10.6.
Cl
10.2.4 Fourier transform expressions
10.3 and
f(t)
== -1
2n
Joo
.
F(w) elwtdOJ (indirect transform)
-00
10.4 wheref(t) is the time varying function and F(OJ) is the frequency spectrum. A good library of waveforms and Fourier transforms is available [8J which can aid the EMC engineer in analysing spectra or predicting the spectrum of a given waveform. Present-day firmware embodiments of the Fourier transform are very fast and can produce a 1024-point FFT in a fraction of a second. While the manufacturers take care to prevent signal aliasing [2J and carefully select window options such as Hanning and Hamming [2J in addition to a simple rectangular 'boxcar' function, the expert user of such advanced instruments will need to have studied the effects of signal convolution [3J with the measurement instrument time window to be able to detect instrument/measurement artifacts.
::)
I-
~ ~
"CORRECT"
~SES FREQUENCY
~
AMPLITUDE SPECTRUM ONLY (no phase data)
'Correct' waveform - reconstructed using phase information.(Sine & Cosine terms in the Fourier Series)
Figure 10.6
Reconstruction of the true waveform jrom spectrum
This explanation has been made to illustrate how powerful the technique of Fourier analysis can be in transposing between the frequency and time domain, and also to show that it is not possible to make rigorous use of the transformation with only amplitude data in a spectrum as generated by a spectrum analyser or EMI meter. I t is possible though to use the full power of the Fourier transform when suitably digitised waveform data are processed using digital fast Fourier transform routines which are now firmware-based and self contained within modern instruments [6, 7J.
10.2.5 Impulse response Consider a simple network (a resonant filter) shown in Figure 10.7. I ts performance is usually determined by a scanning CW technique using a vector network analyser (for amplitude and phase) giving rise to the filter frequency response as either amplitude and phase plots against frequency or sometimes in the form of a Smith chart. The complementary time-domain technique is to determine the filter impulse response by subjecting the input port to a very fast step func tion waveform with a rise time tmin' This minimum rise time is equivalent to the maximum freq uency which has been scanned in the frequency domain method. The filter will respond (ring in this case) in a characteristic way which is observed from the waveform at the output port. This waveform is called the impulse or step response of the filter and contains time details resolvable down to the limit set by tmin' The impulse response and the frequency response are related by the Fourier transform pair. Now consider calculating the output from the filter for a given input signal and proceed in two
ELECTROMAGNETIC TRANSIENT TESTING SWEPT FREQUENCY NETWORK ANALYSER SOURCE
NETWORK FREQUENCY RESPONSE (AMPLITUDE & PHASE)
,,
PHASE
.....
Single frequency scanned to Fmax
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~
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183
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FREQUENCY
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INPUT
NETWORK UNDER TEST
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,
,~
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III
OUTPUT
A TIME DOMAIN MEASUREMENT
STEP OR IMPULSE GENERATOR
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f2
A FREQUENCY DOMAIN MEASUREMENT
a:
a:
~
~
NETWORK IMPULSE RESPONSE
t
---SLOPE
IItm1n "
Fastest slope in response is slower than IIt min
tmin is related to Fmax
Figure 10.7
ll
Network measurement infrequency and time domains INPUT SIGNAL SPECTRUM (phase not shown)
FREQUENCY RESPONSE OF NETWORK (phase not shown)
w
w
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FREQUENCY
t
SPECTRUM OF OUTPUT SIGNAL (phase not shown)
FREQUENCY
F1 (00)
x
F2 (00)
F3(OO)
(MULTIPLY)
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INPUT WAVEFORM
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...... :::J ~ IL+-..........~~I-'-~...,.,....,__-
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OUTPUT WAVEFORM
TIME
c..
«
f2 (t)
f1 (1)
~ fI--t-+---1r----1"--':--+~+.:>Iitrc"...T,-IM-E
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f3 (t)
(CONVOLVE)
Figure 10.8
Multiplication in frequency domain and convolution in time domain
complementary and connected ways, one in the frequency and one in the time domain. Consider the input signal defined by the frequency spectrum F 1 (w) in Figure 10.8. This can be multiplied by the network frequency response
F 2 (w) (linear amplitude) to yield the output spectrunl F3 (w) in the usual way. If the ou tpu t waveform is required, it can be calculated from the output spectrum (amplitude and phase) by the application of the Fourier transform.
184
A HANDBOOK FOR EMC TESTING AND MEASUREMENT narrow range of some variable. All measurements of physical quantities are limited by the resolving power of the instrument and convolution theory is the underlying mathematical description of the smearing process.
f (t)
10.2.7 Advantages of time-domain manipulation
9 (t)
feu)
"
,"'.-g(t-u)
/w. , ,,'
f(
u) g ( t-u) u
F (t)
0fJ
....,
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F (t)
10.9
......... -011
f(x) *g(x)
Convolution and manipulation of data in the time domain is very irnportant if engineers and designers are to be able to understand the significance of complex waveforms and their interaction with electrical networks. This ability is useful in considering for example the impact of a unidirectional NEMP step pulse shown in Figure 10.1 Oa on a target system, and in relating it to the ringing waveform which is specified for NEMP current injection into a target EDT in MIL STD 462 CS10. See Figure 10.10b. It should be clear that the situation is exactly that described in Figure 10.8 where the excitation pulse is modified by the impulse response of the target
Representation of convolution process
Reproduced by permisssion or McGraw-Hill
Approximately
- 50 kVIM
a:
10.2.6 Convolution
w
The output waveform can also be calculated by convolving the input waveform with the network impulse response. Convolution is a mathematical process with wide application and is described by Bracewell [3J. The convolution of two and g(t) is given by
tii
E (t) = A (e-at - e-pt)
~
/
~
o
>
"'V
J~oof(u)g(t -
u)du
10.5
and is written as f (t) *g (t). The process can be seen by inspecting Figure 10.9 where two sample functions of t are drawn out. For a given value of t the function g(t u) is multiplied by f(u)and integrated over u. The value produced is the value of the convol u tion in tegral at that value of t and is represented as a single value of F(t) by a line with a heigh t proportional to the area under the product curve. rThe output function F(t) is evaluated by the above process for all values of t of interest. rThis process is sometimes called the composition product or the superposition integral and at first sight it may seem difficult to evaluate, but is amenable to solution by simple computer numerical techniques. I t has wide application beyond the filter example in Figure 10.8. Convolution is useful in describing the action of an observing instrument when it makes a weighted mean measurement of some quantity over a
10 nanoseconds TIME---............
(a)
IZ W 0::
a:: ::> a. U - .s
TIME
a.
(b) Figure 10.10
Representation of an exoatmospheric NEMP pulse (a) Double exponential (b) US MIL STD 461 CS10 waveform for injection into pins/cables on an EUT. Ip == 1.05 Imaxe-nft/D sin (2nft) where Ip == common mode pin current in amps, f == frequency, Hz, t == time, s, D decay factor
ELECTROMAGNETIC TRANSIENT TESTING system, leading to the ringing type waveforms that are conducted along cables into the EUT. Riad [9] compares the advantages to be gained by solving transient problems in the time domain with those offered using the frequency domain, and using both in a hybrid fashion with Fourier transforms as a connection. This is illustrated by rigorously deriving the 'impulse response of a simple RC network using each of the three approaches. He points out that obtaining solutions to problems in the time domain is not widely taught but offers advantages which are self evident to those engineers prepared to learn the techniques involved. The main advantage of formulating problems in the time domain is that nonlinearities can be properly treated, as can systems which change with time.
10.3 ESD - electrostatic discharge
10.3.1 Introduction Potentially damaging electrostatic discharge to electronic components and equipment caused by contact with human beings, tools and office furnishings has been of concern since at least the early 1970s [10]. The ESD problem can be considered in two areas: (i)
(ii)
ESD to vulnerable components such as MOSFETs in manufacture, storage, handling, transport and assembly into products. ESD to finished products such as desktop computers in the operational environment. This is caused by the electrical charging of humans and furniture etc. in dry airconditioned offices fitted with carpets and chairs made from synthetic materials.
I t is estimated [10] that the cost of ESD damage in the USA alone was around ten billion dollars a year in 1988, and that this may increase as devices become more sensitive with increasing scale of integration. Control of ESD in the first category, the production phase, is amenable to control through the im plementa tion of careful procedures and the use ofspecial antistatic bags and containers. Four generations of antistatic materials have been produced:
First generation
pink polythene
Developed in the 1970s it has a surface resistivity of 9 between 10 and 10 12 ohm per square, but it had low stiffness, poor dimensional stability, short-lived electrical properties and was easily damaged by solvents.
Second generation
antistatic polypropylenes
This material owes its antistatic properties to
185
hygroscopic additives which draw in moisture from the air. I t has the same resistivity as pink poly but persists for longer; for about five years.
Third generation polypropylenes
conductive
This material has carbon black incorporated into the resin and has a surface resistivity of 10 5 ohm per square. The antistatic properties are permanent and the substance has a higher rigidity and dimensional stability than earlier materials.
Fourth generation materials
fibre impregnated
These materials are impregnated with a random network of fibres that provide structural reinforcement and static charge dissipation. They offer permanent antistatic properties, a high resistance to sloughing and to chemical agents. The second category of ESD problems, which affects completed electronic products in the operational environment, cannot be easily controlled by procedure. Resistance to ESD must be designed into the product by careful choice of components, their positions on circuit boards, the use of electrostatic and electromagnetic shields, protection of circuits backing onto connector pins by the use of fast switching limiters, the choice of suitable materials for product cases and the careful design of case joints, edges and apertures. To design in ESD protection the designer must understand the nature, magnitude and frequency of occurrence of ESD events and how they are generated in the operational environment.
10.3.2 The ESD event Human beings can acquire electrostatic charge when walking or shuming along a carpet by a process known as the triboelectric effect. Two dissimilar materials such as wool in the carpet and rubber on the soles of shoes can exchange charge which then builds up on the person. The charged individual then moves towards a piece of electronic equipment such as a personal computer and discharges the stored energy to it via a finger tip or a metal object such as a ring, pen or tool of some kind. The resulting spark to the case produces a fast risetime current which is injected into the equipment and can disrupt or damage the device. Depending on the two materials and the rate at which they are rubbed together, ch~rge will build up at different rates on the two objects. The affinity for materials absorbing charge can be ranked in a table known as the triboelectric series,
186
A HANDBOOK FOR EMC TESTING AND MEASUREMENT
Table 10.1 Positive
Negative
Sample common materials arranged in triboelectric series Air Human hands Asbestos Rabbit fur Glass Mica Human hair Nylon Wool Fur Lead Silk Aluminium Paper Cotton Steel Wood Amber Sealing wax Hard rubber Nickel, copper Brass, silver Gold, platinum Sulphur Acetate rayon Polyester Celluloid OrIon Polyurethane Polyethylene Polypropylene PVC (vinyl) KELF Silicon Teflon
16 15 14 13
>
~
w (')
11
~ ....J
10
>
9
0
w
8
0:
7
(') ~
J:
0
6 5
WOOL
4
/
3 2
1a--.--&...-.a..---a._01---+---a._..a.-.......a---a._.......
5 10 20 30 40 50 60 70 80 90100 RHO/o
Figure 10.11
Typical voltages to which humans will charge as function of relative humidity. Maximum values of electrostatic voltage to which operators may be charged while in contact with the materials shown
Reproduced by permission of BSI
circuit as shown in Figure 10.12 and has been adopted by the lEG for lEG 801-2 and by the DOD for MIL STD-M-385IO. ~1ore complex models have been proposed to account for faster risetimes of the leading edge of the pulse and to produce multiple discharges, which are both sometimes observed under normal office conditions. A crude description of the worst-case waveform and its frequency con ten t resulting from the simple RC model discharge is given in Figure
!zw and an extract from the US DOD- HDBK-263 (1980) is given in Table 10.1. Once the charge begins to build up on a person due to the triboelectric effect a second effect begins to take place which bleeds off charge to the surroundings. This discharging effect depends on the humidity of the air: thus ESD problems can occur more frequently in air conditioned offices where the humidity is low. The voltage to which a person becomes charged depends on the amount of charge acquired and the self capacitance of the individual. In general, the larger the surface area of the body the larger is the self capacitance [11 J. Jones [12J gives data for the voltages to which humans can be charged when in contact with various materials as a function of relative humidity, see Figure 10.11. The basic human ESD model IS a simple RC
12
A
Discharge model
~40A
,....., 500 ohms
----+~ ,.....,200 pF
:::>
20 kV
o w
*
"« 0::
J:
o
en
o time
~
~
w
A
~20dB
o
/perdecade
-I
«
0:: t-
O
W
D-
en
10 MHz
~
200 MHz
10Hz
FREQUENCY Figure 10.12
Typical assumed worst-case ESD waveform
ELECTROMAGNETIC TRANSIENT TESTING Probe tip inductance
- - Ground return inductance
.... A z w ~ 40A ::::>
()
w o ex:
c(
:r: ()
(j)
B time
Figure 10.13
Ground return inductance impairs desired waveform
10.12 [11]. I t can be seen that the risetime is of the order of 1 ns with a tail about 100 ns long. In a portable ESD simulator test equipment the achievable risetime is governed by the inductance of the tip and ground return leads shown in Figure 10.13. Risetime is im portan t because it is known to affect the susceptibility of an EDT to ESD of a given voltage. Thus producing an ESD simulator with a required and repeatable risetime is an important goal of simulator design. When the energy stored in the capacitor of the simple RG model in Figure 10.12 is discharged into a short circuit a current of up to 40 A can flow into the EDT. In nature both positive and negative polarities can build up on human skin. However there is no clear evidence [11] that testing with either polarity produces different effects in electronic EDTs. Most simulators and standards, such as lEG 801-2 (1984), specify only positive voltages.
10.3.3 TypesofESD There are a number of types ofESD which give rise to different voltage and current pulses to which victim devices or equipments may be exposed. The variability and fineness of detail which can occur in ESD pulses has been explored by King and Reynolds [13], amongst others. They measured many hundreds of discharge events for both direct finger contact and conduction through small
187
metallic objects such as rings. Events with leading edges rising as fast as 300 ps were observed, as were multiple discharges at modest voltages of 3-4 kV when metallic objects were in the discharge path. Transition regions were observed at voltages of 68 kV between variable very fast risetime and high current events to the more repeatable conventional slower (> 1 ns) pulses. In some tests [13], fast pulse peak currents of 170 A have been observed at discharge voltages of 4kV. It has been shown by experiment that the variety of ESD pulses is considerable and argument still continues about the need to simulate this variability, particularly with regard to the fast-pulse phenomena. Throughout the investigations however it is clear that the worst case events (high current and fast risetime) are observed when the discharge to the EDT takes place through a metallic object. The multitude of ESD events that occur in the operational environment can be simulated by three types of model [14].
10.3.3.1 Human body model People are the primary source of ESD events and the normal, slow discharge phenomenon ( lO15 ns risetime), can be simulated by the RG network as sho~:t;l in Figure 10.12. The lEe 8012 (1984) standard calls for alSO pF capacitance and the specified simulator circuit is given in Figure 10.14. CAPACITOR CHARGING RESISTOR "'" Rch 100Mn 16.5 k V Cs
CAPACITOR DISCHARGING RESISTOR \
Rd
150 n 150 PF
DISCHARGE ELECTRODE
FUNCTIONAL EARTH
Figure 10.14
ESD probe circuit from first edition of lEG 801-2
Reproduced by permission of BSI
10.3.3.2 Charged device model Device leads, frames and packages can be charged triboelectrically, just the same as humans and can be discharged from the surface of the device to ground via the pins or other conductive parts of the device under test. The charge vol tage and discharge energy will depend on the position and orientation of the charged device with respect to ground. In general the ESD pulse produced with the charged 'device model has a much faster risetime than that for
188
A HANDBOOK FOR EMC TESTING AND MEASUREMENT
the human body model and can be substantially less than 1 ns [14J. Bhar [14J conducted a literature survey which showed that devices such as a 64 K DRAM have an ESD damage threshold of 2 kV when tested using a simulator based on the human body model, but which dropped to only 850 V when testing with the faster pulse produced by the charged device model. I t was observed that generally hurnanbody ESD caused junction damage to the device whereas charged device ESD causes dielectric breakdown [15, 16J.
10.3.3.3 Field-induced model This simulates the effect of charge separation and subsequent discharge on a device when it is exposed to an external static electric field. The mechanism of charge separation and subsequent ESD on an integrated circuit placed in a field generated by a potential gradient between the charged object (not the IC in question) and the ground is described by Unger [1 7J. The field induced ESD model may be visualised as shown in Figure 10.15 with the device lying in the
~+~ +
+
CHARGED OBJECT
l
_
&t~~jj+~
iii
----~~~I~~~~~~~~~;;--/ ++++++++++++++++++++++++
external electrostatic field of the charged object. The discharge pulse is fast, rather like that for the charged device model and parameters have been validated by Enoch and Shaw [18, 19J.
10.3.4 ESD-induced latent defects Experimental work has been carried out [20, 21 J in exploring the effect of ESD which does not modify or destroy the device at the time of application, but does cause an operational failure at a later time. This is called the latent defect mechanism. The existence of such a mechanism is of considerable concern as devices which may have experienced ESD events will not have detectable failures and may be incorporated into equipments which then subsequently fail. This leads to expensive field diagnostics and equipment repair with the attendant poor reliability image which the product may then acquire in the mind of the users. It has been reported that CMOS ICs fail in a latent manner owing to gate oxide rupture [20]. Gate oxide shorts can change character with normal use and sometimes partially recover, but sometimes degrade and cause intermittent or permanen t failure [21 J. There has been some suggestion that latent ESD effects could be beneficial to device operation [22J as discharge pulses have a 'hardening' effect which reduced subsequent sensitivity to ESD damage.
10.3.5 Types of ESD test C
I
1
2 =r=cc =r:::
I
ESD testing is conducted using two different techniques which can be applied directly or indirectly:
Cg
Air discharge method Contact discharge method
direc t tes t } indirect test
EQUIVALENT CIRCUIT
c1 }'igure 10.15
___ Induced ESD on electronic devices Cj == Capacitance between charged object (+) and IC) C2 == IC capacitance between top and bottom surface C3 == Capacitance between lower plate and bottom surface Co == C2 + C3 == IC capacitance to g;ound) R t Resistance oj the test circuit) R d and Cd == Resistance and capacitance oj the device
There was some controversy in the late 1980s about which was the more suitable type of test, some preferring the reality of an air discharge at the expense of repeatability, while others felt that a reliable contact test was more important for testing mass-produced items such as computers, even if it did not simulate all the phenomena observed in operational situations. If the question of the need to simulate the fast subnanosecond prepulse is also considered, it is evident that ESD testing was an area for lively technical debate, which still continues to some extent even though standards such as IEC 801-2 have been re-issued in draft to reflect some of the technical concerns which had been raised.
ELECTROMAGNETIC TRANSIENT TESTING
10.3.5.1 Air discharge test This has been the standard test for a number of years and relies on the discharge taking place in the air between the tip of the simula tor probe and the surface of the EUT as shown in Figure 10.16. The design of the ESD simula tor circuit and shape of probe tip, as defined for example in IEC80 1-2 (1984) attempts to replicate the ESD
Figure 10.16 Hand-held air discharge testing CIRCUIT DIAGRAM
Rd
Rch 100MQ 16.5 kV Cs
DISCHARGE ELECTRODE
150 Q 150pF
....FUNCTIONAL EARTH
----------11...-. DETAIL OF PROBE TIP ___BODY OF GENERATOR
ep8
/ ~.
1
50 ± 0.02
_
AIR DISCHARGES
DISCHARGE CURRENT WAVEFORM
I
~ ~
a:
1-----. 0.9
::::>
()
189
event from the body finger model giving rise to the slow pulse shown in Figure 10.1 7. This test is widely used the world over, but the knowledge gained during testing has led to the realisation that it has some deficiencies [12]. These include The test did not always simulate EDT failures observed in operational use The severity of the test varied with relative humidity Equipment could fail at low voltage levels, but pass at higher ones. The test procedures for discharging to objects near the E UT were not well defined, yet this indirect test is important for items with plastic cases and Iittle RF shielding. I t had been shown [23] that the pulses produced by air discharges were heavily dependent on the rate and direction of approach of the simulator tip to the EUT. Further, complex corona discharge phenomena have been observed [24] which lead to a reduction of the actual probe tip voltage from the expected value by as much as 10kV /s for corona current of 1 f.1A. Thus during the approach time of the probe to the E UT the voltage could fall from say 20 to around 10kV resulting in an undertest. There are advanced air discharge simulators which compensate for this effect by internal tip voltage feedback circuitry [24]. Despite some of these problems· the air discharge method is reported [25J as being specified by the following standards in addition to the original IEC801-2: EIA PN 1361: The Electronic Industries Association (USA) produced a draft standard in 1981 which focused on voice telephone terminals req uiring body finger and body metallic models to be simulated ECSA: Exchange Carriers Standards Association (USA) adopted IEC801-2 with regard to central office telecommunications equipment in 1984 and requires both direct and indirect discharge tests NEMA DC33: National Electrical Manufacturers' Association (USA) issued a draft standard in 1982 which focused on residential equipment and appliances. It was never formally released but called for both direct and indirect air discharges.
UJ
~
«
0.5 ....- . . . . . - + - -
10.3.5.2 Contact discharge test
I
()
C/)
is
0.1
Figure 10.17
ESD probe first edition of IEC801-2 (1984)
Reprod uced by permission of BS I
To overcome some of the unpredictable aspects of the air discharge test, workers [12] have proposed the introduction of a contact discharge simulator where the EHT is switched to the probe tip (which is held in contact with the EUT surface) via a suitable closing switch. A circuit diagram of the con tact discharge tester as specified in the
190
A HANDBOOK FOR EMC TESTING AND MEASUREMENT CIRCUIT DIAGRAM OF NEW GENERATOR Tip is in contact with EUT SUrl~
_
---I===J------,r----e==t---+~ -Iof---''I/. FACE
II
OFEUT
D~~~~~GE
DISCHARGE RETURN CONNECTION
DISCHARGE TIP OF NEW GENERATOR
-f-7~~~:
SHARP POINT
FOR CONTACT DISCHARGES
NEW DISCHARGE WAVEFORM CURRENT I I peak: 100% 90%
-
I at 30 ns
I at 60 ns .... - .....,..--
10%
very fast risetime ---
Figure 10.18
fr
60ns
time
Details oj new IEC801-2 ESD test
Reproduced by permission oCBSI
second version of IEC80 1-2 is shown in Figure 10.18. The most appropriate reliable fast risetime switch was found to be a vacuum relay. The probe tip is now pointed rather than rounded to enable contact to be made through paint to the metal surface of the E UT. The revised specification also stipulates a more complex waveform which has a fast event before the main discharge which can be seen in Figure 10.18. I t has been reported [25] that the fast-edge events occur under operational conditions only 3-5 % of the number of actual events. However, Wood [26J suggests that although the .fast spike may be infrequent and contains little energy it is 'largely responsible for voltage failures and the corruption of IT equipment'. The slow pulse which follows the spike contains most of the energy and is considered to cause current failures and lead to component damage.
In addition to the second version of IEC80 1-2, a number of other standards call for a contact discharge test and some also permit air discharges where appropriate, as the contact method is not suitable for injection onto the joints and seams in the plastic case of an EUT. ECMA TR40: The European Computer Manufacturers' Association published a report in 1987 which changed the approach from air discharge to contact discharge ESD testing. It still permits air discharge tests onto plastics cases near to conducting parts. The contact discharge test is also permitted using a metal foil which is connected to the close-by metal part/case. ANSI: The American National Standards Institute committee ASC C63.4 ScI began work in 1985 to define a standard for testing products and adopted the con tact discharge method alongside the air discharge test. The method used is at the discretion of the tester [25]. Both direct and indirect tests are stipulated and the body metallic and mobile furnishing models are used. Draft 5 (1989) of the standard has been produced and is directed to all electronic equipment. SAE AIR 1499: The Society of Automotive Engineers (USA) has produced several ESD specifications over the years, with this one being the latest. It calls for either air discharge or contact discharge and both body metallic and mobile furnishing models are used. The SAE have also made a proposal to ISO (International Standards Organisation) calling for air discharge testing using a body finger model to be applied to automobile subsystems and components. A more complete list of standards relating to ESD can be found in Chapter 2 and Appendix 1.9.
10.3.5.3 Indirect ESD tests In these tests either an air or contact discharge is made to a conducting plane in the vicinity of the EUT. This plane can be vertical or horizontal and in several positions around the EUT; see Figure 10.19. When the indirect discharge occurs to the metallic plane the fast-varying currents that are set up on the plane produce electromagnetic radiation which .couples to the EUT and is deemed to be important in generating faults in it [27]. The EUT is almost certainly in the near field of the radiation for most of the frequencies which make up the radiated pulse. Thus the coupling to the EUT is very complex but involves magnetic and electric induction fields and radiative coupling. The repeatability of indirect tests has been investigated [27J and it has been found that there are three test parameters which affect the outcome. These are
ELECTROMAGNETIC TRANSIENT TESTING TYPICAL POSITION FOR DISCHARGE TO VERTICAL GROUND PLANE ( VGP )
\.
191
approval' test it is important that no sensitive areas are overlooked, otherwise there may be problems in the field. I t is estimated that between 3,000 to 5,000 pulses need to be applied to a unit such as a desktop computer to ensure all aspects of ESD susceptibility have been covered. The quality control verification testing can be carried ou t wi th perhaps as few as 200 to 500 pulses.
10.3.7 ESD test voltage levels
Figure 10.19
Indirect ESD test for table-top equipment
Reproduced by permission of BSI
•
•
•
The location of the discharge on the flat plate; discharging on the edges of the plate rather than to the centre, makes a factor of two difference in the voltage at which computer E UTs are prone to fail. The position of the simulator ground return wire; this return wire appears to be a significant radiator and its routing therefore changes the field around the E UT. I ts effect is to alter the disruption or failure threshold by the equivalent of a few kV. The location of the radiating plate ground wire; this has been shown to have a small effect on the disruption levels of an EU1-' and is less significant than the two former ones.
10.3.6 Number of discharges per test Jones [12J suggests in line with the revised IEC80 1-2 that at least ten discharges (five positive, five negative) should be applied to each test point selected. Others [25J indicate that many manufacturers/test houses use about 50 discharges per unit area. This may be defined as the side of the case for example, or as discharges per square metre. Other testers may use as many as 10,000 pulses to test an EUT to make a valid statistical analysis of its ESD withstand capability [28J taking into account aspects such as EUT cycle time. I t is necessary to use more pulses to perform an engineering characterisation of a product than to perform quality control sampling. When the engineering team carry out the ESD 'type
These will be contained in the applicable specification relating to the type of product being tested and the country into which it is being sold. The IEC80 1-2 ESD standard is widely used throughou t the world and the levels specified in the new draft are given in Table 10.2. The selection of a severity level for a test depends on the type of materials and environmental conditions that are to be found surrounding the EUT in normal operational use. The IEC recommended ESD levels are to be assessed from Table 10.3. The four severity levels from the lEe 801-2 (1984) standard are carried forward and a new
Table 10.2
IEC801-2 (second draft) Test severity levels
Level
Test voltage cantact discharge
Test voltage air discharge
kV
kV 2 4 8 15 Special
1
2
2
4 6 8 Special
3 4 x(1)
Table 10.3
Selection of test severity levels (IEC801-21984)
ESD test voltage and environmental conditions Class
Relative humidity as low as
Antistatic
Synthetic
%
1 2
35 10
3
50
4
10
Maximum voltage kV
2
x x x x
4 8 15
Test severity levels are selected in accordance with the most realistic installation and environmental conditions. For other materials, for example, wood, concrete, ceramic, vinyl and metal, the probable level is not greater than class 2.
192
A HANDBOOK FOR EMC TESTING AND MEASUREMENT
x-level in Table 10.2 is permitted, which can be agreed between the manufacturer and user [12]. For air discharges, the levels remain the same in the revised standard. However, because the con tact discharge test has been shown to be more s'evere at the higher voltage levels, the voltages differ at the two highest levels. Contact and air discharges are com pIe men tary, not alternative, and should be applied as follows.
likely to grow rather than diminish, as experience to date suggests that there may be other more complex aspects to ESD discharges and their interaction with equipments, which may be revealed as oscilloscopes and waveform digi tisers become faster and permit closer inspection of the phenomenon.
10.4 Nuclear electromagnetic pulse Contact discharges should be applied to areas of the EDT which are accessible to an operator in normal use and customer maintenance. Air discharges should be applied to case seams, slots, air vents and keypads, etc. where the discharge could penetrate to internal conducting components and which cannot be adequately tested in the contact discharge mode.
10.3.8 Assessing EDT performance
Many readers involved with the electromagnetic compatibility of commercial electronics may have no direct need to understand the effect of the radio freq uency pulse prod uced by a nuclear weapon on their equipment. I t is suggested however that this section con tains a good example of a transient RF pulse and how it interacts with equipment. Many of the general points made earlier in the chapter about convolution and equipment impulse response are illustrated.
There are four categories of EDT performance that need to be monitored during an ESD test:
10.4.1 Introduction
(i) (ii)
(iii)
(iv)
Enrico Fermi suggested that electromagnetic effects would be observed in the first nuclear explosion in 1945 [29]. In 1954 Garwin at Los Alamos estimated the parameters of the pulse that would be radiated by an asymmetric gamma source in the exponential growth phase of the NEMP signal. In 1956 other workers in the USA studied the possibility of using the EMP pulse from a nuclear weapon to detonate magnetic mines. The first serious attempt to understand the impact of NEMP on possible target equipments got underway with the Minuteman missile programme in 1960. The electromagnetic pulse is only one of a number of effects produced by a nuclear weapon. In order of promptness following the detonation, the others are gamma, optical and x-ray pulses, EMP, neutrons, thermal effects, blast/overpressure, dust, debris and fallout. Details of weapon effects are of course still classified, but readers involved in this field are
Normal operation within specified limits Temporary degradation/loss of function which is self recoverable, known as transien terrors Temporary degradation/loss of function which req uires opera tor correction (correctable errors) or system reset (non correctable errors) Degradation/loss of function which is not recoverable due to component or software damage, or loss of data, known as hard errors.
The detailed statement of pass/fail criteria based on the voltages at which the above performance levels are recorded will normally be contained in the test plan approved for the product in question. Dash [11] suggests some acceptance criteria for computer-like products, given in Table 10.4. ESD testing is now accepted as an integral part of the wider EMC testing field. I ts importance is
Table 10.4
Suggested tolerablepercentage errors for different test charge voltages
Test voltage
Transienterrors
Soft errors
Hard errors
Correctable
Non-correctable DID
kV
DID
DID
5 10 15 20
0 50 100 100
0 5 15 100
0 0
DID
0 0
5
0.
100
0
ELECTROMAGNETIC TRANSIENT TESTING referred to courses such as those run by RM CS [30]. In this text it is intended only to acquaint the reader with a broad view of issues relating to the NEMP effect, methods of simulating it, and basic test methods which are used alongside those specified for EMC in documents such as MIL STD 461BjC.
10.4.2 Types ofNEMP The nuclear electromagnetic pulses to which electronic equipments may be subjected are produced by several mechanisms, of which the gamma-ray mechanism is perhaps the most important [29]. The EMP generation mechanism and the resulting pulse depend on the height of the weapon burst above the earth.
Surface burst: Occurs at an altitude of less than 2 km and produces an EM field similar to that of lightning [31]. The prompt effects, however, dominate at this range. Air burst: Occurs at heights of 2-20 km and is sometimes called an endoatmospheric detonation. Both the prompt weapon effects and an EMP are observed. lligh-altitude EMP (HEMP): Sometimes referred to as an exoatmospheric detonation which occurs at heights in excess of 40 km and produces the most general EMP effects over a large area. For space-based electronic systems the prompt radiation effects can produce an internally generated EMP known as SGEMP (system generated EMP) by direct interaction with structure and components.
193
What follows concentrates on the exoatmospheric EMP effect because of its potential impact on all types of civil and military electronic equipment over a wide geographical area. Rudrauf [32J reports that if a 1-5 megaton device were detonated at an altitude of 300 km above the Ba7J of Biscay the resulting EMP would contain 10 1 joules and would produce a pulsed field strength of up to 50 kV jm over almost all of Western Europe. Although the pulse would last only a few hundred nano seconds the instan taneous power would be some 500,000 million megawatts.
10.4.3 Exoatmospheric pulse generation This has been widely described [29, 30, 32-34 J and is treated very briefly here. The gamma pulse from the detonation will build up with a risetime of a few nanoseconds and propagate in all directions. That portion of the flux travelling towards the earth will interact with the upper layers of the atmosphere as shown in Figure 10.20. The gamma rays produce a flux of Compton recoil electrons \,yhich constitute an electric current density with a risetime of the same order as the gamma pulse. The time-varying current density then gives rise to the radiated EMP. Each Compton electron receives about 1 MeV of energy from the gamma interaction and can produce about 30,000 electron ion pairs along its track in the air. These secondary electrons make no appreciable contribution to the EMP driving curren t but they make for electrical cond uctivi ty which limits the amplitude of the EMP and influences the waveform [29].
Gamma ray energy from explosion Compton electrons deflected by Earth's magnetic field
UPPERATMOSPHERE~ ~ ~I DEPOSX~
~~
~
RADIATION REGION :I
Figure 10.20
EARTH
Generation of nuclear exoatmospheric electromagnetic pulse
Gamma rays interact with neutral particles in upper atmosphere to produce energetic electrons by "Compton II collision
Reproduced by permisssion of leT Inc.
194
A HANDBOOK FOR EMC TESTING AND MEASUREMENT 12
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(b) NEMP exoatmospheric pulse (a) pulse waveform (b) pulse spectrum
Reproduced by permission of' ICT Inc.
The waveform which propagates to the surface of the earth is roughly represented by a double exponential pulse [34 J of the type illustrated in Figure 10.21, with a characteristic risetime and falltime, and a peak amplitude of about 50 kVj m. The spectrum can be obtained by Fourier transform of the pulse waveform and is also illustrated in Figure 10.21 showing that the rnajority of the energy in the pulse is confined to frequencies below a few MHz [30J. I t is possible that in future new phenomena may emerge which lead to pulses with enhanced higher freq uency com ponen ts. Additional pulse shapes and peak amplitudes derived from the other types of NEMP generation are given by Rickets et al. [33].
10.4.4 NEMP induced currents The exoatmospheric pulse shown in Figure 10.21 can induce enormous currents into long wire an tennas such as overground power lines and represents a major threat to electrical generation, distribution and control equipment. An estimate of the induced current in such a power line is given by Ghose [34J and shown in Figure 10.22. Currents induced into the structures of systems such as aircraft, missiles, radio towers and other simple forms which can be represented by electrical dipoles or monopoles with a reasonable Q, (of around 5-10) are usually found to have the characteristics of a damped sinusoid. Results of the calculation of the induced opencircuit voltage and short-circuit current in an example monopole [33J (7.5 m high and resonant at 10 MHz) are shown in Figures 10.23a and b and illustrate the general damped sinusoid response. Other calculations and measurements [35-37J have been made to explore the induced currents in cylinders, wires inside leaky cylinders and surface currents on other metallic bodies of revolution. All indicate that the usual induced current waveforms are bidirectional damped
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Reproduced by permission of' ICT Inc.
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Voltage and current induced into 7.5 m-Iong monopole e.g. antenna tower or street lamp, by NE.MP stimulus (50 k Vj m) (a) Open-circuit voltage for 7.5-m monopole (b) Short-circuit current for 7.5-m monopole
Reproduced by permission of' Wiley
ELECTROMAGNETIC TRANSIENT TESTING
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sinusoids produced in response to the unidirectional NEMP incident pulse. Damped-sinusoid induced-current responses have been measured and calculated for complete systems such as fighter aircraft [38, 39J and missiles [40]. Figure 10.24a shows the induced current density parallel to the fuselage of an FIll aircraft, measured on the underside just in front of the wings [38J. The plot is actually the time derivative of the current density and needs to be integrated over time to yield the actual current waveform. I t is common practice to record and present time-derivative data as these are what is actually measured by the special wideband Band D dot sensors fitted to the EUT. The time derivative exaggerates the higher frequency components in the actual signal. If the necessary integration is carried out at the sensor, problems of signal breakthrough, dynamic range and signal-to-noise ratio may be encountered. The integration of the time differential signal can be better carried out digitally in the microprocessor of a digitising oscillo'scope at the recording site. If the current density differential record given in Figure 10.24a were integrated to yield the true current density waveform, the fast variations would be suppressed, but its damped oscillatory
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form would be preserved. The Fourier transform of the true current density waveform is shown in Figure 10.24b. This induced current density spectrum shows a resonant peak at about 6 MHz which corresponds to the main ringing in the current waveform and is related to the electrical length of the aircraft fuselage. The tests on the F III were carried out by Kunz and Lee [39J in the USA using a horizontal dipole simulator at the US Airforce Weapons Laboratory, which attempts to simulate the NEMP pulse. Its idealised frequency spectrum is given in Figure 10.25. After recording many measurements of the damped sinusoid type at various locations on the aircraft a picture of its main resonant properties could be built up and the envelope of many spectra such as that in Figure 10.24b was constructed, and is shown in Figure 10.25. This inverted-V curve is an example of the general coupling property of systems with resonant, medium Q, metallic structures and has been confirmed elsewhere [41 J. All systems that have a frequency response of this resonant form will have an impulse response in the form of a damped oscillatory wave. This is exemplified in Figure 10.26 which shows the induced current in the skin of a missile [40J, which is a high-Q, high-aspect-ratio metallic tube and behaves like a fat electrical dipole. I t is because so many real induced current waveforms are damped oscillations that the current injection NEMP tests called up in standards such as MIL STD 462 CSI0/ll specify damped sinusoid waveforms.
10.4.5 NEMP testing NEMP testing can be divided into component, equipment and system testing; each, type of test requires different stimulation equipment and different instrumentation. The order just given reflects the complexity of the test and the cost which can increase almost exponentially from a
196
A HANDBOOK FOR EMC TESTING AND MEASUREMENT incident pulse can be measured by instruments which are either attached directly to the component in a noninvasive way, or connected external to the jig. The jig design may be based on transmission-line techniques, the RF properties of which are well understood, so that the pulse waveform delivered to the component is not distorted, and that the absorbed, reflected power and component dynamic impedance can be determined by measurement external to the jig. Many types of active and passive components have been measured over the years and a sound database exists in various government and industrial establishments. Component tests re sometimes cond ucted with a simple sq uaretopped 1 IlS pulse [34] or a damped sinusoid. Rickets [33] gives examples of upset levels for active components and damage levels for active and passive components in terms of watts for a 1 IlS pulse, see Figure 10.27. Much useful data on in tegra ted circuit disruption and damage due to RF energy are contained in a report by McDonnell Douglas [42]. Figure 10.28, rep rod uced from their report, shows the peak power needed for burnout of a typical bipolar opera tional amplifier integrated circuit as a function of pulse duration. Figure 10.29 gives the measured pulse energies [34] req uired to fail various typical electronic components. Notice that the most sensitive devices require less than 1 IlJ to destroy them.
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Reproduced by permission of' BAe Dynamics Ltd.
few hundred pounds/dollars for a component test to many millions for tests on large complex systems such as an aircraft or ship.
10.4.5.1 Component testing This is carried out by a direct injection method onto the component terminals when mounted in a suitable jig. The jig must accommodate the component in such a way that the true RF power! energy absorbed by the component from the
Figure 10.27 Thresholds for typical active and passive electronic components
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Estimate ofpeak power required to burn out bipolar op. amp. Ie as function of pulse duration.
Reproduced by permission of McDonnell Douglas
PULSE ENERGY Jl J
101
102
104
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-
Examples ofpulse energy required to damage electronic components
Reproduced by permission of leT Inc.
Much has been written on designing equipment to withstand NEMP attack and the data on component hardness are important in this context. Many protective measures can be taken [33, 34] to reduce the ingress of pulse energy to that which components can survive. These include screening cables, shielding equipment, fitting transient clipping devices and filters. The first real test of many of these interconnected design features occurs at the equipment or box level.
10.4.5.2 Equipment testing Some equipments may be installed in the'final system such that the only NEMP induced energy to which they are subjected comes along cables or other conducting pipes or hoses. In this case current injection using damped sinusoids is the appropriate technique. Other equipments may be exposed to both current injection via cables and directly to the NEMP wavefield. In this case both a damped sinusoid current injection test and a radiated test using a simulated NEMP pulse are both needed.
197
Specifications for NEMP testing usually only apply to military equipments, but the requirement may be extended to cover certain items of commercial electronics such as telephone and telecommunications systems, power conditioning and distribution and computer systems. In the UK, DEF STAN 00-35 describes all environments which are relevant to military equipments, including natural ones such as lightning and also includes a section on NEMP. Design advice is available in DEF STAN 084 wi th regard to techniq ues for hardening equipment against NEMP. Testing requirements for equipments are exemplified in two technical standards which are DEF STAN 59-41 in the UK and MIL STD 461Cj462j463 in the USA. Probably MIL STD 461 Cj462 represents the best example of a set of NEMP related equipment tests which are embodied within a wider framework of general EMC testing. It is for this reason that the NEMP tests called up in this standard are examined more closely. The structure of MIL STD 461 C is complicated and different sections relate to tests on different classes of equipment which are procured for use by the Army, Navy and Air Force. It is not intended here to define the exact applicability of the various NEMP tests but rather to indicate the types of tests a'fld their limitations when applied to equipments. The NEMP-related tests in MIL STD 461C consist of two types. The conducted susceptibility tests CS 10 to CS 13 involve the injection of damped-sinusoid waveforms into pins, wires and cables. The radiated susceptibility test RS05 requires equipment and interconnect cables to be subjected to a unidirectional double exponential simulation of the free-field weapon pulse. CS 10: Conducted susceptibility, dampedsinusoidal transients, pins and terminals (pin injection), 10 kHz-I00 MHz (MIL STD 462 notice 5 Navy). CS 11: Conducted susceptibility, dampedsinusoidal transients, cables, 10 kHz-l 00 MHz (MIL STD 462 notice 5 Navy). CSI2: Conducted susceptibility, common-mode cable current pulse, interconnecting and power (MIL STD 462 notice 6 USAF). CS 13: Conducted susceptibility, single WIre coupled pulse (MIL STD 462 notice 6 USAF). RS05: Radiated susceptibility, electromagnetic pulse field, transient (MIL STD 462 notice 5 Navy). These NEMP equipment tests are described briefly to illustrate a few of the more important test requirements and instrumentation which should be used.
198
A HANDBOOK FOR EMC TESTING AND MEASUREMENT ISOLATION TRANSFORMER
TEST SAMPLE _ _ _ _ LISNS
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Test configuration,lor US MIL STD 461 CS-10 NEMP pin injection test
CS 10: The test setup is shown in Figure 10.30. The pin/terminal under test is accessed via a breakout box using a short « 25 cm) low-resistance wire and the damped-sinusoid current is indirectly injected into it via an inductive (ringing frequencies < 10 MHz) and capacitive (ringing freq uencies > 10 MHz) probe. Alternative directcoupling techniques can be used with the agreement of the EDT procurement authority. l'he test waveform is that shown in Figure 10.1 Ob and the value of the peak current at various frequencies is taken from Figure 10.31. The test should be conducted at frequencies of 0.01, 0.1, 1, 10, 30 and 100 MHz and at other EDT critical frequencies. Ten pulses of both positive and negative polarity should be injected at a rate of between 1 per n1inute and 1 per second. CS 11: The test configuration is shown in Figure 10.32 and the test waveform, maximum current, test frequencies etc. are all similar to CS 1O. The screens around cables which are to be tested are
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dealt with in different ways depending on the nature of the cable. The specification should be consulted for details. As with CS 10 the injection signal genera tor is calibrated prior to use to set the maximum test levels. CS 12 and CS 13: The test configurations and injected waveforms are similar to those for CS 10 and CS 11. The peak current as a function of freq uency is shown in Figure 10.31. The pulses in CS 12 are applied at the rate of 1 per second for a period of 5 minutes, after which the polarity is reversed and the test repeated. In the case of CS 13 the pulses are applied at one pulse per second for not less than 1 minute and a minimum of 50 pulses must be applied for each polarity. RS05: The EDT is set up in a 100 ohm parallelplate line as shown in Figure 10.33. The line should have sufficient separation between the plates such that the EDT occupies less than half of the available height. The EDT shall have a footprint which is smaller than the useable area which has been previously mapped out during a calibration exercise. The field strength as a function of time is shown in Figure 10.21 and simulates the standard exoatmospheric pulse. The EDT is exposed to a minimum of ten pulses in each of three orientations at pulse repetition rates of between one per second and one per minute.
10.4.5.3 System testing
~~
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MIL STD 461 .NEMP current injection limits
Testing large systems for hardness against NEMP (or for EM C) is a difficult and expensive undertaking. I t is only carried out on key systems which must be shown to be able to survive the high-level electromagnetic pulse. Such systems might include long-range bomber aircraft, nuclear armed missiles, fighter-bomber aircraft, ships, command and control centres, mobile tactical command and control facilities, telecolnmunications facilities, and key power generation and transmission facilities. There are two key interrelated problems with testing systems such as these:
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Test setup for MIL STD 461 RS05 NEMP radiated susceptibility test
They are large 20-300 m (or greater) in extent. Some systems such as communications centres are fixed and must be tested in situ. Simulating the correct NEMP waveform at the high field strength required over such a large target system. This calls for the generation and precise shaping of a very high-voltage pulse (up to 5 MV) that is applied to the large radiating elements of the simulator.
The technical and cost constraints which flow from these two problems mean that it is not possible to build a single, large all-purpose facility which can test a whole inventory of systems. It has transpired that individ ual simula tors have been built to test specific types of systems. NEMP simulators fall into two technical types: (a)
(b)
Bounded-wave devices, where the energy is constrained somewhat in the simulator structure and, around the system under test. Free-field simulators, where the wave is generated and allowed to propagate freely into free space. With such a simulator it is possible to test a system such as an aircraft in any orientation or even in flight (at subthreat levels).
In general, the field strengths produced in the bounded-wave devices can be at full threat level (50 kV 1m), whereas the larger volumes covered by free-field simula tors necessitate a reduction in
maximum field strength except close In to the radiating elements.
10.4.5.4 Bounded-wave NEMP simulators These may be of the following types: •
Parallel transmission lines made from anum ber of closely spaced wires with a tapered section leading to the parallel-line working volume and than via an opposite taper to a matched load resistor. See Figure 10. 34a for a diagram of the simulator referred to in MIL STD 462 RS05 for testing large systems. As an example, Figure 10.34b shows a schematic of the ALECS parallel-wire simulator (95 ohm line) at Air Force Weapons Laboratory, Kirtland AFB, USA, used for testing the B-1 bomber [34]. The working volume of this facility is 27 X 15 x 13 m cube. The peak voltage across the line is about 1.7 MV, the peak E-field is up to 125 kV 1m and it can fire one pulse per minute. Another example of a parallel-line simulator (125 ohm line) is ARES, which is a Defense Nuclear Agency facility at Kirtland AFB. It has a large working volume of 40 x 33 x 40 m cube and can produce field strengths of up to 100 kV 1m [34]. In the UK, PETS 1 and 2 simulators at AWE Aldermaston are parallel-wire lines and like ALECS and ARES produce vertically polarised waves. It is much simpler to build a vertically polarised simulator than one which
200
A HANDBOOK FOR EMC TESTING AND MEASUREMENT SHIELDED ROOM MARX GENERATOR
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•
is horizontally polarised, as the lower conductor can be laid on the ground for the vertical simulator. However, the actual NEMP from a weapon release may contain predominantly horizontal polarisation and this is more difficult to simulate with facilities in close proximity to the ground. The simulator and test target must be raised above the ground by a significant fraction of a wavelength at a few MHz if a horizontally polarised pulse is not to be distorted by interaction with the conductive ground. The large TRESTLE simulator at AFWL, Kirkland has two 5 MV pulsers producing a horizon tal E- field from two vertical sets of wires on a 61 x 61 m wooden platform which is 36 m above the ground. 1'he cost of such facilities is enormous, many millions of dollars, but if NEMP hardening is to be proven at system level, such simulators are necessary. Tapered transmission lines, ,again constructed from wires but with no parallel working section. An example is shown in Figure 10.35a of a line referred to in MIL STD 462 for testing large systems to method RS05. A drawing of a large 20 m high double-taper simulator manufactured by Elgal EM-IIOI
ANT-II0 [43] is shown in Figure 10.35b. This device can be equipped with a pulsed power source of up to 2.5 MV for full threat simulation. A facility called HEMP which is a dou ble- ta pered line with the wires forming an elongated diamond in plan view is located at Fort Hauchuca in the USA. Each taper is 68 m long and the intersection is 15 m high and 24 m wide. I t can prod uce a field strength of around 26 kV 1m from a 400 kV pulser. In ,France, Aerospatiale and Thomson CSF have developed a large relocatable offset singletaper wire simulator for testing ground installations such as communications facilities [32]. I t can illuminate an area of 40 x 60 m with a field strength of up to 50 kV 1m. A number of other bounded-wave simulators exist including TEFS and SIEGE in the USA. Details of these and other facilities exist in published references [33, 34, 44, 45]. Simulators in France such as CYTHARE, SIVA, SIEM 2, PEGASE and CORNE MUSE are listed by Rudrauf[32].
10.4.5.5 Free-field NEMP simulators Simulators of this type have been constructed mainly in the USA from horizontal dipoles, resistively loaded dipoles and vertical conical monopoles. The TEMPS (transportable EMP simulator) is an example of a 300 m horizontal dipole device. It can produce about 50 kV/m at a distance of 50 m normal to the centre of the dipole. Large horizontal dipoles have been constructed similar to that shown in Figure 10.36. These simulators sometimes have a 15 MV switched pulser located at the centre of the dipole, high above the system under test and can provide near full threat pulses. Two 1000 ft longwire simulators have been built in the USA. Such simulators with up to 100 kV pulsers can radiate a field of about 1 kV 1m at 100 ft from the antenna. A vertical dipole facility formed from a resistively loaded wire conical monopole above a ground plane has been bu:ilt in the USA. It is 27 m high and is driven by a 4 MV pulser located at the apex of the inverted cone, see Figure 10.37 a and b. Finally, one of the largest simulators has been constructed on a floating barge for the NEMP testing of complete ships. The EMPRESS simulator in the USA is a vertically polarised dipole using a large vvire monocone antenna as shown in Figure 10.38. There are regular symposia and conferences [46] dealing with all aspects of high-voltage pulse generators, NEMP and lightning with specialist sections on simulators.
ELECTROMAGNETIC TRANSIENT TESTING
201
System under test situated under the wires
(aj
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Large NEMP simulators (aj Double-taper NEMP simulator (bj Elgal ANT-110 double-taper NEMP simulator
(b) Reprod uced by permission or Elgal
10.5 Lightning itnpulses 10.5.1 Lightning environment
Figure 10.36
Example of horizontal dipole NEMP simulator
Reprod uced by permission or Eigal
The physics and electrical engineering associated with the study of lightning and its impact on electrical equipment is a distinct technical field, with many workers contributing to the current state of knowledge. In the space available here it is only possible to introduce the reader to the subject in ,the context of EMC testing electronic equipment to withstand the large impulsive electrical transients which lightning creates. This natural electrical phenomenon has been studied scientifically since Benjamin Franklin, who remarked in 1752 that 'the clouds of a thunder-gust are most commonly in a negative state of electricity, but sometimes are in a positive state'. The details of how the charge separation
202
A HANDBOOK FOR EMC TESTING AND MEASUREMENT phenomenon has continued, it has been necessary to collect actual data on the nature of lightning strokes and their frequency of occurrence around the world such that protection against typical strikes can be engineered into equipments. These data have been analysed statistically to yield a nominal description of the lightning discharge event which has been used as the basis for a number of standards and specifications. One such is DEF STAN 00-35 which gives the standard description of the discharge as:
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Risetime average risetime == 2 J1S value range == 1-10 J1s Duration average duration == 40 J1s value range == 20-200 J1S Peak curren t average value == 20 kA value range == up to 150 kA EM radiation frequency of maximum radiation == 10 kHz Occurrence storms worldwide at any instant == 2000 daily occurrence == 5000 Other data .may be obtained from References 48 and 49.
(b) Reproduced by permission of I CT Inc.
10.5.2 Defining the discharge
mechanisms operate and the complex microstructure of lightning discharge formation are still being studied and argued over [4 7J . While the debate concerning the physics of the
Aerospace vehicles may experience different direct discharges from ground equipment and this is reflected in the standards which each type of equipment must meet. Figure 10.39 shows the standard double exponential form for the transient
Figure 10.38 7 MV EMPRESS vertical polarised 'dipole' VPD for whole-ship testing. Manufactured by Maxwell Labs, USA
Reprod uced by permission or Maxwell Labs.
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Parameters of typical lightning pulse when it strikes Earth
current pulse observed in a ground strike with average risetimes and fall times and peak currents suggested in DEF STAN 00-35, and cited by Keiser [50J. A 2 J.1S risetime and 50 J.1S falltime doubleexponential pulse with a worst-case peak current of 200 kA has been specified in an early MIL-B-5087B [51 J for aerospace systems and a pulse with similar rate of rise of current (100 kAj J.1s) is suggested for testing aerospace equipment by SAE [52]. The discharge in a natural lightning event has been shown to be a complex phenomenon giving rise to a number of components as shown in Figure 10.40. The whole event may last for a second or so and is commonly perceived as the lightning flash. The testing waveform stipulated
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Figure 10.39
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Figure 10.41 Lightning test waveform (SAE-AE4L). A: initial stroke, peak amplitude == 200 kA ± 10%; action integral == 2 x 10 6 A 2 s ± 20%. B: intermediate current, maximum charge transfer == 10 coulombs, average amplitude == 2 kA ± 10%. C): continuing current, charge transfer == 200 coulombs ± 20 %, amplitude == 200 - 800 A. D: restrike, peak amplitude == 100 kA ± 10%, action integral == 0.25 x 10 6 A 2 s ± 20 %, duration == < 500 /lS
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:::::>
()
TIME
FIRST RETURN STROKE
SUBSEQUENT RETURN STROKES
CONTINUING CURRENT
Figure 10.40
STEP-WISE DART LEADER
Pictorial representation of the components in lightning flash
Reproduced by permission of BAe Military Aircraft Ltd.
in the SAE AE 4L standard, and shown in Figure 10.41 encompasses the parameters of most lightning strikes, and is divided into the four components A to D. Statistical studies of a great number of lightning strikes have been carried out by NASA [53J including both positive and negative discharges. This has led to a model being defined which accounts for 98% of lightning strikes. The idealised curren t profile is similar to that in the SAE standard and is shown in Figure 10.42 [54]. It gives the current at various times during the pulse along with the charge transferred during these times. Details of current lightning standards applicable to a range of equipments and situations may be found by consulting
204
A HANDBOOK FOR EMC TESTING AND MEASUREMENT
SAE Orange Book CLM-R163 MIL STD 1757A MIL STD 1795
FAA AC 20-53A FAA AC 20-136 Do 160/ED14B (section 22) MODAVPl18
There are reported to be a number of new lightning standards under development (1991) [55J which include AC 20-53B Do 160/ED 14C (sections 22 and 23) EUROCAE: Lightning test standard (WG31-SG3) Lightning threat definition Lightning zoning standard
10.5.3 Effects on equipment Lightning can influence equipment in two ways. The obvious means is by a direct strike where the lightning channel attaches to the system at some prominent point and leaves by some other point to complete the discharge path to ground. Effects may also be produced in nearby equipment indirectly by electromagnetic fields radiated by the rapidly changing voltages and currents in the ligh tning channel. The spectrum of a lightning pulse contains less energy at HF and VHF than an NEMP pulse, as can be seen in Figure 10.43. Most of the energy in the lightning pulse is confined to VLF. Direct strike and indirect effects can be considered separately for two broad categories of equipment: aerospace systems and ground equipments. N
~
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LIGHTNING ( 2 / 40 lls )
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conductors, magnetic forces leading to high transient pressures, and fuel ignition. The indirect effects include high transient magnetic field, upset ground voltages across the aircraft, unwanted operation of electrical power trips, and electromagnetic shock excitation of the aircraft structure resulting in oscillatory currents. This last topic is of particular concern in connection with the assumed risetime of the lightning stroke as it attaches to a prominent point on the aircraft. Fast risetimes can occur during the attachment/discharge. For shorter risetimes than the normally specified 2 J1S (where 90% of the energy is below 10kHz [60J), which have been reported as being down to 200 ns [61 J, the lightning pulse begins to have significant energy at frequencies which can resonate with the aircraft structure (5 to 20 MHz) and cause it to ring. This produces a damped sinewave oscillation similar to that generated by NEMP which can then couple into cables and thus into avionics, which increasingly contain large quantities of sensitive digital integrated circuits, and can lead to disruption of function or component burnout. The indirect effects of ligh tning on aircraft are covered in RAE technical memorandum FS(F)457 which is referred to by Carter [58]. Revised lightning specifications relating to aircraft systems and avionics call for testing using a multiple stroke waveform. There is one initial return stroke with a peak current of 200 kA followed by as many as 23 re-strikes of 50 kA distributed over a period of up to 2 seconds with spacings between any two pulses of 10 to 200 ms [59]. As with ESD measurements, it would seem that the exploration of the actual physical events using faster detection and recording equipment reveals faster risetime phenomena, which have previously been unobserved. The slower risetime oversimplified models of ESD and lightning which have been used to test equipments may explain some of the discrepancies which occur between the EUT performance in the test laboratory and that actually experienced in the operational environment.
FREQUENCY
Figure 10.43 Comparison oj RF spectra oj NEMP and lightning pulses
10.5.3.1 Aerospace systems Include civil and military aircraft, high altitude missiles, and space systems transiting the atmosphere. This is a major specialised field in its own right [56-59J and is not pursued further here, other than to point out some of the issues which are of concern. The direct effects of a ligh ting strike on an aircraft can include: arc burnthrough of high resistance parts, acoustic shock, heating of
10.5.3.2 Ground equipment Particularly power distribution and telephone systems, radio communications and general industrial, commercial and domestic electronic equipment. Lightning effects on these equipments are of more general interest to the majority of designers and test engineers involved with EMC. Consider the simplified situation shown in Figure 10.44 where lightning discharges are taking place over an industrialised area with power and telecommunications systems feeding two interconnected equipments. These could, for example, be a
ELECTROMAGNET,IC TRANSIENT TESTING
POWER GENERATOR
- - V g _..··.....
/~L.v. P~ER L1NE~ LARGECURRENTS
GROUND VOLTAGE FLOW INTO DIFFERENTIAL DUE TO THE GROUND HEAVY CURRENT FLOW
Figure 10.44
WI~ ~
~
> lime SURGES, TRANSIENTS AND DROPOUTS DUE TO VARIOUS FORMS OF LIGHTNING STRIKE
Types of lightning discharges leading to direct and induced effects on equipment
couple of desktop computers connected by a modem link via a telephone line. Cloud-to-cloud discharges result in a radiated electromagnetic pulse which can couple into power cables and result in a power line transient. A direct strike from a cloud to the equipment is rare [60J and if the building in which it is situated is protected according to the advice contained in BS6651 the likelihood of a direct strike is even more remote. Discharges from cloud to ground that do not strike an equipment directly are obviously more common than those that do. They can produce radiated electromagnetic field strengths of 6 kV 1m at a distance of 1 km from the lightning channel [62]. The measured spectrum of frequencies produced by a typical discharge can be seen in Figure 10.45 [63J. Note that this confirms that lightning contains less high-frequency energy than a NEMP transient. The peak ligh tning channel current is limited by the ground resistance in the particular geographical area where the strike occurs. In low resistance coastal marsh areas the current may be greater than 30 kA whereas in high-resistance areas where the ground is predominantly granite the 10
r--.,...----r---r----r--------
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......
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10-1
205
curren t will be lower. The ground currents will manifest themselves by developing large potentials, Vg in Figure 10.44, between electrical grounds at various places, and this will drive surges down the power and communication return lines leading to data corruption or damage to interface circuits. Lightning is considered to be a significant cause of power sags and transients. The peak transient voltages can reach several kV [50J before arcing through the insulation of commercial or domestic power wiring at levels of around 10kV [60]. Gerke [64J asserts that 'in too many cases, power disturbances (to electronic equipment) are overlooked or ignored by the manufacturer in the hope that someone else the user or the power company will solve the problem (of transient failures of their equipment)'. However, not all the power line transients or other effects are caused by ligh tning. A resume of these other effects is also given by Gerke and includes power sags, surges, outages, harmonic distortion, frequency deviation, superimposed transients and wideband noise.
10.6 Transients and general power disturbances
10.6.1 Importance of power transients All the disturbances previously mentioned can sometimes occur on power lines as a result of local loading or switching problems rather than from external sources such as lightning. While the undesirable properties of equipments generating switch-off spikes and being susceptible to incoming transients are addressed in military standards, commercial equipment has in the past had to operate in the presence of similar disturbances but without the comprehensive immunity achieved by rigorous design in compliance with a set of standards. Early studies by IBM (1974) showed that most power supply related problems with regard to computer equipment were due to the existence of transients on power cables. Other measurements by AT&T and the US Navy have deduced that vol tage sags and dropou ts were the primary causes of computer disruption. Taken together, this suggests that transient suppression and the provision of internal stored energy in the equipment are two necessary design features of modern digital electronic equipment.
W
a:: a::
10-2
10.6.2 Examples of power supply immunity standards
::> 0
10-3 1
10
100
1k
1Ok
100k
1M
FREQUENCY (Hertz)
Figure 10.45 Typical lightning spectral density
1OM
The CS06 spike test in MIL STD 461/2 has been adopted as a good test for both military and commercial equipment.
206
A HANDBOOK FOR EMC TESTING AND MEASUREMENT
Examples of other relevant standards include ANSI/IEEE STD C62.41 (1980) formerly IEEE STD 587: Guide for surge voltages in low-voltage AC power circuits, essentially a lightning damage specification [64]. I t defines surge/transient voltages and currents up to 6 kV and 3 kA. There are two classes of equipment: Class A, those connected further than 10m from the entrance of the power cable into a building, and Class B, those connected within 20 m of the power entrance. Figure 10.46 shows the applicable transient waveshapes and limits. ANSI/IEEE STD 446 (1987): Recommended practice for emergency and stand by power in the USA. It gives recommended power limits for computer systems as shown in Figure 10.47. Notice that the transient am pli tude increases with decreasing pulse width. Occasionally, levels can reach 400 % of the line voltage for a 20 JiS long transient. ANSI/IEEE STD 519 (1981): 'A guide for harmonic control and reactive compensation of static power converters'. IEC 801: Part 4 of international standard (which applies to European equipments) concerned with power transients, including those induced by VOLTAGE Vp 0.9VP
V peak 0.9V
p
0.5V u 0.1 Vp
(aj
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TIME
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Figure 10.47 Limits on transients and dropouts for power supplies connected to computer systen1S. ANSIIIEEE-STD 446-1987 recommended power limits lightning. Part 5 deals with surges in electrical power systems. Typical waveshapes are given in Figure 10.48. This is probably the standard which must be met by most new commercial electronic equipment. IEC 60-2: 'High-voltage test techniques' IEC 60-2, gives a standardised 1.2/50 JiS lightning impulse vol tage transien t and 8/20 JiS current surge pulse similar to that for ANSI/IEEE C62.41 Cat.B [65J. BS 5406 (1988)-EN6055: Concerned with disturbances to supply systems caused by household apparatus and similar equipment. Part 2 Harmonics, Part 3 Voltage fluctuations. BS 66~2 (1985): Guide to methods of measurement of short-duration transients on low-voltage power and signal lines. BS 2914 (1979): Specification for surge diverters for AC power circuits.
10.6.3 Summary
Figure 10.46 Examples of transient waveshapes on LV power lines (ANSIIIEEE C62.411980) ( a) Category A and B voltage and current ( b) Category B only) voltage ( c) Category B only) current Location category
Waveform
Amplitude
A: Inside building > 10 m from entrance
(a) 0.5 ,us-lOO kHz
6kV
B: Inside building < 20 m from entrance
(b) 1.2 x 50,us (c) 8 x 20 ,us (a) 0.5 ,us-l00 kHz
6kV 3kA 6kV 500A
In this brief introduction to the nature of transients and disturbance to power distribution systems it has only been possible to acquaint the reader with the broad categories of effects and show some examples of applicable standards and specifications. With the fast growing implementation of digi tal microprocessors in to industrial, commercial and domestic equipment this area of electromagnetic compatibility is becoming very much more important than it has been in the past. Consequently, EMC design and test engineers must be aware of this somewhat overlooked aspect of electromagnetic interference engineering, and not allow an otherwise compatible equipment to be compromised by these conducted transients and other power line phenomena.
ELECTROMAGNETIC TRANSIENT TESTING V UJ
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WAVE SHAPE OF A SINGLE PULSE INTO A 50 n LOAD
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FAST TRANSIENT BURST
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Typical transient burst and surge waveforms (lEG 801) (aj lEG 801-4 transient burst (b j lEG 801-5 surge waveform Reproduced by permission of BSI
10.7 References GLEDHILL, SJ.: 'Spectrum analysis' in BAILEY, A.E. (Ed.): 'Microwave measurements'. (Peter Peregrinus, 2nd edn., 1989) Chap. 13 2 'Digital signal processing'. Technical tutorial 1990 catalogue, LeCroy Ltd, 28 Blacklands Way, Abingdon Business Park, OX14 1DY 3 BRACEWELL, R.N.: 'The Fourier transform and its applications'. (McGraw-Hill, 2nd edn.) 4 CHIRGWIN, B.H. and PLUMPTON, C.: 'A
18
19
20
21
22
207
course of mathematics for engineers and scientists volume 5'. (Pergamon) FEYNMAN, R.P.: 'Lectures in physics'. (AddisonWesley) Vol. 1, Chapters 25, 47 and 50 7200 series digital oscilloscope 1990 catalogue. LeCroy Ltd, 28 Blacklands Way, Abingdon Business Park, OX14 1DY TD230 1/TD 130 1 digitiser systems. 1990 product catalogue. p. 162, Tektronix UK Ltd, Fourth Avenue, Globe Park, Marlow, Bucks SL 7 1YD 'Spectrum analysis - pulsed RF'. Application note 150-2. Nov. 1972, Hewlett-Packard, Winnersh, Wokingham, Berks, RG 11 5AR RIAD, S.M.: 'Instructional opportunities offered by the time domain measurement technology' . in MILLER, E.K. (Ed.): 'Time domain measurements in electromagnetics'. (Van Nostrand Reinhold), Chapter 3, pp. 72-94 SCHAFFER, R.: 'Choosing an ESD container: Materials and mechanics'. Interference Technology Engineer's Master, 1988, pp. 122-128 DASH, G.R.: 'Designing to avoid static - ESD testing of digital devices'. Interference Technology Engineer's Master, 1985, pp. 96-110 JONES, B.: 'Improvements to the ESD testing of equipment;. Presented at IEEE symposium on EMC) 1990 KING, W.M. and REYNOLDS, D.: 'Personal electrostatic discharge: Impulse waveforms resulting from ESD of humans directly and through small hand-held metallic objects intervening in the discharge path'. Proceedings of IEEE symposium onEMC) 1981; pp. 577-590 BHAR, T.N.: 'Electrostatic discharge models simulating ESD events'. Interference Technology Engineer's Master, 1988, pp. 112-118 ENOCH, R.D., SHAW, R.N. and TAYLOR, R.G.: 'ESD sensitivity of NMOS LSI circuits and their failure characteristics'. Proceedings of EOS symposium on ESD) 1983 TAYLOR, R.G., WOODHOUSE, J. and FEASEY, P.R.: 'A failure analysis, methodology for revealing ESD damage to integrated circuits'. Proceedings of EOS symposium on ESD) 1984 UNGER, B.A.: 'Electrostatic discharge failures of semiconductor devices'. Proceedings of 19th international symposium on Reliability physics) 1981 ENOCH, R.D. and SHAW, R.N.: 'An experimental evaluation of the field-induced ESD model'. Proceedings of EOS symposium on ESD) 1984 SHAW, R.N. and ENOCH, R.D.: 'An experimental investigation of ESD induced damage to integrated circuits on printed circuit boards'. Proceedings of EOS symposium on ESD, 1985 GAMMILL, P.E. and SODEN, J.M.: 'Latent failures due to ESD in CMOS integrated circuits'. Proceedings of EOS symposium on EMC) 1986 HAWKINS, C.F. and SODEN, M.J.: 'Electrical characteristics and testing considerations for gate oxide shorts in CMOS ICs'. Proceedings of IEEE international conference on Test) 1985 CROCKETT, R.G.M., SMITH, J.G. and HUGHES, J.F.: 'ESD sensitivity and latency effects of some HCMOS integrated circuits'. Proceedings ofEOS symposium on ESD) 1984
208
A HANDBOOK FOR EMC TESTING AND MEASUREMENT
23 DAOUT, B. and RYSER, H.: 'The correlation of rising slope and speed of approach in ESD tests'. Presented at 7th symposium on EMC, Zurich, 1987 24 RICHMAN, P., WElL, G. and BOXLEITNER, W.: 'ESD simulator tip voltage at the instant of test'. Proceedings of IEEE symposium on EMC, 1990, pp. 252-257 25 STAGGS, D.M. and PRATT, D.J.: 'Electrostaticdischarge standardization'. Proceedings of IEEE symposium on EMC., 1988, pp. 175-178 26 WOOD, D.V.: 'Investigation of a new ESD test method using current injection'. Proceedings of IEEE symposium on EMC, 1988, pp. 179-185 27 MAAS, j.S. and PRATT, D.J.: 'A study of the repeatability of electrostatic discharge simulators'. Proceedings of IEEE symposium on EMC, 1990, pp. 265-269 28 BUSH, D.R.: 'Statistical consideration of ESD evaluations'. Proceedings of 7th international symposium on EMC, Zurich, 1987, pp. 468-473 29 LONGMIRE, C.L.: 'On the electromagnetic pulse produced by nuclear explosions'. IEEE Trans. EMC-20, (1), 1978 30 Nuclear hardening course. Applied physics and electro-optics group, Royal Military College of Science, Shrivenham, Swindon, Wilts, SN6 8LA 31 KEISER, B.: 'Principles of electromagnetic compatibility'. (Artech House, 3rd edn.) Chap. 3, p. 48 32 RUDRAUF, A.: 'Electromagnetic pulse simulators - EMP on tap'. Int. Def. Rev., 1984 (1) 33 RICKETS, L.W., BRIDGES, j.E. and MILETTA, j.: 'EMP radiation and protective techniques'. (Wiley) 34 GHOSE, R.N.: 'EMP environment and system hardness design'. Don White Consultants Inc., Gainesville, Virginia 22065, USA, 1984, 1st ed. 35 PAN, W.Y.: 'An experimental investigation of the distribution of current and charge induced on a tubular conducting cylinder by an electromagnetic pulse'. IEEE Trans. EMC-27, (2) 1995 36 RODGER, K.S.: 'An approach to EMP testing a complete aircraft'. Proceedings of IEEE symposium onEA1C, 1977, pp. 95-98 37 MEREWETHER, D.E.: 'Transient currents induced on a metallic body of revolution by an electromagnetic pulse'. IEEE Trans. EMC-13, (2) 1971 38 KUNZ, K.S. and LEE, K.M.: 'A three-dimensional finite-difference solution of the external response of an aircraft to a complex transient EM environment: Part-l - The method and its implementation'. IEEE Trans. EMC-20, (2), 1978 39 As Reference 38, part 2 40 THURLOW, M.H.: 'Evaluation of EMP coupling of linear systems by swept CW methods'. Proceedings of lEE colloquium on Protection of communications equipment against EMP and other hazards, March 1985, digest 1985/28 41 MORGAN, D.: 'Analysis of EMC susceptibility data on 23 systems'. Internal report (Class), BAe Dynamics Ltd, Filton, Bristol, UK 42 'Integrated circuit electromagnetic susceptibility handbook'. Report MDC E 1929, McDonnell Douglas Astronautics Company, St Louis, Missouri 63166, USA 43 Elgal EMP and HV Systems, P.O. Box 494, Karmiel 20101, Israel
44 GILES, j.C.: 'Simulating the nuclear electromagnetic pulse'. Mil. Electron./Countermeas.., August 1977 45 RUDY, T., BERTUCHEZ,j. and WARMISTER, B.: 'NEMP simulation and tests in Switzerland'. Third symposium on EA1C, 1979 46 Pulsed power conferences. IEEE Electron Devices Society, 1987 Washington, 1989 Monterey, USA 47 WILLIAMS, E.R.: 'The electrification of thunderstorms'. Sci. Amer. Nov. 1988, pp. 48-65 48 'World distribution and characteristics of atmospheric radio noise'. CCIR Report 322, 1964 49 HORNER, F.: 'Analysis of data from lightning flash counters'. Proc. IEE,july 1967, 114, p. 916 50 KEISER, B.E.: 'Lightning protection'. Interference Technology Engineer's Master, 1985, pp. 66-68 51 MIL-B-5087B: 1978 'Bonding electrical and lightning protection for aerospace systems' 52 SAE ARP: 1978 'Lightning test waveforms and techniques for aerospace vehicles and hardware' 53 NASA SP 8084: 1974 'Surface atmosphere extremes' 54 PERROTON, G.: 'How to protect hardened facili ties from very high level transients induced on power line by lightning or EMP'. Proceedings of IEEE symposium onEA1C, 1985, pp. 1-4 55 JONES, C.C.R.: 'Latest lightning standards'. Presented at BAe electromagnetics seminar, BAe Military Aircraft Ltd, Warton, UK, Nov. 1990 56 BISHOP, j.: 'EMP and lightning testing of avionic equipments with an introduction to transient effects to aircraft'. Proceedings of colloquium on Protection of communications equipment against EMP and other hazards, March 1985, lEE digest 1985/28 57 DARGI, M.M.: 'Lightning protection testing of full scale aircraft to determine induced transient levels'. Proceedings of international conference on Lightning and static electriciry, September 1989, University of Bath, UK 58 CARTER, N.J. and HOBBS, R.A.: 'Lightning qualification testing for UK avionic equipment'. Proceedings of international conference on Lightning and static electriciry, September 1989, University of Bath, UK 59 WILES, K.G.: 'Lightning protection verification of full authority digital electronic control systems'. Proceedings of international conference on Lightning and static electricity, September 1989, University of Bath, UK 60 CLARKE, G.J.: 'The hybrid barrier: protecting against EMP and lightning'. Interference Technology Engineer's Master, 1989, pp. 162-170 61 FIEUX, R.P. et al.: 'Research on artificially triggered lightning in France, IEEE Trans. P AS-97 (3), 1978, pp. 725-733 62 PODGORSKI, A.S.: 'Composite electromagnetic threat'. Proceedings of IEEE symposium on EA1C, 1990, pp. 224-227 63 HAR1', W.C. and MALONE, E.W.: 'Lightning and lightning protection'. Don White Consultants, Gainesville, Virginia 22065, USA, 1979 64 GERKE, D.: 'Power disturbances and computerised equipment'. Interference Technology Engineer's Master, 1990, pp. 302-309 65 FREY, O. and HAEFELY, E.: 'Electromagnetic compatibility testing of electronic components, subassemblies, measuring instruments and systems'. EMC Technol., 1982, pp. 78-83
Chapter 11
Uncertainty analysis: quality control and test facility certification 11.1 Introduction
fall into in the case of EMC testing where, as will be seen, there are a great number of factors which affect the final measurement uncertainty, but for which there may be no sound theoretical or experimental knowledge of the distributions of individual error terms. What follows considers the general trea tmen t of errors in electrical measurements and then puts this into context as part of the whole quality control process for EMC testing. Finally, how the UK calibration and test laboratory accreditation service (NAMAS) regulates measurement and testing activities to ensure sound practice is examined.
The repeatability of EMC testing depends on many factors that affect the measurement result. Some factors are not well understood or not documented in terms of the nature and magnitude of their contributions to the total uncertainty. The treatment of measurement uncertainty involves the use of statistics to estimate the probable uncertainty and associated confidence level with regard to a particular measurement or set of measurements. Statistics and probability theory is a considerable subject in its own right and far too large to deal with adequately in a book such as this, which is primarily concerned with the varied aspects of EMC testing. Useful summaries of the main theories and analytical techniques are given in a number of texts [1-5 J and are recommended to those particularly interested in this aspect of mathematics, or those who are determined to achieve a sound grasp of the basics before considering how to apply them to the understanding of EMC measurement uncertainty. The application of statistics to experimental measuremen ts has been dealt with specifically by Box et al. [6J and Topping [7], and a good understanding of the subject can be achieved via these texts. Early work carried out in the British Calibration Service by Dietrich [8J resulted in the development of a statistical approach to combining measuremen t uncertainties based on the assumption of probability distributions for all the contributing components. In 1977 a useful code of practice was produced by Harris and Hinton [9J for the treatment of uncertainty in electrical measurements. Test engineers should always endeavour to estimate the errors or uncertainties associated with the measurements which they make. However, a superficial and incomplete knowledge of the statistical treatment of measurement uncertainty may lead to an unwarranted belief that an error estimate is correct. In such circumstances it is possible to become complacent and assume that because some uncertainty analysis is being done that the measurement results are now more reliable in some way. This can occasionally lead to a less searching approach to testing on the part of engineers. This trap is particularly easy to
11.2 Sotne definitions Terms that are commonly used in the analysis of measurement uncertainty [7, 10, 11 J have specific meanings which should be understood prior to a general consideration of the statistical treatment of uncertainty. True value: the actual value of the quantity being measured. This is unknowable via measurement. I t is approximated in practice by the value of the quantity measured that is established by traceability to national standards [1 OJ. Measured value: the result of conducting a fixed test procedure to determine a quantity of interest. In the statistical treatment of errors its value is usually denoted by x. Error: the difference between the measured value and the true value. Mean value: the result of computing the arithmetic mean of a number of measurements. Usually denoted by x. Uncertainty: this term quantifies the indeterminacy in the measurement process by stating the range of values within which the true value of the quantity being measured is estimated to lie [10]. This is normally expressed in terms of ± an absolute number or a % of the mean value. Confidence: because the limits of uncertainty of a measurement cannot be known absolutely it is necessary to qualify them with a statement of confidence. This is expressed as a percentage; for example, a confidence value of 95% means that there is only a 1 in 20 chance or probability that the true value lies outside the stated uncertainty. 209
210
A HANDBOOK FOR EMC TESTING AND MEASUREMENT
Random error probability distribution: the result of plotting the frequency of occurrence of a measured value as a function of the value. If a large number of measurements of a quantity are made using the same procedure and under the same conditions, but with small random variations, it is found that the envelope of the probability distribution approxirnates to the 'bell-shaped curve known as the Gaussian or normal curve. This normal curve was first derived by de Moivre in 1733 when dealing with the problems associated with the tossing of coins. It was also later obtained independently by Laplace and Gauss. I t is sometimes known as the Gaussian Law of Errors because it was applied to the distribution of acciden tal errors in astronomical and other scientific data. The Gaussian curve IS derived from the expression y==A
(x m/
11.1
where A) hand m are constants. The shape of the curve is given in Figure 11.1. Topping [7J suggests that 'it is difficult to overstate the importance of the Gaussian error curve In statistics as its importance is like that of the straight line in .ge~metry'. Definitions of a wider set of terms concerned with uncertainty and statistics can be found in the UK NAMAS information sheet NIS 20 [11 J. Systematic error: one which consistently biases the measurement (and therefore the mean of n measurements) in one direction away from the true value [12J. Examples include a panel meter with the resting place of the needle not aligned with the zero mark or an instrument calibration factor which may be incorrect. Most uncertainties in real measurements result from a combination of systematic and random errors which are introduced via a number of factors. I t is common to treat these two types of errors separately as systematic errors that may be known (within a given uncertainty, by reference to a national standard) and can be removed from the measurement by a correction factor, whereas
the uncertainty resulting from random errors can not. It will be shown that random errors can be combined with systematic errors to yield a single value for the uncertainty of a measurement for a given confidence level.
11.3 MeasureDlent factors Consider a typical EMC measurement of the RF current flowing in a multiconductor cable as depicted in Figure 11.2. There are three main areas where uncertainty can be introduced into this measurement. (i)
(ii)
(iii)
The factors affecting the physical/electrical environment of all components of the test. These are sometimes referred to as the control quantities. The factors involved with the sensor and its linkage to the quantity being measured. These are known as the coupling factors. The calibration of the measuring instrument and the way it is used.
This classification of uncertainty factors follows that given in NAMAS NIS 20 [11] and is illustrated in Figure 11.3. As an example, some of
CABLE POSITION P
CURRENT PROBE TRANSFER IMPEDANCE ( T I )
Figure 11.2 y
Example of uncertainty factors in currentprobe EMC nzeasurement
CONTROL FACTORS
A
A B C 0---"""4
MEASUREMENT INSTRUMENT FACTORS RESULT
I
I I I
n
Figure 11.1
Normal error curve y
DATA
x
m
Ae _h
2
R
I SENSOR
Figure 11.3 (x-m)2
1
CALIBRATION DATA
Classification of contributions to uncertainty in measurement
UNCERTAINTY ANALYSIS: QUA.LITY CONTROL AND TEST FACILITY CERTIFICATION
11.4 Random variables
the factors that affect the measurement uncertainty for the current-probe measurement in Figure 11.2 in each of these are listed:
The nature of random variables in measurement can be demonstrated by reference to Figure 11.4. A number of measurements are made of a quantity and these are recorded as arrows. The arithmetic-mean value of the set of measurements is also shown. This, however, will not necessarily coincide with the mean or central value for the whole population of possible measured values. Such a value will be the mean value of the Gaussian or norlTIal distribution curve relating to the quantity under investigation. Hinton [7J defines the difference betvveen the mean of the me:lsuremen t set and the population mean as an expression of the random component of uncertainty. I t is not possible to correct for the uncertainty in trod uced by random errors; all that can be done is to state a value of uncertainty and an associated confidence level. In practice, it is found that a relatively small number of contributions to random uncertainty which are of similar range can be combined to produce a distribution which is close to the normal or Gaussian curve, even if their individual probability distributions are not normal (i.e. entirely random) [1 OJ. This is important in calculating the uncertainty in EMC measurements where there are a large number of factors con tri bu ting to the final uncertain ty with many of them not having purely random distributions. Pure random variables or measurement errors are those which are independent of other errors
Control factors Stability of source and load parameters on the ends of cable Position of current probe along the wire (1) Exact lay-up of wires in the bundle (w) Angle of current probe to cable axis (a) Height of cable and probe above ground plane (h) Degree to which centre of cable bundle is offset from centre of current pro be (0) Position of the connecting cable from probe to meter (p) Grounding of meter (g) Stability of the power supply to meter Normal physical environmental factors such as temperature, humidity, etc.
Coupling factors Current probe transfer impedance as a function of frequency (1'1) Connector and cable losses as a function of frequency (-C1, C2 and A) Current probe, cable and meter impedances resulting in VSWR for given cable length
(L) Measuring instrument Calibration correction factor and its uncertain ty Signal-to-noise ratio Operator accuracy in setting up the meter and in reading and recording the result (R)
r
I (J)
W
:J
-J
~ LL o
MEAN VALUE OF ALL I MEASUREMENTS WITHIN THE ~ POPULATION
:
CALIBRATION FACTOR SYSTEMATIC UNCERTAINTY (can be removed)
I MEAN VALUE
()
w
a:: a::
:J
() ()
J
I
I.FACTTS .1
NORMAL PROBABILITY . DISTRIBUTION
o LL o
---J -I
UNCERTAINTY IN SYSTEMATIC
W
Z
INDIVIDUAL MEASUREMENT VALUES IN A MEASURED SET OF n
~
I
I I I
>()
zW
:J
I
I
I
THE CAliBRATION LABORATORY VALUE ASSUMED WORST CASE RECTANGULAR UNCERTAINTY DISTRIBUTION
I
VALUE OF QUANTITY
a w
fE ..-
.-1
~
_ _ _ _ error for __-__ a
Illustration of measurement uncertainties
~,
measurement no. 3
RANDOM UNCERTAINTY IN THE MEANS OF THE MEASURED SET
f'igure 11.4
211
THE TRUE VALUE (unknowable by measurement)
212
A HANDBOOK FOR EMC TESTING·AND MEASUREMENT
(i.e. not related or linked to the same causal factors) and produce additive effects due to independent random causes. The mean value of n measurements is
FREQUENCY OF OCCURRENCE OF MEASURED VALUES OF x
......-o----t--a--~ 1 STANDARD DEVIATION
11.2 where x == mean value, n == number of measurements, and Xi == value of the ith measurement. For a random distribution of the variable x it is possible to derive meaningful quantities known as the variance and standard deviation which for a sample population are given by Reference 10 as 11.3 where a 2 == variance of the sample and a == standard deviation. Each time a set of n samples is taken the value of a will change slightly. The best estimate of the standard deviation for a whole population of results based on a single sampIe of n measurements is
a est
so that
In [n - 1
6
....---+---+-30---+----30"+----+--..... 3 STANDARD +---+---+----~---+__-_+_-~ DEVIATIONS x ...---68.3%---10+ .------95.5%------tlIoI . . . . - - - - - - - 9 9 . 7 % - - - - - - -.. % CONFIDENCE THAT THE TRUE VALUE OF x LIES IN THIS RANGE OF x
Figure 11.5
Standard deviations and percentage co1?fidence for normal distribution of values
Table 11.1 Relationship between K and % population for normal distribution %population within limits
(x
a a[_n ]~ est
2 0 " - -.........--20"-+--.........
-
n- 1
11.4
11.5
2.58a
and the difference between a est and a is only significant when the number of measurements n is small. This standard deviation relating to the uncertainty in a quantity may be calculated from a set of measurements, or estimated. The probability density may follow a normal or Gaussian distribution, a log-normal, Poisson, binomial or other distribution [12]. Information regarding the nature of these and other types of probability distribution can be found in Meyer [13J and in a useful NATO restricted document [14]. Standard deviation is useful because it is an indicator of the range of measurement uncertainty for a given distribution. In Figure 11.5 it can be seen that for the Gaussian or normal distribution there is a 68.3% probability that any particular measured value will be within the range ±a of the population mean. Further, 95.5% will be within 2a and 99.7 % within 3a. Defining the multiplying factor for a asK, then some further useful relationships between K and 0/0 of population wi thin K a limits can be seen from Table 11.1 The uncertainty U which corresponds to a specified % probability or confidence factor can now be defined as
U == Ka
0.675a l.Oa 1.96a 2.0a
11.6
3.Oa
50 68.3 .95 95.5 99 99.7
There is a relationship between the standard deviation and the parameter h (sometimes called the precision constant) in eq n. 11.1. I t can be shown [7J that
1 2 -a 2
11.7
and that A in eqn. 11.1 is taken as A
h
yin
11.8
and so it is possible to rewrite eqn. 11.1 as __ 1_
y -
0"J2i[
e- (x-x )2;:2 20"
11.9
The value of the standard deviation a for a given popula tion mean determines the tigh tness of the density distribution. For example, in Figure 11.6 it can be seen that as a decre~ses the tightness or precision of the error distribution increases.
lJNCERTAINTY ANALYSIS: QUALITY CONTROL AND 1'EST FACILITY CERTIFICAl'ION y
t.-----------
For small samples the normal distribution may underestimate the probabilities for large deviations [13]. Assuming that the density distribution function for the population is Gaussian, then to calculate the random uncertainty of a finite sample of measurements to a given confidence level, it is necessary to use Student's tdis tribu tion [13, 15 J. The random uncertainty of the mean of a sample of measured values is
·1
2 (21t)-'2
(21t
)~
t
==
o Figure 11.6
x .....
Normal distribution curves for different values of (J y
11.4.1 Student's t-distriblltion When the measurement of a quantity is only made a few times the mean value for the set is unlikely to be the same as that for the whole population of measurements which could be made. Thus there is an additional uncertainty associated with the mean for the set, as shown in Figure 11.2. This uncertainty decreases by the square root of the number of measurements made but can be significant for small values of n, say less than ten. Table 11.2
213
(Jest
Vn
11.10
where (Jest is the estimate of the standard deviation derived from eqn. 11.4 and t are the values specified in Table 11.2 for given confidence levels and as a function of n, the number of measurements made. I t is clear that as n tends to infinity the mean for the set tends to the popula tion mean and U m tends to zero. Finally, in connection with random distributions, the standard deviation (Je of the combination of a number of random distributions each expressed by a standard deviation (J, is the root sum square of the contributions: 11.11
11.5 System.atic uncertainty In determining uncertainty it is necessary to consider the measuring apparatus,
Studenf s t-distribution table Specified confidence level
.......
v en v
~
.......
.S en ....... ~
v
ev $....i
;:)
en
rj
v
e
4--f
0
$....i
v
..D-
e ;:)
Z
~
0.500
0.683
0.950
0.955
0.990
2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20
1.000 0.817 0.765 0.741 0.727 0.718 0.711 0.706 0.703 0.700 0.697 0.695 0.694 0.692 0.691 0.690 0.689 0.688 0.688 0.675
1.84 1.32 1.20 1.14 1.11 1.09 1.08 1.07 1.06 1.05 1.05 1.04 1.04 1.04 1.03 1.03 1.03 1.03 1.03 1.00
12.7 4.30 3.18 2.78 2.57 2.45 2.36 2.31 2.26 2.23 2.20 2.18 2.16 2.14 2.13 2.12 2.11 2.10 2.09 1.96
14.0 4.53 3.31 2.87 2.65 2.52 2.43 2.37 2.32 2.28 2.25 2.23 2.21 2.20 2.18 2.17 2.16 2.15 2.14 2.00
9.92 5.84 4.60 4.03 3.71 3.50 3.36 3.25 3.17 3.11 3.05 3.01 2.98 2.95 2.92 2.90 2.88 2.86 2.58
ex:
0.997
9.22 6.62 5.51 4.90 4.53 4.28 4.09 3.96 3.85 3.76 3.69 3.64 3.59 3.54 3.51 3.48 3.45~
3.00
r - - - - Values of't'
214
A HANDBOOK FOR EMC TESTING AND MEASUREMENT
the operational procedure and the item under test. Systematic bias in a number of individual measurements can be detected by planned variation of the measurement conditions and process, and by averaging the results. The bias can then be corrected and thus removed froIn contributing to the total measurement uncertain ty. The calibration of a measurement instrument with respect to a national standard will reveal any systematic factor which the instrument or meter may contain, see Figure 11.4. This can then be corrected or allowed for by applying the calibration factor specified in the certificate of calibration. The calibration laboratory value and the rnean of the population as shown in Figure 11.4 can then become aligned. The correction factor given in the calibration certificate has an error associated with it which is the systematic uncertainty and is assumed to have a worst-case rectangular distribution. This uncertainty, and any other systematic uncertainties which can be identified and quantified, are first grouped together in accordance with their known or assumed probability distributions. If only the limits of the systematic uncertainties are known it is safest to assume they have rectangular distributions [10]. They are then combined and the standard deviation is added to that of the random uncertainty to yield the full measurement uncertain ty. It has been shown [8J and quoted [10, IIJ that the standard deviation (Js of a systematic uncertainty having a rectangular distribution with limits of ± a is
a
11.12
If there are a number of uncorrelated systematic uncertainties with assumed rectangular distributions with values ± al a2 a3 ... an the combined standard deviation of the contributions is
aT
+ a~ + a~ ... a~ 3
11.13
11.6 COInbining random. and systen1.atic uncertainties There are two cases that must be treated separately vvhen combining systematic and random uncertainties: when a dominant systematic term exists, and when no dominant systematic term exists. When a dominant systematic term ad exists in eqn. 11.13 then Us == K (Js may exceed the arithmetic sum of the
semirange values of the individual contributions. This can be checked by n
Us>
L
am
m=l
If this is the case, the dominant systematic uncertainty contribution should be brought outside the RSS (root sum square) addition. rrhus the correct total uncertainty is I
U=
ad
+
[U/ + U;] 2
2
11.14
where U~, == K (J~ and (J~ is obtained from eqn. 11.13 without including the dominant term ad. U r is the random uncertainty. When no dominant systematic term exists the random and systematic uncertain ties can be combined by the RSS method as expressed in eq n. 11.11. For all EMC test engineers and designers of electronic equipment that may be subject to EMC testing it is advisable that they should consult the references listed in this chapter and gain a working understanding of the statistical basis for the treatment of measurement errors as outlined to understand and have confidence in the results of EMC tests.
11.7 Uncertainties in EMC n1.easuren1.ents
11.7.1 Contributions to measurement uncertainty There are two main problems to be overcome in generating worthwhile uncertainty statements to accompany EMC measurements. The first is the sheer number of variables involved in these complex measurements and which must be investiga ted and segregated into control, coupling or instrument factor categories. There may be up to 50 identifiable contributing sources of uncertainty in, for example, a radiated emission measurement [12]. This contrasts with a much more simple measurement of quantities such as temperature or weight etc. where there may be less than a dozen significant sources of uncertainty. As a rule of thumb, used to simplify matters, one might ignore uncertainty contributions that are smaller by a factor of ten than the largest contributor. The second problem relates to obtaining a meaningful understanding of the nature of the probability distributions attaching to each of these uncertainties. This can be done either by experiment, where each contributing factor is isolated and studied in a series of careful and
UNCERTAINTY ANALYSIS: QUALITY CONTROL AND TEST FACILITY CERTIFICATION probably time-consuming controlled experiments, or by examination of the theory relating to the factor in question. The experimental EMC uncertainty database is generally considered to be poor [12], and as the generation of definitive data is expensive for commercial test houses they tend to do only that necessary to sa tisfy the requirements of the national accreditation authority, NAMAS in the UK. A progressive programme to provide industry with a well researched EMC measurement uncertainty database could be undertaken by the national metrology authority, or possibly by combining efforts with those of other nations to provide a really comprehensive foundation for the treatment of EMC measurement uncertainty.
separately when combining the uncertainties relevant to that test regime, although in many cases a common database can be used for components such as VSWR mismatch, polarisation and antenna positioning uncertainties. Slightly different uncertainty analyses will result for variations in a basically common test method when there are detailed variations required by particular test standards. For example, consider two open-range radiated emission tests, one specified by FCC and the other by VDEjCISPR. The field averaging uncertainty (Chapter 10) in trod uced by virtue of the size of the tuned dipole antenna will be different for the two tests at frequencies below 80 MHz owing to the CISPR requirement that the dipole length remains tuned to 80 MHz for frequencies below this, whereas the FCC requires the dipole to be tuned down to 30 MHz. Returning to the uncertainty analysis of a typical radiated emission test in a screened room, consider a standard measurement of radiated emissions from an EDT at 1 m as shown in Figure 11. 7. The factors that contribute uncertainty to the measurement in the groups identified earlier, in line with NAMAS NIS 20, i.e. control factors, coupling factors and instrument factors are listed in Tables 11.3-5. There is no absolute determination as to which group some uncertainty factors should be placed in and some control factors may also be considered as coupling factors. The definition used in this example is that coupling factors are those which influence the measurement
11.7.2 Identification of uncertainty factors Now examine a typical EMC radiated emission measurement made in a semianechoic screened chamber and attempt to identify the more obvious sources of uncertainty as an example of the complexity of uncertainty analysis in EMC testing. Other test regimes such as open-site testing or bulk-current injection will have factors contributing to measurement uncertainty that are particular to them, and no general uncertainty analysis can be developed to cover widely differing tests methods. Each must be considered POWER LINE NOISE (external)
Penetration of ambients through shielded room
"-...
~
P '~ - ower cable position
Angle subtended by EUT & cables ...... /
+- / - -----,
x+
~I
I I
y
Anglesublend~by EUT
I
-=-
t=
I _
----r----
Sign~1
I
cable position
1
d
_-------
EUT GROUNDING CONDITIONS
EUT position
PEUT
·
RAM PERFORMANCE reflections from walls, floor & ceilings
~-
r
I.;!
V
ANTENNA ANGLE
~T:NNA
0
TO ITS REFLECTIONS NEARFIELD BOUNDARY?
SWRAT LOAD
\ r-----[2]~...
EMI METER -------f'<'-L --;---.. CABLE ~----~
/I\. ..
______
- ANTENNA _ ----MUTUAL IMPEDANCE
AMBIENT SIGNAL PENETRATION
SWR CONNECTOR
- ------------..:
-----=---:SWR ANTENNA _______
___ SCREENED ROOM
CALIBRATION FACTOR (AF)
~--t-.-...-POLARISATION
ANGLE P -------- M
TYPE & CONFIGURATION OF GROUND PLANE BENCH
SIGNAL CABLE EXTERNAL NOISE
Figure 11.7
215
Sources of measurement uncertainty in typical radiated emission test
CABLE ATTENUATION "A"
216
A HANDBOOK FOR EMC TESTING AND MEASUREMENT
of the electric field immediate region of Ineasurement instrument.
from within the antenna to the
Table 11.5 Instrument uncertainties EMI meter calibration calibration shifts are assumed to be corrected EMI meter stability with humidity /altitude In1pulse generator (internal gain stability EMI meter input VSWR various input attenuator uvl,L.LL.LC::,~ EMI meter overload (saturation and harmonic/ intermod distortion) EMI meter spurious signals EMI meter linearity NB and BB EMI meter bandwidths and shape factors EMI meter detector weigh ting functions against pulse repetition frequency NB and BB range ..... TTl-,i-·L1.,""'"i-'"
Table 11.3 Control factors EUT orientation EUT humidity EU1" grounding EDT signal cable layout EUrr signal cable penetrations EDT power cable layout EUrr power cable LISN terminations Ground -plane ra tio Ground-plane height Ground- plane cond uctivi tylintegri ty Ground-plane bonding to screened room wall Ingress of external noise through screened room walls, through penetration panels and along EUT power and signal cables Antenna height Antenna distance from EUT Antenna mutual impedance to screened room Antenna correction Antenna cable position External noise on antenna cable Path loss due to angle subtended by EUT and cables Multipath reflections from screened room walls, floor and ceiling Screened room size Screened room absorber efficiency Position of EDT and antenna in room
Table 11.4 Couplingfactors An tenna correction factor '-~ calibration shifts are assumed to Antenna size Antenna polarisation (elevation Antenna azimuth angle Antenna polar response (beamwidth and spot Antenna balun efficiency (differential/commonmode rejection) Antenna \lSWR IJ
'-'-d,A.A.,-"," '- ••
Measurement antenna connector loss and VSWR Measurement cable VSWR and length Measurement cable loss Measurement cable layout Measurement instrument connector loss and VSWR
Instrument operator related uncertainties Scan speed/bandwidth choices Meter reading errors/computer system data recording errors In-built software errors EMI meter parameter recording errors (e.g. atten. settings, bandwidth records 2/10 dB amplitude scale confusion etc.) Misnumbered test runs Incorrect recording of amplifiers in line to EMI meter Proced ural or· da ta recording errors at shift handover times in test facili ty
11.7.3 Estimation
uncertain ty
For each of the identified the EMC test engineer needs to estimate the level of random and uncertain ties and express these in terms of a standard deviation for a particular type of function. In some cases the factors may rise to a contribution to both random and types of uncertainty. The total uncertainty for the random and errors can be calculated by .I.''Fthe standard deviations for each as shown if they are substantially independent of each even if the density functions are not Gaussian. This is a useful property of the central limit theorem [13]. Depending on whether there is a dominant systematic contribution, the random and systematic uncertainties can be accumulated according to eqns. 11.11 or 11.14. I t is unreasonable to carry out a detailed uncertainty analysis for all EMC tests taking into account all of the possible influencing factors such as those listed, for the radiated emission test in a screened room. The experienced EMC test engineer should be able to rank the uncertainties 1J'-'-'J.A.A."""c"'",,-",,
"-'''-'J.LI.U'",,-LI..I..
UNCERT'AINTY ANALYSIS: QUALITY CONrrROL AND TEST FACILITY CERTIFICATION introduced by the various factors so that only the major ones need to be included in the formal statistical analysis. This will greatly reduce the work needed to arrive at a satisfactory estimate of measurement uncertainty. The NAMAS executive [11 J require EMC test houses to formally present, during a quality audit surveillance visit, at least one such analysis carried out on a test during the last year. Examples of the magnitude of the uncertainty associated with the most important factors relating to the radiated emission test in the screen room follow. Strictly, one should not use the logarithmic term decibel to represent an uncertainty range, as figures should be expressed as a linear factor. However it has become common practice in test laboratories to use the dB and it is widely quoted in References 12 and 17. •
EUT placement: Very dependent on the type of EUT and its radiation pattern. Default value (J == 6 dB. Screened room reflections: Dependent on size of room, location of antenna and EUT and anechoic damping. For a room with no anechoic lining and below first resonance (J 2 dB [16, 17J. Above first resonance and up to 5x this frequency (J == 7 dB [16, 17]. An exam.ple of the difference between EUT to an tenna coupling for an open site and a screened room, given the same test configuration, is shown in Figure 11.8 [18, 19]. It can be seen that the worst-case differences are up to ±40 dB. I t would be interesting to analyse the density distribution of the occurrence of differences to see if it was normal, log-normal or another well known type. For a screened room with RAM lining giving 6 dB reflection loss at 30 MHz it is possible to reduce the reflection uncertainty to (J == 3 dB above 30 MHz and (J == 2 dB above 100 MHz [1 7J .
•
(j) (j)
~
~~
50-----------.....,.-....-----..,...---., 30
Zz
20 10
w w co
•
•
•
40
ww
~~
•
0t.=:=======:=:II~t-'I,..,PI
~~'O -10 ~~ -20 00
-30
~~
-50
~~
6~
-40
• L..-_ _- - - I _.........
....._ ~ - _ - ~ - - -
Go Z
Figure 11.8
Coupling between antennas in shielded room as function offrequency-' with 1 m spacing *Coupling normalised by r~ference to opensite antenna coupling values
217
Polarisation uncertainty: White [12J shows that the uncertainty in measuring a maximum E-field due to an element of cross polarisation is composed of an offset mean of -6 dB and an uncertainty with (J == 6 dB. This comes about because the probability of making a measurement, other than for perfectly aligned polarisation, must always result in a lower than true reading. Assuming that a dipole-like antenna has nulls at -20 dB below the peak response and that all polarisations of waves from the EU1' are equally likely, it can be shown that the figures quoted above are realistic even though the distribution is not Gaussian. VSWR mismatches: If a typical biconic antenna has a VSWR of 3: 1 it can be shown [17J that the uncertainty can be represented by (J 2.9 dB. For a receiver with an input VSWR of 2: 1 there is another additional uncertainty of 1.9 dB. Taken together the combined uncertainty will be represented by (J == 3.5 dB. Antenna calibration uncertainty: There is very little reliable information regarding the uncertain ty associa ted wi th the use of commercial passive antennas. I t is clear that varia tions in the manufacturers' calibration cannot be~ .ignored as these calibration curves must usually be used. I t is very difficult to calibrate one's own antennas by comparison to industry reference standards or to calibrate thern by generating known fields. The NPL published uncertainty [20J in calibration for antennas below 1 GHz is ±1 dB excluding large loops (assuming a rectangular distribution for systematic uncertainty) , ( J 0.5 dB approx. This is almost insignificant, but when other factors such as ageing in use, loose elements, worn connectors are included, White [12J suggests it is prudent to allow for (J 3 dB. Antenna cable/balun effects: Uncertainties due to antenna cable, antenna element and antenna cable and poor balun effects have been estimated by DeMarinis [21 ] to introduce uncertainties with a maximum value of up to 12 dB. Assuming this to be representing a 3(J case one may estimate the standard deviation to be around (J == 3-4 dB. Field averaging: Estimates of worst-case uncertain ty for field averaging introd uced into an open-range measurement with a tuned dipole at 30 MHz have been produced by Brench [22]. He gives values of -8.5 dB lower than would be expected from a point measurement. If some systematic correction is made, for say half this range, it may be
218
•
•
A HANDBOOK FOR EMC TESTING AND MEASUREMENT reasonable to set a value for (J 2 dB at < 80 MHz falling to < 1 dB above 80 MHz. The 1 dB value is relevant to screened room testing where only short dipoles are used. EM! meter uncertainty: This is generally quoted as having a worst-case value of ±2 dB. This would' lead to an estimation for the standard deviation of < 1 dB. Near-field uncertainty: White [17] quotes values for the uncertainty in a 3 m open-range FCC test with a dipole at 30 MHz as being (J < 1 dB. It is unlikely that the figure V\Till be so low for MIL S1-'D measurements made at 1 m in a screened room.
11.7
Estima te of total urlcertainty
White [12] a comprehensive account of estimating EMC measurelnent uncertainties on the basis that most probability density functions for con tribu ting factors are largely unknown or not documented, and over a limited range of ±(J may be represented by a log-normal distribution. l'his is a distribution where the random variable x has a logarithm which is normally distributed [13]. Thus White gives standard deviations expressed in dEs and sun1med according to eqn. 11.11. While this is certainly convenient, one must be careful to establish its validity in each particular circumstance. Combining the estimates of standard deviations given earlier for the major factors which contribute to measurement uncertainty of our example test in a screened room, using eqn. 11.11 (assuming no totally dominant contribution) we arrive at a value of (Jtotal == 10.6 dB. This is higher than the figure given by Dash [23J, produced by analysing actual test data from open-range sites derived from measuring the same EUl' at a number of government facilities in different countries. l'his should not be surprising, as the extra contribution to uncertainty from standing waves in the shieldedroom case will increase the total measurement uncertainty. The standard deviation quoted by Dash for the open site is 8.5 dB. The example calculation of uncertainty in the measurement of radiated emissions fron1 an EU'f at 1 m in a screened room has been carried out as an illustrative exercise to demonstrate the nature of factors and to suggest reasonable magnitudes their standard deviations. 1-'he reader should take the values suggested as being indicative only, and should carefully evaluate the statistical distributions and estimate standard deviations for contributing factors with reference to particular test configurations and operating
11 . 8 Test laboratory llleasuretnent uncertainty 11.8.1 NAMAS The authority responsible for test/calibration laboratory accreditation in the UK is the National Measurement Accreditation NAMAS, formed in 1985. The organisation developed from the National Testing Laboratory Accredita tion Scherne and the Bri tish Calibration Service. NAMAS document MIO [24J, 'General criteria of competence for cali bra tion and laboratories' replaces the BCS EO 102 'Approval calibration laboratories' and the NATLAS Nl document. '[he principles on which document M lOis based are consistent with those given in ISO Guide 25: 'General requirements for the technical competence of testing laboratories', and EN4500I: 'General criteria for the operation of testing laboratories', which standard calls up the relevant requirements of BS 7581: 'Measurement and calibration systems'. Laboratories accredited NAMAS meet the requirements of ISO 25 and EN4500I and are considered as meeting the req uirernen ts concerned with the adequacy of calibration and testing contained in IS09000, EN29000 and BS5750 series of specifications, relating to assurance in manufacture and similar Details of contact names and telephone numbers for staff in the NAMAS executive are available [25]. NAMAS does not accredit laboratories to approve products [11]. This is the role of the appropriate national certification bodies such as the DTI Radio Communications ])ept in the UK, the FTZ in Germany and the FCC in the USA. The reports produced by accredited competent laboratories concerning the EMC test measurement of a product is usually the basis on which the certification body will grant approval for sale and operation.
11.8.2 NAMAS and measurement uncertainty The broad policy and details of statistical methods to be used by accredited laboratories when assessing measurement errors are con tained in publications NIS 20 [11 J and B3003 [26]. NAMAS requires that test laboratories shall have sufficient understanding of measurement, such that their measurements, or statement of compliance or noncompliance 'can be defended beyond all reasonable doubt'. NAMAS policy aims to minimise doubt which could result from a difference of opinion with
UNCERTAINTY ANALYSIS: QUALITY CONTROL AND TEST FACILITY CERTIFICATION
respect to the compliance or otherwise of a particular product. Laboratories are therefore required to be able to make corrections for error where appropriate, and having done so, to evaluate the overall uncertainty when carrying out testing. The role of NIS 20 [11] is to enable laboratories to understand the NAMAS requiremen ts with regard to uncertainty in measurement and to be able to evaluate the total uncertainty using a statistical approach. Test reports must carry a statement of total measurement uncertainty and therefore laboratories are required to have a documented policy in line with NIS 20 on measurement uncertainty, which guides staff in the preparation of such statements. Laboratories must produce and meet a NAMAS-approved laboratory schedule of limits of error and uncertainty (known as Schedule A), for all quantities for which measurement accreditation is claimed. The applicant laboratory decides the level of uncertainty for which it wishes to be accredited, bu t the limits cannot exceed those specified in appendix 4 of NIS 20 and each limit set out in Schedule A must be justified by an uncertainty budget and calculation. At least one uncertainty evaluation for an accredited test must be available for inspection each year during the NAMAS surveillance visit. Within the UK, NAMAS prefers that the uncertainty in a given test be evaluated at the 95% confidence level. This is a convenient point to look briefly at the statistical approach which is widely adopted to the statement of compliance of a sample of massproduced items with the interference limits stipulated in EMC standards. The statement of compliance is made after the test, or possibly multiple tests, of a batch of units, with the results being analysed according to a set of statistical rules.
11.8.3 Limits and production testing In the wider field of sample testing for product approval, the CISPR publication 16 (section 9) [27J requires that 'compliance of mass-produced appliances with radio interference limits should be based on the application of statistical techniques to assure the consumer with an 80% degree of confidence that 80% of the appliances being investiga ted are below the specified limit'. This is the so called 80% /80 % rule and details of the statistical techniques are given in Reference 26. Dvorak [28] gives data on practical measurements made on samples (up to ten) of angle grinder and hand-drill electric power tools and discusses the statistical techniq ues used to reach a statement of compliance. He suggests that very
219
Iittle of value has been published on a rational engineering approach to the problem of setting limits to ensure interference control. However, this is crucial if technically derived limits are to be implemented within realistic economic constraints for mass produced products. Absolute limits (no interference from any product shall exceed a stated value) are not unreasonable for small numbers of specialist devices, but a statistical analysis is far more appropriate to the EMC clearance of mass produced items. Many of the equations given in Section 11.4-11.6 are applicable to the calculations which need to be performed. Compliance with a limit L can be judged from
xn + Ka n < L where x == arithmetic mean of n samples, K is a factor related to confidence level for a given sample size, and an == standard deviation of the sample of size n. To generate a high degree of manufacturer confidence that the testing carried out on equipment does lead to a credible statement of compliance, it is necessary to test samples of up to ten randomly selected examples of the product and perhaps to repeat the testing of the sample a few times [28J. This will inevitably be rather time consuming and therefore expensive, but should lead to the absolute minimum of product compliance rejections without sound cause. An early CISPR Recommendation [29] gives the procedure to be used for testing appliances in large-scale production.
11.9 NAMAS requirem.ents for laboratory accreditation 11.9.1 Requirements for accreditation Companies which conduct EMC testing and wish to gain NAMAS accreditation must be prepared to submit to the requirements set out in NAMAS documen ts M 10 [24] and M 11 [30] . The princi pal req uiremen ts follow. The laboratory must be legally identifiable. Paying due fees to NAMAS. Giving such undertakings as NAMAS may reqUIre Being subject to reassessment every 3-4 years, and surveillance visits every year, with the possibility of unscheduled surveillance visits. The laboratory cannot make use of unreasonable or irresponsible subcontracting. The laboratory must abide by the law of the land. The laboratory must be impartial, and the
220
A HANDBOOK FOR EMC TESTING AND MEASUREMENT staff shall be free from any commercial, financial or other pressures which might influence their technical judgment. The laboratory must afford the client whose product is being tested reasonable cooperation so that the performance of the laboratory can be monitored. The laboratory must afford NAMAS reasonable accommodation and co-operation to enable their representative to monitor compliance with the regulations. Key staff in the laboratory must have job descriptions and be aware of their roles and the extent and limitations of their responsibilities. Specification of the qualifications, training and experience necessary for the key managerial and technical staff shall be contained in the job descriptions. The laboratory shall normally use only staff who are permanently employed or on contract to the laboratory. There shall be a documented policy to ensure that new and existing staff maintain relevant academic expertise, technical skills and professional expertise. ManageriaJ staff shall have the authority and resources needed to discharge their duties. The laboratory must have a documented policy and procedures to ensure the protection of proprietary rights and confidential information concerning products being tested. The laboratory should have restricted physical access for unauthorised individuals. There must be named laboratory and quality managers with responsibility for all work undertaken. The quality manager must have direct access to the highest level of management which is concerned with the company policies which affect the laboratory. The laboratory shall have nominated authorised signatories recognised by NAMAS. A q uali ty system shall be formalised in a quality manual which meets the requirements ofNAMAS document M16 [31]. The manual must be maintained up to date. The laboratory must establish and maintain proced ures to control the distribu tion, updating and retrieval of all documentation that relates to the calibration or tests it performs. Amendments to the documentation shall be subject to strict management control. The laboratory quality manager must plan and ensure that periodic internal audits of proced ure and operation are carried out. They shall be conducted as freq uently as necessary to achieve the required level of quality control. The period between audits shall not exceed 1 year.
The laboratory shall normally use only equipment that is owned by, or on long lease or loan to the laboratory. It shall be suitable for all the tests for which it is to be used and must be capable of achieving the accuracy req uired. I t should be of established design if possible and be operated only by named authorised staff. Each items of equipment should be uniquely identifiable. i\n up-to-date calibration history of equipment req uiring calibration shall be maintained and available for inspection. The laboratory shall, wherever possible, ensure that any data recording and analysis computer software is fully documented and validated before use. The standard of accommodation and facilities shall be such as to facilitate proper performance of the tests. Effective monitoring and control of environmental factors shall take place. Due attention shall be paid to factors such as mains voltage, electromagnetic interference, humidity, temperature, dust and vi bration levels. The laboratory shall have an effective documented system for identifying test items. There must be a systematic and documented record of all information of practical relevance to the tests performed. All records must be maintained for a period not less than 6 years.
11.9.2 Advantages of laboratory accreditation The examples of the requirements that a NAMAS accredited EMC test laboratory must meet are stringent and involve the laboratory in some time, trouble and cost. The accreditation may be taken as an indication that the req uiremen ts set out are being met by the laboratory on a daily basis. For a manufacturer reqUIrIng EMC conformance testing the use of an accredited laboratory offers a number of advantages in terms of confidence in the q uali ty of test reports obtained. A major one is the acceptance of third part certification by customers, avoiding the need for each customer to carry out a separate assessment. A list of accredited laboratories (at the time of writing) is given in NAMAS Concise Directory M3 [32]. Not all EMC tests need to be carried out in NAMAS approved facilities; adequate development or preconformance testing can be done in other laboratories, or by the manufacturers themselves. Under the terms of the EC harmonisation directive 89/336/EMC self certifica-
UNCER1'AINTY ANALYSIS: QUALITY CONTROL AND TEST FACILITY CERTIFICATION
tion of product EMC performance by manufacturers using their own facilities is permitted. It may be in the interests of self certifying Inanufacturers to obtain NAMAS accreditation for EMC testing if they wish to convince customers they are working to the highest q uali ty standards, or offer a testing service to other companies. NAMAS-accredited laboratories may become the core of competent bodies referred to in the harmonisation legislation who can assess the EMC performance of a new equipment by studying the technical file which is built up during the development programme. If the assessmen t is posi tive it is possible to certify compliance with the relevant standards called up in the legislation wi thou t recourse to a full/ / EMC test. Thus the generation and submission of the technical file to a N AMAS/ liboratory could be the preferred route to EMC compliance if the equipment is large, difficult and expensive to test. The electrical measurements division of the National Physical Laboratory and the NAMAS executive are approachable and will help in the understanding and interpretation of technical and quality requirements which relate to EMC testing in the UK. EMC test facility managers and engineers, together wi th managers and development engineers in firms producing products which require EMC clearance, should study carefully the NPL/NAMAS documentation referred to in this chapter to appreciate the framework within which EMC compliance testing is carried out in the UK.
11.10 References
2 3 4 5 6
7 8 9
SPIEGEL, M.R.: 'Probability and statistics' (McGraw Hill, 1988, SI edn.) FELLER, W.: 'An introduction to probability theory and its applications' (Wiley, 3rd edn.) BEAUMONT, G.P.: 'Probability and random variables' (Wiley) HARPER, W.M.: 'Statistics' (Macdonald & Evans, London, 1965) TUCKWELL, H.C.: 'Elementary applications of probability theory' (Chapman & Hall, London) BOX, G.E.P., HUNTER, W.G. and HUN'TER, j.S.: 'Statistics for experimenters: An introduction to design, data analysis and model building' (Wiley) TOPPING, j.: 'Errors of observation and their treatment' (Chapman & Hall, London) DIETRICH, C.F.: 'Uncertainty, calibration & probability' (Adam Hilger, 1973) HARRIS, LA. and HINTON, L.J.T.: 'The expression of uncertainty in electrical measurements'. BCS guidance publication 3003, issue 1, 1977
221
10 HINTON, L.J.T.: 'Uncertainty and confidence in rneasurements' in BAILEY, A.E. (Ed.): 'Microwave measurements' (Peter Peregrinus, London, 2nd edn., 1989) Chap. 25 II NAMAS information sheet NIS 20. NAMAS Executive, National Physical Laboratory, Teddington, Middlesex, TWll OLW 12 WHITE, D.R.J.: 'A handbook series on electromagnetic interference and compatibility, volume 2 EMI test methods and procedures'. Don White Consultants, Germantown, Maryland, USA, Chap. 5 13 MEYER, S.L.: 'Data analysis for scientists and engineers' (Wiley) 14 'Statistics, probability and reliability' . NATO ES/ CISG/71, draft issue C 1986, Chap. 4 15 GOSSETT, W.S.: 'Student's 't' distribution'. (Statistician to Guinness Brewery, UK) 16 Shielded enclosure performance measurernen t program. APG contract DDAD05-77-9551, USA 17 WHITE, D.R.J. and MARDIGUIAN, l\r1.: 'Errors in EMC compliance testing and their control'. Don lvVhite Consultants Inc, Gainesville, Virginia 22065, USA 18 HEIRMAN, D.N.: 'Education and training of the industrial regulatory compliance test team'. Proceedings of IEEE symposium on EMC) 1981, pp. 365-368 19 FREE, W.R. and S'TUCKEY, C.W.: 'Electromagnetic interference methodology, communications equipment'. Technical report ECOM 0189-F, NTIS AD 696496, Oct. 1969 20 'RF and microwave measurements'. NPL 00/1.5K/ NI/9/90, National Physical Laboratory, Teddington, Middlesex, TWll OLW, UK 21 DE MARINIS,j.: 'The antenna cable as a source of error in EMI Measurements'. Proceedings of IEEE symposium on EMC) 1988, pp. 9-14 22 BRENCH, C.E.: 'Antenna differences and their influence on radiated emission measurements'. Proceedings of IEEE symposium on EMC) 1990, pp. 440-443 23 DASH, G.: 'Computing equiplnent standards -- An update on cable and peripheral placement'. Proceedings of IEEE symposium on EN/C, 1987, pp. 332-337 24 NAMAS accreditation standard: General criteria of competence for calibration and testing laboratories, MI0. NAMAS Executive, National Physical Laboratory, Teddington, Middlesex, TWll OLW 25 'NPL points of contact'. NPL0004/8K/Nj /6/90, National Physical Laboratory, Teddington, Middlesex, TW11 OLW, UK, pp. 36,37 26 'The expression of uncertainties in electrical measurements B3003'. NAMAS Executive, NPL Teddington, Middlesex, 1'W11 OLW (Related to Ref. 9) 27 'Specification for the radio interference measuring apparatus and measurement methods, section 9: Statistical considerations in the determination of limits of radio interference'. CISPR 16 28 DVORAK, T.J.: 'The problems of limits interpretation in type approval testing and production control'. Proceedings of IEEE symposium on EMC, 1981, pp. 264-268
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A HANDBOOK FOR EMC TESTING AND MEASUREMENT
29 'Compliance with limits in large scale production'. CISPR Recommendation 19, publication 7 30 'Regulations to be met by calibration and testing laboratories'. Publication M 11, NAMAS Executive, NPL, Teddington, Middlesex, TWll OLW, UK 31 'The quality manual: Guidance for preparation'.
Publication M16, NAMAS Executive, NPL, Teddington, Middlesex, TWll OLW, UK 32 'NAMAS concise directory'. Publication M3, August 1990, edn. 5, NAMAS Executive, National Physical Laboratory, Teddington, Middlesex, TWll OLW, UK
Chapter 12
Designing to avoid EMC problems EMC has not been quantified. Production variability can then lead to EMC problems manifesting themselves in some proportion of the units manufactured. Design is an iterative feedback process which involves continued interaction between required performance and technical and financial constraints. I t involves the synthesis of system archi tecture and the development of stra tegies and techniques by which the product performance goals can be met in the optimum cost-effective manner within the imposed project constraints. In general, it is the designer who is technically responsible for every significant aspect and result of his design. But EMC problems can often arise from obscure causes which may not be significant in terms of the design strategy adopted to meet the primary design goals. The designer can call for assistance in the form of an EMC specialist, possibly an external consultant who will provide the background knowledge and expertise to advise the designer and his team about aspects of their proposed design which may lead to EMC problems. Together they can then work out a set of modifications which improve the EMC performance of the design without compromising the original design goals. Whilst this approach does work, it necessarily sets the EMC expert apart frorn the main design effort and can cast him in the role of the opposition, an extra qualification on the design which is being developed. In these circumstances EM C engineering tends to con tin ue as a discipline outside that of mainstream electronic design and the situation is repeated again and again, as EMC knowledge stays with the EMC expert and is not transferred and integrated into the product design team. To do so requires company management to realise that EMC problems and solutions do not fit neatly into watertight design compartments, nor can it be dealt with at a single level of technical staff and management. EMC is a vital part of product design and the best way to ensure success is to set aside time and funds to train existing designers and to recognise the value of using multidiscipline ElVIC skills as just one part of the formal design process. See Figure 12.1. Consultants can still occasionally be brought in to supplement the EMC knowledge required for
12.1 Intrasystetn and intersystetn
EMC Electromagnetic compatibility is likely to play an increasingly important role in both electronic engineering and commercial law during the 1990s [1]. rrhe overuse of the EM spectrum, the rapid inclusion of microprocessors into a very wide range of industrial, commercial and domestic equipment, and the increased awareness of possible effects of EM energy on biological systems will bring this abou t. Electromagnetic compatibility has emerged from the world of military and space systems manufacture, to contribute to the performance of everyday electronic equipment and the profitability of companies engaged in its supply. There are two kinds of electromagnetic compatibility which a system or equipment designer must consider: intrasystem compatibility, which is concerned with EMI internal to an equipment, and intersystem compatibility, which is concerned with EMI external to the equipment. This involves compatibility with other equipments or systems in its immediate electromagnetic environment and the safeguarding of the wider EM spectrum.
12.1.1 Intrasystem EMC Internal electromagnetic compatibility is important for the equipment to function properly in its own right. Without careful EMC design, circuits can interfere with each other and this leads to unreliable and degraded operation, unnecessary expensive field support engineering and possibly a loss of customer confidence in the product. Electronic designers have a great number of problems to consider and sometimes only recognise EMC when it becomes a problem, usually close to the end of the design stage when it is far too late to effect low-cost solutions. In the past, electronic designers have sometimes designed with only the obvious product design goals in mind, and have not recognised the need for a formal approach to EMC to be considered as an integral part of the design process. Thus equipments have sometimes been produced where the margin between successful operation and degraded performance due to a lack of internal
223
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A HANDBOOK FOR EMC TESTING AND MEASUREMENT
ELECTRICAL DESIGN
I I r------i
y REQUIRED EQUIPMENT PERFORMANCE
H
SOFTWARE DESIGN
ENVIRONMENTAL ENGINEERING
Figure 12.1
EMC is just one important product design feature
certain tasks, but the long term goal of the company should be to access education and training programmes [2-5J to build an EMC design knowledge base wi thin the normal design teams. The need for management to provide training support for staff to ensure good EMC design, especially for companies producing commercial electronics, is sometimes not understood. This may stem from the multifaceted nature of EMC and the difficulties which arise in quantifying the benefit to be gained by a formal EMC programlne being carried out by the design team, when compared with the investment which is needed for their training and support. The situa tion has been a little clearer for companies in the military and aerospace sectors which have been contracted to meet EMC standards, and formally demonstrate product compliance. Legally enforceable EMC standards are being introduced in the UK in the first half of the 1990s to regulate the EMI from commercial electronic products. When this happens, and the penalties for noncompliance are known, the cost benefit in pursuing good EMC design will be much more obvious.
12.1.2 Design for formal EMC compliance Wi th the adven t of these mandatory regulations concerned with the electromagnetic compatibility of almost all commercial electronic equipment, EMC has come to be associated by some principally with those procedures and practices which will ensure compliance v\lith standards. l'his is perhaps a narrow view and fails to recognise the reliability and other performance/ quality benefits which good intrasystem EMC design can bring to the product to increase its competi tive edge in the market. A successful EMC design requires more than slavish adherence to a set of design rules and practices, useful though these are. Designers must have the knowledge to design in EMC from the
earliest stage in the development process with flair and invention. The existence of a specified external EM environment in the relevant standard is the starting point for the intersystem EMC designer, who knows what performance must be met. In the case of designing purely for internal compatibility, one must attempt to specify both the emission and susceptibility levels for the equipment and set an affordable margin of compatibility. Designers of military equipment are familiar with the formal control of the EMC design and certification process as required by the relevant MIL or DEF standards. Much of the approach is transferable to commercial equipment design teams without imposing unnecessary technical constraints or costing too much. The general approach is outlined next.
12.1.2.1 Contractual assistance EMC expertise is required to establish and agree a satisfactory contract for the equipment to be produced. In the simplest case this may only involve interpreting the technical and cost impact of the legal requirements flowing from EEC/89/ 336 and appropriate technical standards for equipment to be sold within the EEC. For /equipment to be sold worldwide, careful assessment of the multitude of country and trading block EMC standards and regulations must be made to establish equivalences and understand which regulations take precedence. Only when this has been completed can the worst-case composite specification be defined and its impact reflected in the contract and price.
12.1.2.2 EMC control plan The evolution of a satisfactory EMC design that can meet the contractual req uiremen ts should be ensured by the control afforded by adequa te documentation, design reviews and for large projects, the referral of key design decisions to an EM C control board. The top-level control plan will specify how the design is to be controlled, by whom and with what means.
12.1.2.3 System specification The contractual requirement for EMC will be interpreted into design req uiremen ts in the form of system and subsystem EMC budgets as shown in Figure 12.2. These documents can be simple in the case of a commercial product that only has to meet, for example IEC 801, or may be complex in the case of a military equipment or large commercial system. The subsystem specifications
DESIGNING TO AVOID EMC PROBLEMS
225
Figure 12.2 Top-level EMC design process EMC DESIGN BUDGET CONSTRAINTS SIMPLE EMI COUPLING MODELS EMC TECHNICAL APPROACH EMC TECHNICAL DATABASE
(Case histories! Expert system/ Consultants)
- a design methodology for cost-effective balanced hardening
EMC DESIGN HANDBOOKS
- detail technical options
CO-ORDINATED EMC DESIGN AND TESTING PROGRAMME CONTROLLED BY EMC CONTROL AND TEST PLANS
will reflect the need of the product to meet the imposed external EMC specifications and the need to achieve internal compatibility. Clear understanding of the technical aspects of EMC is important at this stage as the apportionment of the contractual and internal EMC requirements down to subsystem level affects the cost of optimum design for compliance with imposed standards and correct equipment functioning. The systems engineers or designers must have a good understanding of EMC hardening methodologies [6-8J and be capable of running trade-off studies between various possible hardening solutions to select the best overall system philosophy.
12.1.2.4 Design handbooks Often design engineers engaged on EMC will make use of a wide range of general design hand books to aid their work. Some are produced by the Military [9-12J and various books and notes [8, 13-15 J have been written which include information on EMC design. Although some of the basic techniques are reviewed in this book, it is not intended to deal in depth with EMC design techniq ues, as the subject is well covered elsewhere. Companies which have a longstanding need for cornpetence in EMC design often produce their own handbooks for the guidance of electronic and
other engIneers. However, these are company confidential documents as they the results of hard-learned lessons in the EMC design. Design handbooks will typically include the following topics:
usually contain field of data on
Bonding: types, corrosion, vibration Grounding: LF/HF/VHF solutions Filtering: passive, lumped/distributed, absorbative EMC aspects of mechanical design: box fabrication, seams, materials Shielding: materials, penetrations and apertures Cable design: coupling, screening, routing, segrega tion Connectors: type, use, shielding, connection to cable screens Circuit board design: ground planes, decoupling, loop areas, inductance and stray capacitance Component placement and orientation IC logic selection: speed, noise emission and susceptibility Power supplies: regulation, noise emission, susceptibili ty Optoisolators and fibre optics Transformers: mains, audio, video, "RF Common-mode /differen tial- mod e interfaces Interface circuit protection Electrical safety req uiremen ts.
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A HANDBOOK FOR EMC TESTING AND MEASUREMENT
12.1.2.5 EMC computer modelling
eg. own subsystem, other system, external
lEM environment Figure 12.3
I
real coupling USUallY} { involves parallel paths
l
eg. own subsystem, own system, other system, the general EM environment
Extensive EMC system-level computer models have been constructed in the past for military and space programmes such as SEMCAP [16] (systen1 and electromagnetic compatibility analysis program) bu t the use of such a program is perhaps inappropriate to all but the largest projects. Smaller versions have been produced which are better for smaller projects, but these complex software packages have not been universally adopted as the best way of estimating potential EMC problems. Other EMC system software has been developed largely in the US, and is described by Keiser [13]. These models include IPP-1 (interference prediction process) and IEMCAP (intrasystem electromagnetic compatibility analysis program) which determine whether a coupling path exists between any two nominated subsystems/units as shown in Figure 12.4. Appropriate transfer function or coupling models are then used to determine the level of potential incompatabilities between the various subsystems. The incompatibility identification process can be visualised in the form of a set of ma trices as shown in Figure 12.5. The matrix is composed of columns of subsystems defined as ABC D etc. which are
I
Block diagram of the generalised EMI situation
The configuration of these design elements into a complete EMC equipment design is a task req uiring considerable theoretical and practical knowledge and experience. In a real system the transfer of EM energy can occur at subsystem level (wi thin the system), or between the system and its EM environment and between the system and other systems. Depending largely on the distances between the coupled elements, the circuit impedances and the frequency of the EM energy being coupled, the coupling paths can be considered to fall in to three broad types, as shown in Figure 12.3. All possible paths or combinations of paths must be addressed in a competent EMC design.
Figure 12.4 Illustrative example oj intrasystem coupling paths
EXAMPLE SYSTEM (transmitter)
POWER AMPLIFIER SUBSYSTEM ICI
POWER SUPPLY OSCILLATOR SUBSYSTEM IN ~-.:...._----.. SUBSYSTEM IB I
PROBABLE COUPLING PATHS LIST OF CONDUCTED EMISSIONS LIST OF RADIATED EMISSIONS
SUBSYSTEM 'A'c CULPRIT (Psu)
CE RE SUBSYSTEM 'B l c CULPRIT (oscillator)
LIST OF CONDo SUSCEPTIBILITY LIST OF RAD. SUSCEPTIBILITY
SUBSYSTEM IA'v VICTIM (Psu)
cS RS SUBSYSTEM'SSv VICTIM (oscillator)
CE
CS RS
RE SUBSYSTEM 'C'c CULPRIT (PA & antenna)
SUBSYSTEM ICIV VICTIM (PA & antenna)
CULPRITS - - - COUPLING PATHS - - - - VICTIM NOTE: SUBSYSTEMS ARE TAKEN TO BE COMPATIBLE WITH THEMSELVES
DESIGNING TO AVOID EMC PROBLEMS
227
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Intrasystem compatibility matrix
considered to be potential culprits capable of emitting unwanted electromagnetic energy by conduction or radiation to other subsystems. The rows in the matrix contain actual or estimated susceptibility data for each subsystem/unit as a victim of EM interference. The frequency and amplitude details of any potential incompatibilities between any subsystems/units are recorded in the appropriate matrix elements. For clarity, three rna trices can be used: one for conducted interference incompatibilities and one each for E- and H- field radiated incompatibili ties as shown in Figure 12.5. The subsystems or units are assumed to be self compatible and thus the 1:1 diagonal line in the matrix is void. Many EMC system level programmes can be used to 'cull' the many possible frequency matches which will be found between culprits and victims, on the basis of expected incompatibility level. Thus potential subsystem incompatibilities can be ranked into 60 dB problems, 30 dB problems or relatively trivial problems of less than 10 dB. This analysis can be very useful in directing the attention of the design team to the key EMC issues at a very early stage in the design. Detailed analysis of serious incompatibilities can then be carried out with computer models such as NEC [17J and EMAS [18J or other finite element/difference/method of moments or other electromagnetic codes. Whatever computer software packages are used
to carry out system-level EMC assessments, a great deal of reference data and a number of case histories are usually used as background material to support the current assessment. I t is therefore important that the designers and managers of projects with an EMC content, formally and carefully, record data generated on their project and make them available to future projects by contributing to a company wide EMC database. In recent years, some design teams have attempted to codify EMC system and detailed design knowledge in the form of EMC expert systems which can be run on personal computers [19, 20]. This technique may prove to be more practical than the earlier large models and more suitable as a tool for equipment designers rather than EMC specialist engineers.
12.1.2.6 Test plan The requirement for this document would be written by the EMC project management team in the case of a large project and would require the designers and EMC test specialists to define what testing is required at the various stages of the equipment development. The plan itself would be produced by the designers and EMC test engineers. The test plan would include state,ments on the following tests. Risk reduction tests: These short tests would be
228
A HANDBOOK FOR EMC TESTING AND MEASUREMENT
undertaken to resolve a design question in favour of one technique or another. They are carried out at a very early stage of the design and are in some ways the most important tests which will be carried ou t on the project. They will set the course for all future EMC engineering solutions and may be concerned with deciding on a grounding and bonding philosophy or selecting the types of integrated circuits to be used. Development tests: These semiformal tests would be required at certain well-defined stages of the developing design to check that the EMI predictions which will have been made are being achieved. Such tests may be conducted before design drawings are formally adopted and other elements of the overall design are allowed to proceed on the basis of what has already been achieved. Occasionally additional informal development tests may need to be conducted by the designer to quickly verify the EMC benefits of a particular circuit, shield or technique. Such tests need not be done by submitting the problem to a test house, external or internal to the company. They are best done on the bench by the designer responsible for that element of the design who may be assisted by test personnel and may need to use some of the specialist equipment such as current probes and spectrum analysers. Conformance tests: The test plan for these tests is usually written by the EMC test laboratory personnel in conjunction with the design team. It requires intimate knowledge of the standards and specifica tions which have to be met and the limitations of test techniques which are specified. In certain circumstances, on small projects for example, a consultant may be called in to liaise with the external approved test laboratory, to help generate the test plan and to oversee the testing which is carried out. The consultant may also be in a position to advise the designer as to the validity of the results obtained and to point out improvements which should be made to the design if the equipment has failed the test. On large projects the certification testing may involve formal demonstration of the equipment or system functioning in its operational environment, either at its place of installation or on a specially prepared EMC test range. Often these final proving trials are witnessed by a representative of the customer's organisation and may be important in obtaining a stage or final contract payment.
12.2 Systetn-Ievel EMC requiretnents
12.2.1 Top-level requirements Depending on the nature of the product or system and any specific clauses relating to EMC in the contract to purchase, together with any EMC
legal obligations relating, to product sale, export and use, the system-level EMC requirement can range from simple to very complex. If complex, it is necessary to consider a hierarchy of req uirements and define areas of precedence for certain specifications and standards for the product as a whole. For example, electrical safety requirements are usually paramount. Once the customer/contractual/legal/export/ licence for operation and electrical safety req uirements have been defined, it is then necessary to estimate the probable levels of RF emissions and susceptibilities of the proposed design. These data can then be compared with the requirements, and emissions reduction req uiremen t or hardening requirement (for immunity) can be defined. These EMI immunity hardening or emission reduction requirements must then be apportioned to the various subsystems such as power supplies, mother board design, clock and processor circuits, cable harnesses, interface boards/circuits and mechanical design of the case or container, based on the need to achieve these req uirements for the minimum cost and impact on the optimum equipment design.
12.2.2 Determining EMC hardening requiremen t The system hardening requirement for an equipment which must meet a full range of interrelated electromagnetic specifications may have to accommodate the following features. EMC:
Transients: NEMP: Lightning:
Radiated susceptibility Radiated emissions Cond ucted susceptibili ty Cond ucted emissions Imported transient immunity Exported transient generation Radiated susceptibility Cond ucted suscepti bility Direct effect immuni ty Indirect effect immunity
In addition, some equipments or systems may be subject to extra requirements, such as that for secure communications (Tempest). If all these requirements have to be met simultaneously to some degree, as for example with the modern military aircraft, the task of reconciling the differing technical solu tions to meet these req uirements can be considerable. The generation of typical system hardening requirements is demonstrated in what follows with reference to radiated susceptibility and emissions. When evaluating conducted emissions and susceptibilities the situation is more straightforward and simple transmission-line models can be used.
DESIGNING TO AVOID EMC PROBLEMS
Large projects may use complex computer models to define a system hardening requirement given the specifications of the external electromagnetic environment and an estimate of the internally generated interference. For most projects, EMC systems engineers or designers can use a number of simple models to yield a worstcase estimate of the hardening requirement. The sophistication of the models available will depend on the expertise of the design team and the financial resources wi thin the project. An example follows of the use of simple modelling tools to establish an early estimate of the scale of the EMC design task on a project. Such information guides the technical and management framework within which the electromagnetic design engineering of equipment is carried ou t. Consider Figure 12.6 which shows a system EMC radiated immunity requirement for project X. This will have been obtained from the EMC standards and specifications called up in the contract, with an additional element derived from the engineering estimate of the potential interference levels likely to be self generated within the equipment. This intrasystem compatibility requirement luay consider, for example, the local fields due to high power RF radiation from an antenna which may be associated with the equipment being developed. To calculate the hardening requirement it is necessary to estimate the worst-case coupling from the specified field to the equipment being designed. Two features of the equipment tend to dominate the EM field coupling: the length and type of cable runs, and the characteristics of equipment boxes and conductive structures in which they are mounted together with any apertures, as shown in Figure 12.7. A number of models can be used to evaluate the worst-case coupling via the box and its 1000 . - - - - - - - - - - - - . . . . - - - - - - - - - - . - - - - - ,
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COMPOSITE FIELD STRENGTH SPECIFICATION
1MHz
10MHz
100 MHz
1GHz
10GHz
FREQUENCY
Figure 12.6
Example of specification for composite RF environment in which system must operate
229
CABLE
~---"" 7.5m 1 0 n g - - - - - ! ------~:fj)
CURRENT INDUCED IN CABLE
k
COMPLEX RF CURRENTS HA/m FLOWING IN BOX ....- - ~ - - ~ E VIm STRUCTURE Z (wave-
impedance) --~--
FIELD INCIDENT ON BOX AND CABLE
Figure 12.7
Example of box-and-cable pickup
apertures and a simple dipole model may be used for field-to-wire coupling.
12.2.3 Simple coupling models This section demonstrates how simple considerations of key physical aspects of a system, such as its size and the length of conductors inside it, or cables attached to it, can be used to predict its coupling properties to external fields. Designers working on projects which have access to sophisticated compu tel' models will of course make full use of them, bu t they may also carry out calculations using simple models to crosscheck their results. Many examples of simple models exist [13, 14, 21, 22J which enable predictions of certain aspects of EMI problems to be made. These include Common-mode coupling of fields into the boxcable-box loop area for balanced and unbalanced systems Differential-mode coupling into box-cablebox loop area Differential-mode coupling into coaxial cables Capacitive coupling between circuits Inductive coupling between circuits Coupling rejection of twisted pair cables Shielding effectiveness of coaxial cables Shielding effectiveness of metallic screens Filter performance models Active component rectification models Attenuation of EM waves through buildings. The selection and combination of these simple models to 'scope' EMI problems in a system req uires experience. Methodologies derived by White [22J and implemented in the form of computer software can be of assistance. As examples of simple models, the circuits on which the capacitive and inductive coupling models are based are shown in Figure 12.8. The field -to-cable common-mode coupling circuit is shown in Figure
230
A HANDBOOK FOR EMC TESTING AND MEASUREMENT EM wave couples into loop area
(Received noise)
(Source of noise/EMI)
Cross talk capacitance
s
(Source of noise/EMI)
"---~~--~--Mutual
impedance
COMMON-MODE COUPLING INTO BOX-CABLE-BOX-GROUND LOOP AREA
Ico
-10---r----r----r--_-....--
_
-20
"0
Figure 12.8
for f
-;
1 ) 1 + J wCcv < vlm
=
0..
-50
8
-60
w
o
o :E Z o :E :E
o()
wCw
« 1/2nCcv < zlm
where 2nfxfrequency in Hz Cev wire-to-wire coupling capacitance per metre < v = parallel combination of victim source and load impedances and wiring capacitance Cv to ground = l/(1/
-40
:::>
( a) Circuit representation of capacitive coupling between parallel wires over ground plane
= (
-30
~
::J
(b) Typical equivalent circuits used to model cable crosstalk above ground plane
Vv/Vc
~
= =
Reprod ueed by permission of I CT Inc.
12.9 together with examples of the calculated coupIing response for various loop areas as a function of freq uency. Engineers at McDonnell Douglas [21] have cond ucted many experiments on field -to-wire coupling of representative avionics cables, including single wires. They showed that the coupled power from the field to the conductor depended on wavefield frequency wire orientation to the wave wire geometrical factors: length, dialueter, shape, rou ting terminating load impedance In a real system there is little control over the routing and orientation of cables with respect to any particular subsystem and the internal field-tocable coupling can be considered as random. Measurements made by McDonnell Douglas using different lengths of cable and load impedances
Figure 12.9
Two-box EMI problem
Reprod uced by permission of ICT Inc.
showed little effect on the pickup probability distribution function, which has a log-normal form with a standard deviation of between 3 and 6 dB. Shielded wires of course will normally tend to pick up less than unshielded wires, but the distribution means may not be as widely spaced as the assumed shielding figure (of perhaps 30 dB) would suggest. Practical measurements of real cables with imperfect shield terminations, poor braids etc, show that the worst-case pickup from a shielded cable can be comparable with pickup levels for an unshielded cable. See Figure 12.10.
cF-
SHIELDED WIRE PICK-UP PROBABILITY DISTRIBUTION FUNCTION
difference in means corresponds to 'average shield performance'
) I (
w
o
zW
14-----'----
UNSHIELDED WIRE PICK-UP PROBABILITY
.-/ DISTRIBUTION FUNCTION
0: 0:
::::>
o o
o
LL
o
~
co
« CD o
0:
_ _----1......_ - - 1
O'~_~
I0000-~.......~ - -..........~
Overlap in distributions indicates that occasionally pick-up in a shielded wire is as great as in an unshielded one.
Figure 12.10
Probability density functions for shielded and unshielded wires
Reprod ueed by permission of McDonnell Douglas
DESIGNING TO AVOID EMC PROBLEMS O~----------,r--------.....,.
i-wave
w
cr:: .... cr::....., :,)
dipole aperture = 0.131..'
Maximum power induced at close to first cable or structure resonance
·10
"
~..s .20 ~3:
OCO
~§
-30
~
·40
0:::"0
LLO
a......J
LL
It,.-o ~
:::>~
OLL
.. ·0.16 m long wires • - 3.05 m long wires
0.1
""'"
dipole model
1
10
3:0
~~
0:::
Measured maximum effective apertures ( A e ) of various wire lengths. The halfwave dipole is a reasonable upper bound for these experimental data
,,
i
I I
2
Intercept point at close to first cable or structure resonance Pa
..!.. f2
Pa ')..}
~20 dB / decade I
2
LL-
FREQUENCY GHz
Figure 12.11
UJUJ
"
Electrically short mismatched
Paf Pa 1
O:::z --I
"
UJUJ
a.....-
~ cable, or system structure.
--------·1
-3:
5~
-50~
cable/structure is electrically short
I
cable/structure is approximately resonant
cable/structure is electrically long
1....- - - -
0.1 MHz
1MHz
10MHz 100MHz FREQUENCY
1GHz
10GHz
close to first cable or structure resonance typically 5 - 50 MHz for average system
Reproduced by permission of McDonnell Douglas
The McDonnell study showed that the pickup could be characterised by calculating the effective aperture of the wire by dividing the power picked up and delivered to the load by the wavefield power density:
Figure 12.12
12.1 where P == power picked up by the wire (watts), P d == EM-field power density (W/m 2 ), and A e == effective aperture of the wire (m 2 ). Measurements of the maximum effective aperture of real wires produces a plot as shown in Figure 12.11 for which a least squares fit is close to that for the theoretical aperture for a matched halfwave dipole, which is given by 12.2 where A == wavelength. This matched-dipole formula is not, however, useful at low frequencies where the length of a halfwave dipole would be greater than the actual length of any cable or system structure. Thus the resonant matcheddipole model should be abandoned at frequencies much below the fundamental cable or system structure resonance. A t such frequencies the condition for a tuned halfwave dipole is not physically achievable in the real system and the model dipole becomes detuned and electrically short, with a rising reactive source impedance still driving the fixed load impedance. Under these conditions the power delivered to the load tends to fall with freq uency owing to the mismatch [13, 21]. From the McDonnell Douglas study two things may be deduced: (i)
Coupling for matched dipole (tuned length) is unrealistic below first resonance length of
"
We;'
~.!
231
Complex coupling in real systems is best treated in a statistical fashion without
(ii)
Composite wavefield to structure/cable model: inverted V
making an attempt to calculate individual coupling path coefficients for the enormous number of possible coupling combinations. The exception to this approach arises when there is a clear dominant coupling path from a strong interference culprit to a sensitive victim subsystem. In such a case the coupling to the particular cable should be calculated individually. The simple matched dipole formula can be used to crudely estimate the worst-case fieldto-cable coupling. For frequencies below the cable first resonance a limit must be applied to the matched dipole model and a roll off with frequency can be used resulting in the combined curve appearing as an inverted-V as shown in Figure 12.12.
12.2.4 Susceptibility hardening case study 12.2.4.1 Cable hardening requirement Consider the equipment shown in Figure 12.7 with a box of electronics of 1 m side connected to an unshielded cable approximately 7.5 m long, which will have its first resonance in the region of 20 MHz. Let the specified RF field strength in which this equipment must operate be as shown in Figure 12.6 and reproduced as curve A in Figure 12.13. As an example of a general coupling model, use the simple matched-dipole worst-case pickup model at frequencies where the cable is electrically long (above 20 MHz). One can estimate the induced power shown in Figure 12.13 as curve B (shown dashed above 1 GHz). The coupled power at low frequencies rolls off
232
A HANDBOOK FOR EMC TESTING AND MEASUREMENT
z
Figure 12.13 Development of cable hardening equipment
o
:r:~
""0 0 -
Z~
ClUJ 0
...Ja:w ~ .... a. U-(J)CJ)
1000 VIm
10 VIm
1 VIm 30
CURVE IB I
20
/
10
~"0
0
,.,..BURN ....... --OUT ( linear & digital
g, -10
... --'1'" /
~
() -20
c::
ffi
-30
~
-40
0..
-50 -60 -70
----
... --------
---... -~----"" . .-
I, ~CMOS ~ • • TTL •
~ integrated circuits ) ~
~ CMOS DEVICES_ ~ (leVelchange)~ TTL DEViCES. . .
• _. --
------/--(Ievelchange)
........ ~
~L1N IC ~
~.
-
.~ CURVE'E'~ _./
_ - - - .....
LINEAR ICs
z
~o m~
CURVE 101
:::>
10dB
~
fZ
w
0
~
UJ
...J
SYSTEM CABLES
~
w co c:: "0 5 aw 80 c::
'20dB
«CO ()
SIMPLE WORST CASE HARDENING REQUIREMENT
70
z Z 60 w 0
c::
«
50
:c 40
w
...J
CD 30
« 0
~
w .... CJ)
>-
CJ)
20 10 0
1 MHz
with frequency from the value at 20 MHz (close to the first cable or structural resonance). Applying this roll off to the specified field strength (curve A) in this frequency range generates curve C. Because the system power and data cables in which the pickup is occurring are not specially designed to transfer RF signals, there is an additional cable loss above 1 GHz which should be introduced as shown in curve D. This modifies the pickup (curve B) above 1 GHz and results in curve E. The final simple estimate of maximum power picked up in the cable and delivered to circuit (loads) inside the box is given by curves C and E.
10 MHz
100 MHz FREQUENCY
10Hz
100Hz
For specific system configurations, other simple coupling models, such as the box-cable-box model (referred to earlier [22J) may be appropriate and can be used in a similar vvay to predict the power extracted from a field and delivered to a potentially susceptible circuit in the box. The delivered power may now be compared with susceptibility levels for the active electronic devices used in the equipment to yield the hardening req uiremen t. Various susceptibility levels can be de.fined for different types of active device such as linear bipolar ICs, linear MOSFET's, TTL logic,
DESIGNING TO AVOID EMC PROBLEMS CMOS logic etc. [21 J. These are shown as the family of dotted/dashed lines in Figure 12.13. A composite worst-case curve could be produced for all IC types used in the equipment, for both functional upset and circuit damage. This could then be used to generate the hardening requirement by comparison with the pickup power. In this example the most severe susceptibility curve for linear I Cs has been chosen and subtracted from the power pickup curve (C and E) to give the system cable hardening requirement which is the lower curve in Figure 12.13.
12 ~ 2.4.2 Equipmen t case hardening req uiremen t The process that is undertaken to derive the approximate worst-case hardening requirement for the equipment case (Figure 12.7) is similar to that for generating the cable hardening requirement. The fundamental simple model will of course be based on the screening performance of the case material [13, 22J. However, penetration through a well constructed metal case will be dominated by leakage through joints, slits and other intended apertures. Microwave EM energy will radiate through suitably sized apertures directly to the susceptible device and associated wiring/PCB tracks. The transmission of radiation through shield imperfections can be modelled as waveguide apertures or slot antennas. The preceding example illustrates the use of simple approximations to quickly assess the level of EM C design required for a particular equipment which helps to direct early design effort to where it is most needed.
12.2.5 Emission suppression requirement The foregoing has demonstrated how simple considerations can lead to a rough assessment of a system hardening requirement for system susceptibili ty. Similar considerations of voltage swings and current flows generated by active components around circuit board conductors can lead to an estimate of the likely radiated signal strength from the equipment at a particular distance (which may be that stated in the emission standards, say 1, 3, 10 or 30 m). A rough spectrum of the signals generated by digital and other circuits can quickly be estimated from their waveforms by using Fourier transforms or transform look-u p tables. It must be emphasised that these techniques are only a rough guide to setting the system
233
hardening and suppression requirements. Some designers will have access to more sophisticated computer models which will lead to a more detailed understanding. For those who do not have these software tools, the approach outlined here may be useful in demonstrating that some progress may still be made.
12.2.6 System hardening flow diagram A process for deriving a set of EMC system hardening requirements is summarised in Figure 12.14 where a simple flow chart is presented. A key decision to be made during the process is when to cease calculation and to begin preliminary assessment testing in order to further reduce the engineering risk in the EMC design. A general rule of thumb is based on the hardening requirement being greater or less than 30 dB. If the initial calculations indicate that the EMC hardening or suppression required is less than 30 dB, further calculation effort to refine the estimate may not be as useful as a quick and simple test. The simple calculations are generally only accurate to about 10-20 dB and the tests should be made to generate information about the actual pickup or emission levels within the system. (Bear in mind that these measurements will also be subject to some uncertainty.) However, the extra practical information will undoubtedly help to guide the design process. If the initial system-level calculations show a hardening requirement greater than 30 dB the engineering design on which they are based should be re-examined and various mitigation approaches involving specific design features should be tried until the hardening requirement is closer to 30 dB. If the calculations show that the hardening requirement is close to zero, then to save development time it may be worth risking the cost of a system or preconformance test against the specified requirement.
12.2.7 Subsystem apportionment and balanced hardening Once the system hardening requirement has been established for radiated emissions, susceptibility, conducted interference and transients, it is then necessary to apportion the overall requirement down to subsystem or uni t level. The details will be specific to the system in question and the task is not necessarily an easy one. Success relies heavily on the experience of previous design cases, and expert system software which incorporates this knowledge can be of help here. EMC is achieved through a number of technical measures applied at various levels from component
234
A HANDBOOK FOR EMC TESTING AND MEASUREMENT
Figure 12.14 Typical hardening task flow chart for system susceptibility
.....-----4
YES
YES
SYSTEM DESIGN INPUTS
PICK UP LEVEL known?
HANDBOOK coupling information or expert system
HANDBOOK Component susceptibility information or expert EMC system
Perform more detailed VULNERABILITY ANALYSIS
COUPLING MODELS
greater than 30 dB ( usual case)
Refine SYSTEM HARDENING REQUIREMENT
Determine. HARDENING REQUIREMENT
less than 0 dB ( rarely achievable)
( sometimes achieved)
detailed DESIGN TECHNIQUES & MODELS
Reproduced by permission
or BAe Dynamics Ltd.
selection and positioning on a circuit board up to efficient global shielding, for example afforded by a metallised plastic case which might surround a personal computer. For cost-effective EMC measures which meet the system hardening requirement without reducing the system functional performance, balanced hardening is necessary. This req uires careful selective apportionment of the EMC requirements for each electronics box, unit or subsystem, including the cabling design/installation and the mechanical design of the hard ware boxes and vehicle or system structure. The apportionment must be made on the grounds of what is technically achievable for an affordable cost based on using complementary EMC design techniques at the various levels of construction. Table 12.1 indicates some of these techniques and the levels at which they can be applied. Often an effective hardening programme requires a careful combination of screening, filtering, grounding, bonding, isolating, frequency
planning and component selection. Generally the techniques must be used together if they are to be effective at all levels where they are employed. For example, there is no point in placing a noisy circuit in a well-designed screened box if the input and output cables are not filtered to the same standard. Equally, one would not allow a high-power RF transmitter to operate at a frequency which was known to be in-band to other equipment in the system which is sensitive to that frequency. Bandpass filters would be used at the Tx and Rx subsystem ports in conjunction with frequency planning to minimise unwanted interference. EMC techniques such as these must be employed in a coordinated way across the whole system being designed. The process should be under the direction of the chief designer who controls and monitors the EMC tasks from component selection to the final system test to achieve the best EMC performance for the minimum cost, weight, space and impact on system reliability and maintainability.
DESIGNING TO AVOID EMC PROBLEMS
235
Table 12.1 EMC hardening techniques EMC technique
Applied at
EMC technique
Applied at
Component selection Circuit level I C type selection Component placement Component screening Component filterIng Component-case grounding Spectrum limiting: fast-edge slugging Use high signal levels for good noise immunity Frequency management: selecting oscillator, Tx, Rx, IF, frequencies etc. Use low-power circuits where possible - for low emissions Bandlimit RF inputs and outputs to minimise spurious, intermod. and overload signals
Screened power and signal cables Subsystem level Use lossy filter line Subsystem grounding and bonding plan Cable routing: minimise cable runs and loops Cable grounding scheme: single point, both ends, multiground Use isolated power supply Positioning of subsystem components on structure Use differen tial interfaces Use signal ground reference Transformer couple Use optoisolators on key interface lines Employ fibre optic signalling
Large RF ground plane Use RF layout techniques for digi tal boards Minimise track loop area Minimise HF signal current Employ loop-area compensation Close mounting of ICs to PCB Use of surface mount components Decoupling chip capacitors Board grounding Board edge/section filtering: with chip capacitors or lossy ferrites/ pastes
Use global shield System level Ensure all structure components are bonded Minimise structure penetrations Use waveguide beyond cutoff access ducts Employ correct RF gaskets Use system grounding plan Use single RF ground if appropriate
Board level
Screened box with minimum Box level apertures Box with RF compartments Use of RF gaskets Screened cables Cables not routed in corners of box or behind slots and apertures Board ground planes well-grounded to box Box grounded to structure Fil tered connectors Connectors with 360 0 backshells for shield termination No unfiltered cable penetrations
12.2.8 Staff support for EMC The design, manufacture, and field service personnel all have a role to play in ensuring successful EMC of a product. I t is not just the responsibility of the electrical designer and his immediate team. The
Break u p electrically resonant structures Use RAM or lossy coatings Cover windows, air vents etc. with suitable RF screening Minimise exposure to external high power RF fields during operation Site equipment away from sensitive receIvers
mechanical engineers, wiremen, installers and service teams must all contribute. This will necessitate these staff having an understanding of what EMC is and how it is achieved and maintained. Many company managements are unprepared for this aspect of meeting the EMC requirements which
236
A HANDBOOK FOR EMC TESTING AND MEASUREMENT
are increasingly being placed on their products through measures such as the EC Directive 89/336 on EMC. It is probable that proper EMC training for each type of staff involved in designing and supporting eq uipment will be necessary in the future. Prospective EMC engineers should ensure that they receive professional training before engaging in this fascina ting bu t sometimes difficult engineering field.
12.3 Specific EMC design techniques This book is concerned primarily with EMC testing; the higher level aspects of product or system design to meet EMC requirements have been discussed in some detail so that the reader can appreciate the nature of the overall EMC design task. This clearly involves more than specifying a mains filter connector, fitting a few chip capacitors on a PC board, or spraying the inside of the plastic equipment case with metal. It is not the intention to detail all the techniques mentioned for achieving good EMC or to give the basic formulas that would be needed to enable a design. Many other books and papers have been written [8, 11-15J on the whole range of design techniques and it is suggested that these and other references are consulted for information on the following topics: Shielding theory [8, 13, 14, 23, 24J Surface transfer impedance [25 J Cable screening design [8, 13J Cable crosstalk coupling, capacitive and ind uctive [8J Transmission line theory [26J Filters, types and application [8, 13, 14J RF grounding and bonding [8, 13, 14J Corrosion control of bonds [23, 24J Gasketting [13, 14,23, 24J Frequency planning [8J PCB design [8, 27-33J Many of the references cover more EMC design topics than are listed. The books and papers themselves also have extensive references and bibliographies. With these it should be possible for the reader to begin to explore the full range of EMC design techniques and their applications.
12.4 References HILLARD, D.E. et al.: 'Social and economic implications of EMC: A broadened perspective'. Proceedings of IEEE symposium on EMC, 1990, pp. 520-525 2 OTT, H.W.: 'EMC education - the missing link'. Proceedings of IEEE symposium on EMC, 1981, pp. 359-361
3 PEREZ, R.J.: 'First year graduate level course In Electromagnetic compatibility'. Proceedings of IEEE symposium on EMC, 1990, pp. 232-239 4 RILEY, N.G. et al.: 'A university post-graduate course in EMC'. Proceedings of IEEE symposium onEMC, 1990, pp. 240-242 5 HERIMAN, D.H.: 'Education and training of the industrial regulatory compliance test team'. Proceedings of IEEE symposium on EMC, 1981, pp. 365-368 6 'A structured design methodology for control of EMI characteristics'. Presented at IEEE symposium on EMC, 1990 (late submission) 7 SULTAN, M.F. et al.: 'System level approach for automotive electromagnetic compatibility'. Proceedings of IEEE symposium on EMC, 1987, pp. 510-520 8 WHITE, D.R.J. and MARDIGUIAN, M.: 'E1\I1I control methodology and procedures'. ICT ISBN 0-944916-08-2 9 NAVAIR AD1115 (obsolete but very useful). Department of Defense, Washington DC, USA 10 NWS 1000, part 1. Ministry of Defence, UK. Chap. 5, section 10 11 'Electromagnetic compatibility design handbook'. US airforce systems command, 1975, DR 1-4 12 'Electromagnetic (radiated) environment considerations for design and procurement of electrical and electronics equiprnent'. MIL-HDBK-235, DoD, Washington DC, USA, june 1972 13 KEISER, B.: 'Principles of electromagnetic compatibility'. (Artech House, 3rd edn.) 14 DUFF, W.G.: 'Fundamentals of electromagnetic compatibility'. Interference Control Technologies, Inc, Gainesville, Virginia, USA 15 'The achievement of electromagnetic campati bili ty' . Report 90-0106, ERA Technology, Leatherhead, Surrey, LT22 7SA, UK 16 JOHNSON, W.R., COOPERSTEIN, B.D. and THOMAS, A.K.: 'Development of a space vehicle electromagnetic interference Icom pati bili ty specification'. NASA contract 9-7305, final engineering report document 08900-6001-TOOO, TRW systems, Redondo Beach, CA, USA, june 1968 17 BURKE, G.J. and POGGIO, A.J.: 'NEC-2 numerical electromagnetics code Method of moments: A user-oriented computer code for the analysis of the electromagnetic response of antennas and other metal structures'. NAVELEX 3041, Washington, DC 20360, USA 18 'EMAS electromagnetic code'. MacNealSchwendler Co Ltd, 85 High 81., Walton on Thames, Surrey KT12 IDL, UK 19 PRICE, M.J.S. and MACDIARMID, I.P.: 'Developing an expert system for EMC design'. Proceedings of IEEE symposium on EMC, 1988, pp. 331-336 20 LOVERTI,j. and PODGORSKI, A.S.: Evaluation of HardSys: A simple EMI expert system'. Proceedings of IEEE symposium on EMC, 1990, pp. 228-232 21 'Integrated circuit electromagnetic susceptibility handbook'. Report MDC E]929, phase 3,
DESIGNING TO AVOID EMC PROBLEMS
22
23
24
25
26 27
McDonnel Douglas Astronautics Company, St Louis, Missouri 63166, USA WHITE, D.: 'Montreux workshop on applications of programmable calculators and minicomputers for solutions of EMI problems'. Don White Consultants Inc., 14800 Springfield Rd., Germantown, MD 20767, USA 'Design guide to the selection and application of EMI shielding materials'. Tecknit EMI Shielding Products, Cranford, NJ 07016, USA, 1982 'EMI shielding engineering handbook'. Chomerics Europe, Globe Park Industrial Estate, Marlow, Bucks SL7 lYA, UK, Jan. 1987 RICKETTS, L.W., BRIDGES, J.F. and MILLETTA, J.: 'EMP radiation and protective techniques'. (Wiley) SMITI-I, A.A.: 'Coupling of external electromagnetic field to transmission lines'. (Wiley) VIOLETTE, M.F. and VIOLETTE, J.L.N.: 'EMI con trol in the design and layou t of printed circuit
28
29
30
31
32
33
237
boards'. EMC Technol.) l\1arch-April 1986, pp. 1932 MARDIGUIAN, M. and WHITE, D.: 'Printed circuit board trace radiation and its control'. EMC Technol. Oct. 1982, pp. 75-77 MARDIGUIAN, M.: 'Prediction of EMI radiation from PCBs', R F Design, July/August 1983, pp. 2636 KOZLOWSKI, R.: 'Follow PC board design guidelines for lowest CMOS EMI radiation'. EDN) May 1984, pp. 149-154 COOPERSTEIN, B.: 'Radiation from printed wiring boards'. Xerox Corporation, 701 S Aviation Blvd. El Segundo, CA 90245, USA WAKEMAN, L.: 'Transmission line effects influence high speed CMOS'. EDN, June, 1984, pp.171-177 POLTZ, J. and WEXLER, A.: 'Transmission line analysis of PC boards, VLSI Syst. Des., March 1986, pp. 38-43
Chapter 13
Achieving product EMC: checklists for product development and testing 13.1 Introduction
In the last few years, with more market growth in prospect. The existence of this large market su pports the choice of the personal com pu ter (PC) as a good example with which to consider the task of coping with EMC product development for the first time. The general issues addressed will also apply to manufacturers of other electronic products including industrial, scien tific and medical eq uipmen t and household electrical/electronic appliances. The specifications and details of test methods however will be differen t for each class of products as has been pointed out in the preceding chapters.
13.1.1 Chapter structure This chapter is written as a set of checklists in the form of flow diagrams which engineers and managers can use to assist in generating their own EMC product development and test programiues. It highlights the importance of the issues previously discussed and relates them in terms of an overall EMC programme.
13.1.2 Example adopted To focus the discussion, assume the point of view of a manager in an electronics company manufacturing personal or small business computers who has recently heard about EMC and the possibility of the European Harmonisation Directive having some impact on the company's business. Assume that the manager has no particular background in electromagnetic engineering, and that there is no one to turn to in the company who has direct experience of EMC. The issues that would need to be considered are dealt with in the form of a top-down appraisal, starting with general questions such as 'What is EMC and how does it affect my operation?' (including assessment of relevant standards). Consideration is also given to the issues involved in setting up an in-house test facility to self certify new products. Specifically, the discussion centres on the following broad issues:
13.2 Inform.ation about EMC Managers and engineers need information on a number of aspects of EMC to plan and properly execute a product development and certification programme. A check list of possible useful sources of information follows.
13.2.1 Customer sources The primary motivation for a manager In an electronics company to understand EMC is the need to meet the expectations of his customers. In turn, their needs may be driven by regulatory authorities which implement national or international legislation on EMC. Manufacturers can only sell and customers will usually only buy products which meet the requirements of the regula tory au thori ty, and in that sense the regulations are the main driver. Customers may also have requirements for aspects of EMC which are not covered by mandatory government regulation. For example, enforceable regulations covering PCs and other IT products in some countries may only relate to their EM emission performance, but the customer/operator may also be concerned about reliable operation in a hostile RF environment and thus be interested in equipment performance with respect to radiated or conducted susceptibility Among the first questions the product development manager must ask are:
Where to obtain information about EMC Determining EMC requirements for new products Developing an approach to EMC design Setting up an in-house test facility
13.1.3 Personal computers and information technology Personal/business computer products represent a large part of the electronics ind ustry and choosing this relevant example permits the discussion of EMC issues to be focused for increased clarity. The sales of such equipments into the home and office have increased rapidly
238
Who are my main customers? Which countries are they in?
ACHIEVING PRODUCT EMC: CHECKLISTS FOR PRODUCT DEVELOPMENT AND TESTING
Do my customers demand EMC performance? Are their requirements legislation driven? Are their requirements operationally driven? Do they demand special EMC performance which it would be uneconomic to provide in tha t sector of the market? The information gained will lead to a number of specifications being cited depending on the product type and customer countries. For example, to sell worldwide, the PC manufacturer may have to meet the requirements of FCC part 15j (RF emissions) + voluntary immunity standards for USA customers VCCI (RF emissions) + any special voluntary immunity standards for customers in Japan EN 55022 (RF emissions) for customers in Europe EN 55101-2 Immunity to ESD EN 55101-3/4 Immunity to radiated and conducted EMI for customers in Europe There may also be specific single customer EMC requirements which must be taken into account if the manufacturer sells high-cost customised systems. In consultation with the legal and sales and marketing departments, the example product manager in the PC company must attempt to draw up a precedence hierarchy for EMC specifications which are to be met by the new product, depending on volume sold, technical difficulty of compliance, plans for future market, competitors' EMC policy, etc.
13.2.2 Regulatory authorities The specifications cited will be contained within EMC standards produced by various bodies such as the CENELEC, IEC, CISPR,VDE, BSI, SAE, IEEE, etc and called up by the clauses in the appropriate regulations. In the UK, the DTI Radiocommunications Division is the responsible legislative authority and produces papers for guidance on EMC [1--4]. 1'he Department have an active awareness campaign (1993 onwards) designed to provide industry with a great deal of technical information on EMC. At the time of preparing this text information can be obtained from DTI Implementation and interpretation of regula tions Manufacturing Technology Division Department of Trade and Industry, Room 1/112 151 Buckingham Palace Road London SW1 W99SS o17 1 2 15 1403.
239
Notified bodies Appointed by the DTI and operate the type examination procedure on their behalf for certain classes of equipment e.g. radio transmitters Radiocommunications Agency, Room 106 Waterloo Bridge House Waterloo Road London SE 1 3UA 0171 215 2084 concerning RF matters DRA ARE Fraser Ranger Fort Cumberland Road Eastney Portsmouth P04 9LJ Air Traffic Service Standards Aviation House Gatwick Airport Gatwick West Sussex RH6 OYR
Other sources: Contact point for type approval of radio transmitters: Radiocommunications Agency, Room 514A Waterloo Bridge House Waterloo Road London SE1 8UA For copies of the EC directive Alan Armstrong Ltd 2 Arkwright Road Reading RG2 OSQ 01734 751771 Progress on harmonised standards BSI 2 Park Street London W 1A 2BS 0171 629 9000 BSI Standards Sales Dept. BSI Linford Wood Milton Keynes MK14 6LE 01908 221 166 Technical help to exporters BSI Standards 01908 226 888 For information relating to the EC commiSSion contact the London office: Tel. 0171 222 8122 For information relating to the European Parliament contact the London office: 0171 2220411
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A HANDBOOK FOR EMC TESTING AND MEASUREMENT
Information relating to EMC quality assurance and laboratory accreditation in the UK can be obtained from NAMAS National Physical Laboratory Teddington Middlesex TW110LW 0181 943 7140 0181 943 7094 Information is also available concerning RF measurements and calibration of EMC antennas from Division of Electrical Science National Physical Laboratory 0181 943 7175 For details of services and staff at NPL consult Reference 5 EMC contacts for military equipments are usually arranged through MOD PE for the specific equipment in question. Helpful advice can usually be obtained from Dr N Carter, Mission Management 5 RAE Farnborough Hants, GU14 6TD particularly on matters relating to aircraft. A list of US EMC contacts for military systems is given in the ITEM handbooks (see 13.2.7).
13.2. 3 Industry sources Information on EMC matters can be obtained from trade associations. In the example of a PC manufacturer this would include ECMA (European Computer Manufacturers Association). ECMA Rue DuRhone 114 CH-1204 Geneva Switzerland 22-353634 Association of the Electronics, Telecommunications and Business Equipment Industries (EEA) 8 Leicester Street London WC2H 7BN 0171 437 0678 Information on EMC standards, design techniques and testing is also available from ERA Technology Ltd Cleeve Road Leatherhead Surrey KT22 7SA 01372 374151
ERA have produced 'Guidelines international recommendations relating to EMC' presented at a exhibition 'EMC 91 direct Feb. 1991
to national and and standards conference and to compliance',
Manufacturers with an interest in Civil Aircraft Avionics can contact the EMC Club, Secretary D.A. Bull, RAE Farnborough Mission Management 5 Hants, GU14 6TD Most NAMAS-approved EMC test houses will give advice on standards, design techniques and testing. A list of such facilities can be found in the NAMAS M3 document [6] and in Appendix 13.1 Competent Bodies will also offer advice on EMC design and testing to meet the EC Directive, a list is provided in Appendix 13.2 A list of over 40 organisations offering EMC test or consultancy services can be found in a special EMC edition of New Electronics [7].
13.2.4 Equipment, component and su bsystem suppliers There are a large number of EMC equipment suppliers for everything from screened rooms to spectrum analysers; they can be helpful with general information which need not be strictly related to their products. They are often among the first to appreciate the impact within the electronics industry of new EMC legislation or test methods. If one particular person does not know the answer to a question, they will often know of a contact who does. Any product manager or manager setting up an EMC test facility will find the representatives of equipment suppliers a good source of information on EMC matters. An efficient way to make contact with firms is to attend one of the national/international EMC conferences and exhibitions which are held in most years. Electronic component and subsystem suppliers may also be helpful in providing specific product information if their goods have been designed with EMC performance in mind.
13.2.5 Professional bodies and conferences 'The lEE and IEEE regularly sponsor national and international conferences on EMC. They are well worth attending and valuable conference proceedings are published, usually quite quickly after the event. 'The IEEE also produce a specialist journal of their transactions on EMC.
ACHIEVING PRODUCT EMC: CHECKLISTS FOR PRODUCT DEVELOPMENT AND TESl'ING
The Electromagnetic Compatibility Society of the IEEE is designated S-27 and ,has its headquarters at 345 East 47th Street New York, NY 10017, USA The lEE Electronics Division Technical Information Unit Savoy Place London WC2R OBL o171 240 187 1 The Society of Automotive Engineers has an EMC committee designated AE-4 with headquarters at 400 Commonwealth Drive Warrendale, PA 15096, USA The Electronic Industries Association have an EMC committee (G-46) at 2001 I Street, NW Washington, DC 20006, USA The dB Society is a special organisation for persons with considerable experience in the field of EMC. It is a worldwide organisation and the contact point in the UK is at Mission Managemen t 5 Room 302, Q153 Building RAE Farnborough Hants, GU14 6TD
13.2.6 EMC consultants and training Information available from EMC consultants can be extensive, but is rarely free. There are a growing number of consultancy organisations within the UK and they can usually be found through company and trade directories. Some individual consultants, including those within companies, may be contacted through The Association of Consulting Scientists 11 Rosemont Road London NW3 6NG 0171 794 2433 A number of companies offering EMC consultancy are listed in Reference 7, with about an additional 30 offering consultancy along with other EMC services. Using a consultant is a quick way to gain initial EMC information about legislation, standards, design techniques and testing. It may be expensive and a manufacturer who takes EMC seriously for the long term competitiveness of his business will probably wish to develop some inhouse capability. Consultants can still be brought in as needed, to assist the in-house team in tackling
241
a particularly difficult EMC problem, or venturing into a new market with its own EMC requirements. A list of organisations offering consultancy and training in EMC is given in Appendix 3.3
13.2.7 Electronics and EMC technical press There are a number of specialist EMC publications and a few periodicals which have a special interest in EMC. Among these are ITEM, Interference Technology Engineers' Master, directory and design guide for the con trol of EMI. Published by Robar Industries, Inc. R & B Enterprises Division 20 Clipper Road West Cosnshohocken PA 19428-2721 USA EMC Technology, Published by Don White, Interference Control Technologies 5615 West Cermack Road Cicero, IL 60650-2290, USA New Electronics Franks Hall Horton Kirby Dartford Kent DA4 9LL DTI EMC awareness campaign EMC helpline (1993) 0161 954 0954 Managed by Findlay Publications EMC Awareness Campaign Administrator Findlay Publications Franks Hall Horton Kirby Kent DA4 9LL A list of useful publications (available 1993) related to EMC is given in Appendix 3.4.
13.3 Detertnining an EMC requiretnent Consider again the example of a PC manufacturer who has just realised that EMC exists and that it will probably apply to his future products but has no experience and no contacts in the field of EMC. The broad questions that should be considered were listed briefly in paragraph 13.2.1; these are now examined in the framework of a logical flowchart which will act as a checklist to enable the 'PC product manager' to get started in defining the system level EMC requirement for a new product.
242
A HANDBOOK FOR EMC TESTING·AND MEASUREMENT Any specific EMC requirements based on operational environment
_____
Customer's stated EMC requirements
.......- - - - 1
EMC standards/legislation relating to product type or country. (Mainly for emissions)
t
t Immunity to RF ESD, lightning & faults on mains power lines/ transients - leads to low cost of ownership of the equipment
NationaVInternational standards called up within legislation
t Professional bodies concerned with EMC Standards. (ENELEC, IEC VDE,SAE,IEE,CISPR etc.j
Product manufacturerCompilation of all stipulated EMC requirements EMC requirements for reliable product. for the new product (eg. immunity requirements additional to those specified ~L----r----j.4--nE~lec;;rrtricairoilssdaff8ei\;ty;----' by customer/legislation.) requirements Consider competitors' policy on EMC ~-----'
Consider cost of implementing EMC
r-
Generation of ....._---1 Consider manufacturer's technical EMC capability EMC requirement hierarchy for product being developed Export marques EMC requirements considered
. Consider marketing advantages of lower through-life cost of ownership
Marketing input: Sales volume/Customer/Countryl EMCspec.
Consider company quality image is enhanced by good EMC System level EMC specification for new product
Figure 13.1
Generation of EMC requirement for product development (Example: manufacturer of desktop computer products)
Figure 13.1 shows a typical flow chart for generating product EMC requirements. There is insufficient space available here to discuss individually each item in the chart, but the short titles act as a reminder to consider that topic as part of the checklist. Some insight into the central process of assessing individual customer requirements and developing a system level hierarchy of EMC test standards which must be met, is given by Barrett and Scherdin [8J, in the case of ind ustrial controIs products produced by Texas Instruments. They found that products had to meet up to 30 test standards to satisfy all their customers' needs. After considering the issues shown in Figure 13.1 they were able to narrow this down to a minimum of 15 test standards. In the case of a PC manufacturer who intends to sell world wide the product may have to meet 5-10 standards to cover the full range of EMI emission and immunity, ESD and power line transient requirements.
13.4 Developing an approach to EMC design 13.4. 1 Process flow chart Once the system-level EMC requirement for a new product has been determined the product manager and her technical team must decide how to approach the EMC design process as part of the overall product development programme. There are many ways to do this depending on the product, the company, and the individual talents and expertise available. One example, which again acts as a checklist, is given in Figure 13.2. Two features of this flow chart meri t further discussion.
13.4.2 EMC strategy There are two fundamental approaches to ~eeting the emission regulations for products. The first has traditionally been favoured by the military for
ACHIEVING PRODUCT EMC: CHECKLISTS FOR PRODUCT DEVELOPMENT AND TESTING
System level EMC requirement
Develop an approach to system & subsystem EMC design
Technical resources: . tackle project in-house? Use consultants? Do both?
t IN-HOUSE Consider: Staff expertise Training in EMC Capital equipment Location of EMC within company structure
I
I
+ Mainstream product design constraints
~
243
Figure 13.2 .Developing an approach to EMC design
EMC STRATEGY: Containment? Source suppression? Tackle immunity before emissions? Do the minimum?
CONSULTANTS Balance of shortterm advantage to long-term reliance. Cost Accountability Selection of:
Technical design of 'EMC' sub syfems (Technical plan)
their procurements and is based on the concept of containment. 1--'hus the EMC subsystem designs will be based around the techniques of screening, filtering and shielding of cables with less attention being paid to suppression of the interference signal sources. Such an approach is viable for military products which usually have stout metal cases and require strong semiarmoured cables, and where cost may not be the overriding consideration in product design. EMC designers of commercial products, which are usually lower cost and may have features such as plastic cases and unshielded cables, have tended to suppress EMI at source wherever possible by careful signal shaping/band limiting, the use of screened/filtered subsystems and low radiation PCB designs. Staggs [9J suggests that around 1984 the optimum balance for PC-type products was to use a 50/50 strategy for containment and source suppression. However with the advent of very fast 32 bit chip sets running at 25 MHz or more (resulting in high peak switching currents) source su ppression becomes more difficult and the balance shifts more to a 60/40 containment/suppression ratio.
13.4.3 Immunity first? For IT products, current enforceable EMC regulations are limited to radiated and conducted emissions and therefore meeting these regulations will have a very high priority in the system EMC requirement for a new IT product. It should be remembered that the FCC in the USA has the power to introduce immunity standards if the voluntary system to ensure product EMI immunity fails to work. In Europe there are a
Project EMC control plan! management plan
number of published draft standards which will cover product immunity to EMI in future years. The performance of the product in use can be seriously reduced by EMI if it is susceptible, and this will result in a poor reputation for reliability for the product and the company. The field service and warranty costs can be high if equipments are continually in need of repair owing to disruption or damage from EMI and ESD/power transients. It has been estimated [9J that for a machine population of 20,000 units over five years the total gross savings in field repair and warranty could be as high as $2.77 M, with the cost of implementing EMC at $42 per unit, this results in a net saving of almost $2 M. For all these reasons it n1akes good sense for the product development team to insist that new products are designed with EMI immunity as a high priority. For the example of a fast 32 bit PC where the emphasis is on containment rather than source suppression, this strategy becomes doubly attractive as much of the shielding, filtering and screening needed for immunity reduction will be required anyway for emission suppression. Staggs [9J comments that, 'by solving the immunity problem first, and consequently lowering the emission levels, the incremental cost of specific emission solutions is relatively small'. For PC-type equipments he estimates that 80% of emission problems will be solved by meeting immunity standards first.
13.4.4 Example of EMC design process The considerations taken into account In developing the overall approach to achieving EMC for a new product (Figure 13.2) are
244
A HANDBOOK FOR EMC TESTING AND MEASUREMENT
Approach to and constraints on mainstream electronic design
Consideration of bought-in items. (Some may have EMC features built-in)
Sub system level EMC specifications eg. for case, keyboard, interfaces, chipsets, PSU etc.
.....- - - - t
I
_ _ _ ...I
System level EMC strategy: containment source suppression immunity/emissions technical resources
EMC project control plan - including the generation of a technical construction file
Detailed EMC design activity on each sub system (including EMC development testing and practical computer modelling)
EMC test plans for - development - conformance - technical file FOR MINOR PROBLEMS MODIFY ONE OR MORE SUBSYSTEMS
Subsystem integration and pre-conformance testing
Figure 13.3
,If fail, re-examine 1--_ _
--11.... design of sub systems
Example of EMG1 design process
contained within the larger design process. Figure 13.3 illustrates the issues that need to be addressed to progress from the system-level EMC requirement to a preproduction unit which is ready for EMC certification. Managers in small companies may shy away from including mathematical modelling as part of the design process. If staff have the skill to use computer models and are able to interpret the results so that competing design solutions can be evaluated, and only the best option tested and compared with the predictions, the use of modelling can often be beneficial. If the decision is made to use mathematical models the question arises as to which models to use. They range from those req uiring large mainframe machines down to those based on simple assumptions (such as the models described in Chapter 12) and which req uire only a calculator or PC. The question of the appropriate level of models has been addressed by Atkinson [10]. He considers that practical and useful EMI models lie somewhere between the big-machine models and the simple models used for scoping EMC problems. He proposes the use of models based on spreadsheet techniques that can run on a PC and
enable the engineers to describe and visualise the EMI characteristics of the system being designed. The successes (and the failures) of such models to predict solutions to EMI design problems can be used to support building a compu ter based expert system which then becomes a valuable part of the design process itself and which is applicable to the design of the next generation of systems.
13.5 Technical construction file 13.5.1 Routes to compliance
options
During the development of an approach to EMC design (Figure 13.2), and the subsequent desig!l process (Figure 13.3) a decision will have been made in most cases to opt for confirmation of a successful EMC design by one of two methods: by conformance testing, or by the submission to a recognised assess men t body of a technical construction file. A decision to test or submit a technical file has implications for the EMC programme cost and time to market. I t depends on a number of factors related to the availability of appropriate EMC standards, product type, size, numbers made and installation features. If a
ACHIEVING PRODUCT EMC: CHECKLISTS FOR PRODUCT DEVELOPMENT AND TESTING technical file is to be generated it will usually contain some test data, but not at conformance test level. The requirement for the generation of a technical file which would be acceptable to meet the needs of the EC EMC directive 89j336 is now examined.
•
13.5.2 Circumstances requiring the generation of a technical file An early document containing guidance to manufacturers for the preparation of a technical construction file which would meet the requirements of the Ee EMC directive has been prepared by working party WP-3 of NAMAS WG4 [4]. It lays out the circumstances where it may be appropriate to use a technical construction file in aid of the EMC certification of products. These are: (i)
(ii)
(iii)
(iv)
(v)
For apparatus for which harmonised EC EM C standards do not exist or are not appropriate. For apparatus for which standards do exist but which are not applied in full. I t may be possible for the manufacturer to claim that the fund amen tal req uiremen ts of the standard are met without performing any or all of the required tests. For installations where testing to harmonised standards is not practicable due to the physical properties of the installation. For installations where testing of each installation is not practicable due to the existence of large numbers of similar installations. Combinations of the preceding circumstances.
13.5.3 Contents of a technical file Each of the four basic circumstances listed results in a slightly different requirement for the contents of a suitable technical construction file. Take the example of the second condition where the standards are not applied in full. For this situation the technical file shall contain the following as a minimum. •
•
A general overview. Statements should be made that seek to demonstrate why tests for certain phenomena were not felt to be necessary. The key activity must be to demonstrate what special. properties the construction of the apparatus has which render unnecessary the required EM C tests in the appropriate standard. Identification of apparatus. Typical contents should include Brand name Model number
•
245
Name and address of manufacturerj importer Purpose of equipment Performance specification (EMC-relevant) External photographs of the equipment Technical description of the equipmen t. Typical contents should include, but not necessarily be restricted to, Full technical description including block diagram showing the interrelation of the functional areas Set of technical drawings Components list, with special regard to microprocessors, RAM, ROM, logic families, oscillators, power supplies, motors and relays Signal data including frequencies, risetimes, switching curren ts, grounding schemes Description of physical characteristics of the equipment, SIze, weight, power consumption Listing of available purchase options of the equipment List of other equipment likely to be connected to the device being assessed Description of any design measures taken specifically to control EMI and enhance EMC performance of the product A copy of any assembly or installation manuals supplied to the customer which may affect its EMC performance. Technical rationale. This must explain in detail why the harmonised ECEMC standards have not been applied in full. It should be supported where possible by information derived from theoretical and practical studies. Details of all tests performed should be included. Explanation of the quality control procedures that apply to the product must be given to show that future samples of the equipment will also comply with the directive. The detailed contents of the technical rationale should include, bu t not be limited to, the following: The logical process used to determine that certain tests need not be performed. For each type of test described in the standards and which has not been performed, a description and explanation of the results of any relevant development tests. Support for the decision not to perform certain tests with any theoretical studies which show that the apparatus must inherently comply. The description of any EMC design measures relevant to the phenomenon being assessed in tests
246
A HANDBOOK FOR EMC TESTING AND MEASUREMENrr which have not been performed. A detailed analysis of the method of operation, relevance and expected effectiveness of any EMC protection measures incorporated into the equipment. A list of all formal tests and reports which have been carried ou t. An account of how the EMC performance of production samples will be verified, and how the sampling levels will be chosen. An account of variant and build standard control in production and an explanation of the procedures used to assess whether a design change requires the apparatus to be retested/requalified.
13.5.4 Report from a competent body 1'he technical construction file must be submitted to an organisation which is deemed by the regulatory authorities to be competent to judge the performance of the apparatus in relation to the harmonised EMC standards. Competent bodies are likely to include NAMAS approved EMC test houses and some EMC consultancies. The report from the competent body produced as a result of assessing the technical file should include Reference to the exact build standard of the apparatus assessed and a cross reference to the technical file. Statement on the work done to verify the contents of the technical construction file. Comments on the procedures used by the manufacturer to ensure compliance with the EC directive for each phenomenon described in the relevant standard for which formal tests have not been performed. Comments on any tests which were carried out, and an analysis of test methods employed. Comments on q uali ty assurance procedures which the manufacturer intends to apply to the product. T'he report from the con1petent body can be used by the manufacturer of the equipment to support his statement of compliance with the EC directive on EMC.
13.5.5 1'esting or technical file? I t would be incorrect to suppose that the generation of a technical file and its submission to a competent body for scrutiny is necessarily an easy way of obtaining EMC clearance to Inarket a product under EC 89/336. The testing option may prove less costly in some cases and if suitable test facilities have been booked in good time there
should be little extra delay in bringing the prod uct to market. Experience will be gained during the 1990s as to when the technical file rou te as opposed to conformance testing is most appropriate for product approval. For large, oneoff type eq ui pmen ts it maybe impossible to carry out approved tests and the technical file is then the only route to EMC clearance. In the years to come all Inanufacturers and EMC practitioners will watch with interest the developments surrounding this question.
13.6 Self certification 13.6. 1 Need for an in-house facility The option to self certify products under the EC regulation can be very attractive to some manufacturers. If they originate a continuing series of new products or new variants of existing products, then it may be cost effective to develop an inhouse capability to carry out the necessary EMC conformance tests. The particular list of EM C tests required by standards will be product specific, and a manufacturer's in-house facility need not cover such a wide range of tests as would an all-embracing third party EMC test house. The exact list of EMC tests with which the in-house facility must cope will have been determined during the development of the product design. rrhe basic types to be considered are emission tests -- conducted and radiated immunity tests conducted and radiated screened enclosure testing open-area test site ESD and transient testing. The technical details of all these types of tests have been covered in preceding chapters. An additional consideration for defining the requirement of an in-house test facility is the size of the equipment to be tested. Generally, this is the key cost driver, the greater the EUT size the greater the cost of the test facility. The break point comes for EUTs about 1 m cube. If they are larger than this the facility will be very expensive if radiated immunity/susceptibility testing is to be carried out using semi-anechoic screened chambers. Anyone considering the technical definition and financial investment required to develop an EMC facility should consult as widely as possible existing facility operators dealing with their product types to avoid making potentially costly mistakes. Time permitting, the safest route to constructing the most suitable and cost-effective in-house facility is to build it up in a series of levels of capability.
ACHIEVING PRODUCT EMC: CHECKLISTS FOR PRODUCT DEVELOPMENT AND TESTING
13.6.2 Gradual development
The risks associated with these issues can be reduced by a gradual approach to developing an in-house EMC test capability which has three basic levels:
There are four components which need to be considered when developing an in-house EMC facility:
(i)
(ii) (iii)
(iv)
Defining and obtaining the correct accommodation, facilities and test equipment to perform the range of tests specified in the chosen EMC standards. Having sufficient well motivated and qualified test personnel available. Putting in place sound workable operating and q uali ty control proced ures to guide the technical staff using the equipment to carry out the tests. Ensuring the right amount of work for the facili ty - too li ttle, and morale will fall, along with any continued investment in staff training and new equipment; too much, and the facility will be overloaded, resulting in potential errors in test work and pressure to compromise quality standards. In such circumstances the staff would have no time for training and attending conferences, etc. which would result in little or no development of their skills. The latest information relating to test methods and equipment would not find its way into the facili ty and benefit the conformance testing which helps get the products to market.
Level 1:
Bench testing to serve product development Developing dedicated facilities i.e. an open-area test site or a suitable screened enclosure plus the associated test equipment Upgrading these facilities to conformance test standards Obtaining accreditation for the facility.
Level2a:
Level2b: Level 3:
This process is shown In Figure 13.4. Self certification of products is clearly possible at level 3 but may also be substantially risk-free at level 2b if care is taken to operate in the professional manner req uired by N AMAS (at level 3). rrhe information contained in this and other books on EMC can be helpful in determining the tests/standards which must be covered (for various product types) and the technical knowledge required to specify and use EMC test equipment in approved ways to measure the required parameters of current, field strength and immunity level, etc.
Bench-test capability for development testing includes: conducted emissions ESO & transients near-field radiated emission probes
Up-grade 'OATS to conformance test standard. (Gain experience)
Obtain accreditation: can now self-eertify products with confidence. (Sell spare capacity)
~
H
Screened room test facility for radiated emission and susceptibility development testing
Introduce strict lab. procedures, calibration and quality control to 'NAMAS' standard
~
------------------ LEVEL 20
Up-grade to conformance test standard (Gain experience)
-------- LEVEL 2b
Obtain accreditation Seek NAMAS II--_~ (up-grade capability accreditation I with higher susceptibility field strengths and --------- LEVEL 3 accommodate larger test objects) (Sell spare capacity)
+ Figure 13.4
-------------------------LEVEL 1
THIS ROUTE FOR MILITARY AND SOME COMMERCIAL PRODUCT TESTING
THIS ROUTE FOR EMISSION TESTING OF COMMERCIAL PRODUCTS
Open-area test site for radiated emission development testing
247
Steps to consider when setting up in-house.EMC test facility
+
248
A HANDBOOK FOR EMC TESTING AND MEASUREMENT
13.6.3 Es tilllates of facility cost
When all the consultants have gone, the purchaser is left with an impressive facility but perhaps without the necessary trained staff, facility operating procedures, or backup support, unless they have been included as specific req uirements in the turnkey package, at an extra cost.
An estimate of the typical equipment cost to set up an EMC test facility with various capability levels in line with those previously described 1 to 3), is given in Figure 13.5. l'hese estimates do not include the additional costs of land or buildings which facilities would occupy. No cost has been included for the recruitment and training of EM C test personnel. l'he of facilities is often a critical within companies wishing to develop an EMC test capability. There is usually a shortage of qualified and experienced staff in the UK and often only premiurn conditions will attract the best people. Developing the necessary expertise completely on the basis of existing company staff is also expensive due to the cost of training, which takes from 1 to 3 years.
l'he conclusion is clear: anticipate the EMC product development needs of the company, plan for a phased build-up of facili ties and expertise to meet the need, recruit and/or train the appropriate in-house staff to run the facility.
13.7 Conclusion Electromagnetic compatibility is a fascinating and rapidly developing field of electrical engineering. It poses important, interesting and wide ranging multidisciplinary challenges to equipn1ent designers. I t affords many technical challenges which must be met by engineers, to Inake consisten t and meaningful measurements of complex EMI quantities. The managernent and quality aspects of EMC work are equally in1 portan t if the technical effort expended is to be realised in terms of added value to the products being developed. In its widest role, EMC is part of the growing realisation that the benefits of industrialisation can only be enjoyed to the full if one takes care to minimise the unwanted impact of the technology employed. For example, without EMC controls, what would be the net benefit of the advent of powerful desktop business computers if the electrical emissions _from them disturb and compromise each other or the communications systems which carry the data generated to other uses?
13.6.4 Turnkey facilities If time is pressing and a complete facility must be put in-house in a short time, it is possible to contract a consultant or possibly an EMC equipment manufacturer to deliver an adequate turnkey £0.5 M-£l M) within about 6-9 months. Developing the requirement specification for such a programme would be the main activity which the purchasing company would have to undertake. I t is to use a second set of consultants to do this and to monitor the performance of the ITlain contractor supplying the turnkey facility. rrhere are two main weaknesses inherent in this It is cover
to be costly: the additional cost to factors could be substantial (10 (Yo ---20 % of the turnkey price). risk
£10k
£100k
£500k I
£1 m
!SIMPLE SPECTRLM ANALYSER & !URRENT PROBE(& NEAR-FIELD 1r -
BENCH
~OP C.E. & E.S.D. ~ SOME TRANSIENT TESTING I
I
I
£10m
£100m
P~OBES FOR BENC~ TOP TESTING
I
I
I
*SMALL SCREENED ROOM C.E. & E.S.D. -& TRANSIENTS & R>E> & R.S. (1 Vim to 1GHz) I I I EMI METER) *O.A.T.S., R.E. UP ITO 1GHz (SPE. ANALYSER OR CHEAP I I I lICOMPREHENSIVE CHAMBER-BASED FACILITY CAPABLE OF R.S. UP TO 10V/m & R.E. UP TO 1S-GHz & C.E. & ESD & TRANSIENTS I
I
I
*MULTI-CAPABILITY COMMERCIAL TYPE EMC TEST HOUSE FACILITY I
I
I
*HIGH CAPABILITY MULTI-PURPOSE FACILITY FOR COMMERCIAL & MILITARY TESTING OF SYSTEMS UP TO 4 X 3 X 2m
I
I
* A NATJONAL LEVEL TEST FACILITY FOR A MAJOR MILITARY OR CIVIL PRODUCT
...::1---.....
BENCH TOP LEVEL 1
13.5
...
DEDICATED FACILITIES CONFORMANCE TEST FACILITIES til LEVEL 2 ... LEVEL 3 .. ....
cost against capabilify (1991
£
Sterling)
I
I
MAJOR NATIONAL FACILITIES---.........
ACHIEVING PRODUCT EMC: CHECKLISTS FOR PRODUCT DEVELOPMENT AND TESTING
EMC will be a major area of expanding interest and influence in the decade of the 1990s as legislation drives the commercial world to strict compliance with national and interna tional standards. Electromagnetic compatibility is then a branch of electrical engineering which has a clear commercial and social impact, and its practitioners will con tri bu te in a small way to the future quality of life in an environment which increasingly interacts with technology.
13.8 References 'The Single Market Electromagnetic Cornpatibility'. HMSO, 2/90. Dd 8240964 INDY Jl077NE 40M, Feb. 1990 2 'Electrical interference: a consultative document, The implementation in the UK of Directive 89/336/ EEC on electromagnetic compatibility'. Available from L.B. Green, Radiocommunications Division, DTI, Rm 106, Waterloo Bridge House, Waterloo Road, London SE1 8UA 3 'The Single Market: EMC Update'. DTI Radiocommunications Division, Waterloo Bridge
4
5 6 7 8
9
10
11
249
House, Waterloo Road, London SEl 8UA, June 1990 and March 1991 'Guidance document for the preparation of a technical construction file as required by the EC directive 89/336, A draft for discussion'. From A. Bond, MT Division 4e, DTI, 151 Buckingham Palace Road, London SW1 'Points of contact'. NPL0004/8K/NJ/6/90, NPL, Teddington, Middlesex, TW11 OLW, 1990/91 'Concise directory - M3'. NAMAS Executive, NPL, Teddington, Middlesex, TWll OLW, Aug. 1990 Special edition on EMC, New Electronics, October 1990 BARRETT, J.P. and SCHERDIN, S.: 'The development of EMC laboratory standards and procedures'. Proceedings of IEEE symposium on EAfC) 1990, pp. 329-332 STAGGS, D.M.: 'Corporate EMC programmes'. Proceedings of IEEE symposium on EMC) 1989, pp. 320-325 ATKINSON, K.: 'Graphical EMI modelling spreadsheet'. Proceedings of IEEE symposium on EAfC) 1990, pp. 175-179 'Help is at hand - useful contacts for advice and technical information'. DTI EMC awareness campaign, . Findlay Publications, Franks Hall, Horton Kirby, Kent DA4 9LL, May 1993
Appendix 1.1
Signal bandwidth definitions NARROWBAND SIGNAL (CW)
1 Narrowband/broadband signals
3 dB BANDWIDTH
Signals measured during radiated and conducted emission tests are often extremely complex, consisting of a mixture of individual signals from many separate sources within the equipment. Some of these may emanate from stable oscillators for example, producing a sinusoidal waveform at a single frequency. The frequency extent or bandwidth of this signal is small, perhaps less than 1 Hz, and it is clearly narrowband. Other circuits which generate fast switching waveforms such as switched mode power supplies or digital processing circuits produce signals which contain many frequency components which may be spread across tens or hundreds of megahertz. These. signals have a wide bandwidth with the highest frequency being determined by the switching-edge risetime/falltime, and the spacing of frequency components in the spectrum by the switching repetition frequency. Such a signal is referred to as broad band.
« o
en
W
r--
~ I
W,
...Ji
Narrowband signal frequency f 1
I
OdB--
Bandwidth 11kHz
Cl
::::>
Bandwidth 210kHz
:.J a.
/Bandwidth 3 30 kHz
..... ~
«
FREQUENCY BROADBAND IMPULSE SIGNAL 6 dB IMPULSE BANDWIDTH
I W
..J
«o en
I
r-
I
Wide bandwidth 10MHz Wide bandwidth 1MHz
w
Wide bandwidth 100 kHz
Cl
::::>
Wideband impulse signal spectrum with many frequency components
I-
::J 0-
~
«
2 Measuretnent of narrowband and broadband signals
I
FREQUENCY
1 t
Measuring the amplitude of a narrowband signal with a tuned radio receiver or EMI meter is simple, as once the receiver is tuned to the signal frequency at say 11 MHz, the measured signal strength is independent of the bandwidth used to measure it. This is because all the power in the signal is always contained within the 3 dB passband of even the narrowest IF (intermediate frequency) and post-detector filters as shown in Figure Al.la. This is not the case with broad band noise or impulsive signals where the power in the signal is distributed over a range of frequencies which is much greater than the receiver bandwidth which is used to measure it. Figure Al.lb shows a typical impulse spectrum from a digital switching waveform which has frequencies out to perhaps 100 MHz. It is represented as a series of frequency components spaced at 1/ T (where T is the signal PRF) and a first null at lit (where t is the risetime /fall time). The signal power intercepted under the IF filter bandwidth increases as the bandwidth increases. Thus the measured power and indicated signal voltage referred to the
Figure A 1.1
Narrowband/ broadband signals
receiver input, increases with the bandwidth used. The shape factor of the IF filters becomes important as it affects the intercepted signal power and therefore the indicated signal voltage. 'To derive a signal level that is universal, irrespective of the bandwidth used, the measurement must be expressed in power or voltage per unit bandwidth. This is called the signal spectral density and for EMC measurements it is usually expressed in /lV/kHz or /lV/MHz. When the calibration of the sensor connected to the receiver is taken into account the measurements of physical quantities are expressed as dBIlV /m/kHz or dB/lV /m/MHz for radiated emission field strengths and dB/lA/kHz or dBIlA/MHz for conducted emissions. For pure broadband thermal n~ise where the signal power at each frequency is phase incoheren t, the measured signal vol tage increases as the square root of bandwidth used. For coherent impulsive noise (where there is a defined 250
SIGNAL BANDWIDTH DEFINITIONS
phase relationship between adjacent frequency components) the measured signal voltage increases proportional to bandwid th [1] . Some broadband signals will not fall neatly into either category and for these the change in measured signal voltage wi th a change in receiver bandwidth will lie between that for incoherent and coherent signals. The precise defini tion and consisten t measurement of broadband signals is a major issue in EMC testing. Some EMC standards overcome the difficulty by specifying the precise receiver IF bandwidths which must be used for measurement at different frequencies. This means that all measurements will be made in the same way without the test engineer making a judgment
251
as to whether the signal is narrowband or broadband. The discussion in the EMC community about narrowband/broadband measurement seems to be moving to a conclusion where most commercial and military standards will adopt fixed bandwidths. Those EMC test engineers who are req uired to measure broadband signals normalised to 1 kHz or 1 MHz bandwidth should exerCIse care with this aspect of measurement technique. DUFF, vV.G.: 'Fundamentals of electromagnetic compatibility'. Interference Control Technologies Inc., Gainesville, Virginia, USA, sections 2.6, 2.7, pp. 2.38-2.44
Appendix 1.2
UK EMC legislation (up to 1 January 1996)
Statutory instrument
Title
1952 2023
The Wireless Telegraphy (Con trol of interference from ignition apparatus) Regulations 1952 The Wireless Telegraphy (Control of interference from electro medical apparatus) Regulations 1963 The Wireless Telegraphy (Con trol of interference from radio frequency hea ting a pparatus ) Regulations 1971 The Wireless Telegraphy (Control of interference from household appliances, portable tools, etc.) Regulations 1978 The Wireless Telegraphy (Control of interference from f1 uorescen t lighting apparatus) Regulations 1978 The Wireless Telegraphy (Control of interference from CB radio apparatus) Regulations 1982
1963 1895
1971 1675
1978 1267 as amended
1978 1268
1982 635 as alnended
objective, but not designed by the manufacturer (s) for supply as a single technical unit. Spare parts, subject to regulation 14(2) of the EC EMC Regulations whereby nothing shall be taken to affect the application of the regulations to relevant apparatus into which a spare part has been incorporated. 'Spare part' means a component or combination of components intended for use in replacing parts of electrical or electronic apparatus. Supply of apparatus to authorised representative responsible for complying with the regulations. Second-hand apparatus is excluded, with the exception of such apparatus which has, since it was last used, been subjected to further manufacture; and second-hand apparatus which is either supplied or taken into service in the community for the first time having previously been supplied or used in a country or territory outside the community. Second-hand apparatus means that which has previously been used by an end user. Electromagnetically benign apparatus is excluded where the inherent qualities of the apparatus are such that neither is it liable to cause, nor is its performance liable to be degraded by, electromagnetic disturbances.
2 Specific exclusions The EC EMC regulations do not apply to: Apparatus for use in a sealed electromagnetic environment so long as it is accompanied by instructions stating that the apparatus is suitable for use only in a sealed electromagnetic environment. Radio amateur apparatus which is not available commercially is excluded. Military equipment defined as apparatus which is designed for use as arms, munitions and war material within the meaning of Article 223.1 (b) of the Treaty establishing the EEC (notwithstanding that it may be capable of other applications), but does not include apparatus which was designed for both military and non-military uses.
Exclusions from. EC harm.onised EMC regulations rrhis information is taken from the DTI publication 'Product standards electromagnetic compatibility (UK Regulations April 1993)'.
1 General exclusions The EC EMC regulations do not apply to: Apparatus for export to a country outside the EEC where the supplier believes with reasonable cause that it will not be used in the UK or another member state. Some installations are excluded where two or more combined items of equipment are put together at a given place to fulfil a specific
3 Apparatus wholly covered by other directives The EC EMC regulations do not apply to: Active implantable medical devices within the
252
UK EMC LEGISLATION
meaning of Article 1.2 (c) of the Council Direc tive 90/385/EEC. Medical devices covered by. the EC directive in preparation (1993) and comprising any instrument, apparatus, appliance, material or other article, including software for the purpose of (a) (b) (c)
diagnosis, prevention, 'monitoring, treatment or alleviation of disease, injury or handicap investigation, replacement or modification of the anatomy or of a physiological process control of conception.
4 Apparatus partly covered by other directives The EMC regulations do not apply to Electrical energy meters as regards the immunity
253
thereof (regulated by Council Directive 76/89/EEC). Spark ignition engines of vehicles in so far as the electromagnetic disturbance generated thereby is liable to cause radio interference (such interference is regulated by Council Directive 72/245/ EEC). Spark ignition engines of tractors in so far as the electromagnetic disturbance generated thereby is liable to cause radio interference (such interference is regulated by Council Directive 75/322/ EEC and as amended by Article 1 of 82/890/ EEC). Nonautomatic weighing machines as regards the immunity thereof (regulated by Council Directive 90/384/EEC). rrelecommunications terminal equipment (TTE) to the extent that EMC requirements are determined by Council Directive 91/263/EEC.
Appendix 1.3
European EMC standards Listed in Table Al.3.1 are commonly used European EMC standards, their applicability and equivalent national standards. In some cases a near equivalent USA standard is also identified. Table Al.3.1 EMC standards (emissions)
Equipment
Euro standard
Generic emission ISM
EN50081 EN55011
Radio and TV receivers
EN55013
Household appliances
EN55014
Lamps and lighting IT equipment
EN55015 EN55022
Electrical supply networks Vehicle igni tion systems Motor cycles and vehicles with 3 or more wheels Vehicle brakes Metrology (weights and measures)
EN60555 75j245jEC UN ECE Reg.10
Equivalents BS4809, VDE0871, CISPR11 (FCC pt18) BS905 pt1, VDE0872, CISPR13 (FCC ptl5) BS800, VDE0875, CISPR14 (FCC pt15) 'BS5394, VDE0875, CISPR15 BS6527, VDE0871, CISPR22 (FCC ptl5) BS5406, IEC555 BS833, CISPR12
71j320jEEC, UN ECE Reg.13
NW0320
EMC standards (immunity)
Generic immunity standard Broadcast receivers Industrial process control
EN50082 EN55020 HD481
IT equipment
EN55101
BS905 pt2, CISPR20 BS666 7, IEC80 1-1, IEC80 1-2 (BS6667 (1985)), IEC80 1-3 IEC801-5
254
EUROPEAN EMC STANDARDS
255
Table A1.3.2 Existing EC harmonised standards: emission standards (up to 1993) Compiled from References 16 and 17 of Chapter 2 CENELEC reference
Draft for public comment
British Standard
Mains signalling on low voltage electrical ins tallations.
EN50065-1 * EN50081--1 *
90/26273 DC
EN55011*
BSEN550 11: 1991
EN55013*
BS905: 1991: ptl
EN55014*
Equipment covered
90/20911DC [proposed rev]
BS800: 1988
EN55015*
BS5394: 1988
EN55022*
BS6527: 1988
Any equipment in the domestic, commercial and light industrial electromagnetic environment (class 1 environme'nts) - generic emission standard. Limits and methods of measurement of radio interference characteristics of ISM RF equipment (excluding surgical diathermy apparatus). Based on CISPRII Limits and methods of measurement of radio interference characteristics of sound and television receivers. Based on CISPR 13 Limits and methods of measurement of radio interference characteristics of household electrical appliances, portable tools and similar electrical apparatus. Based 'on CISPR 14 Limits and methods of measurement of radio interference characteristics of fluorescent lamps and luminaires. Based on CISPR15 Limits and methods of measurement of radio interference characteristics of information technology equipment. Based on CISPR22
EN60555-2*
88/27854DC
BS5406: 1988: pt2
EN60555-3*
90/28296DC
BS5406: 1988
Disturbances in supply systems caused by household and similar equipment (harmonics). Disturbances in supply systems caused by household and similar equipment (voltage fluctuations) .
BS4727: ptl: Gp9
EMC definitions
Standards not yet adopted IEC50 chap161
* Denotes standards which have been referenced by the Official Journal of the European Communities and are therefore notified for use in self certification.
256
A HANDBOOK FOR EMC TESTING AND MEASUREMENT
Table A1.3.3 Existing *t and possible future EC harmonised standards: immunity standards Compiled from References 16 and 17 of Chapter 2 CENELEC reference
Draft for public comment
EN50082-1 *
90/26272DC
Any in the domestic, commercial and light industrial electromagnetic environments (Class 1 environment). Generic immunity standards.
prEN50082-2
91j21828DC
Any in the industrial electromagnetic environment.
EN55020*
91 j25187DC [proposed rev] 89j34172DC
prEN55101-2
British Standard
BS905: 1991: pt2
Equipment covered
Sound and TV broadcast receivers and associated equipment. Information technology equipment. Electrostatic discharge.
prEN55101-3
89j34171DC
ITE
Radiated RF disturbances.
prEN55101-4
90/30270DC
ITE
Conducted RF disturbances.
HD481 (IEC801) HD481.1SI
EMC for industrial process measurement and control equipment. BS6667: 1985: pt1
General introduction.
HD481.2SI
BS6667: 1985: pt2
Method of evaluating susceptibility to electros ta tic discharge.
HD481.3SI
BS6667: 1985: pt3
Method of evaluating susceptibility to radiated electromagnetic energy.
IEC801 IEC801-3 (Draft Rev) IEC801-4: 1988 IEC801-5 (Draft) IEC801-6 (Draft)
Electromagnetic compatibility for industrial process measurement and control equipment. Immunity to radiated RF EM fields.
90j29283DC BS6667: pt4
Electrical fast transient/burst requirements.
90j21076DC
Surge immunity requirements.
90/27512DC
Immunity to conducted RF disturbances above 9KHz
* Denotes standards which have been referenced by the Official Journal of the European Communities and are therefore notified for use in self certification. tup to 1993
EUROPEAN EMC STANDARDS
257
Table A1.3.4 Proposed product-specific EMC standards (introduction into EEC by 1996) Industrial 1. 2. 3. 4. 5. 6. 7. 7a. 8. 9. 10.
CENELEC ref.
Industrial measurement and control equipment Machine tools (electronic control of manufacturing machinery robots) Power electronics (convertors, rectifiers etc) Industrial electroheat equipment Electrical welding Industrial transport equipment (cranes) Power capacitors Related filters LV switchgear and control gear Rotating machinery FusesLV
Residential) commercial and L V professional 1. Audio, video, audiovisual equipm.ent for domestic entertainment 1a. Broadcast satellite receivers 2. Audio, video, audiovisual lighting control equipment for professional use 3. Domestic appliances and similar household appliances (including toys) 4. Lighting 5. Alarm systems (without mains connection) 6. Mains signalling in low-voltage 7. Building automation 8. Small power electronics (power supplies) 9. Lifts 10. LV circuit breakers and similar equipment 11. Residual current devices 12. Electronic switches Information technology equipment ITE (including telecommunication terminal equipment) ISDN
prEN55024 ENV55102-2
Traffic) transportation 1. Electric traction equipment 2. Motorway communication equipment and traffic control equipment 3. Electrical installation of ships 4. Navigational instrumentation Utilities 1. HV switchgear and control gear (secondary systems) 2. Protection equipment 3. Telecontrol, teleprotection and associated telecommunication for utilities 4. Measuring, metering and load control apparatus (electronic) 5. HV fuses Special 1. Medical equipment 2. Electrical and electronic test and measuring instruments (including scientific instruments) 3. CATV cable distribution equipment
prEN50082-2
258
A HANDBOOK FOR EMC TESTING AND MEASUREMENT
Table Al.3.5 CISPR publications CISPRI CISPR2 CISPR3 CISPR4 CISPR5 CISPRI-6 CISPR7 CISPR8 CISPR9 CISPRI0 CISPRII CISPR12 CISPR13 CISPR14 CISPR15 CISPR16 CISPR17 CISPR18 CISPR19 CISPR20 CISPR21 CISPR22 CISPR23
RFI measuring set 0.15-30 MHz RFI measuring set 25-300 MHz RFI measuring set 10-150 kHz RFI measuring set 300 MHz-l GHz Peak, quasipeak and.average detectors inclusive have been superceded by CISPR 16 Recommendations of CISPR Reports and study questions CISPR and national limits Organisation, rules and procedures of CISPR (1981 and 1983) ISM limits and measurements (1975, 1976) Ignition limits and measurements (1978, 1985) Sound and TV receivers limits and measurements (1975, 1983) Household equipment limits and measurements (1985) Fluorescent lamps and luminaires limits and measurements (1985) RFI measuring sets specification and measurements (1977, 1980, 1983) Filters and suppressors measurement (1981) RFI of power lines and HV equipment part 1: description (1982) Microwave ovens measurements above 1 GHz (1983) Immunity of sound and TV receivers (1985) Interference to mobile radiocommunications (1985) II' equipment limits and measurement (1985) Derivation of limits for ISM
Appendi)( 1.4
Gerntan decrees and standards Table
~41.4.1
German decrees relating to Ell-lC
Decree
.A.pplicability
\lfg 523/1969 \lfg 1046/1984
Individual permit for HF equipment - Class A Decree ISJ\tI and similar equipment (EDP, etc) - Class B general permit decree (includes self certification ) [specifies VD E08 71 ] General permit for household appliances [specifies VDE0875] EC directives implementation and harmonisation of legislation [specifies 82/499/EEC and 82/500/EEC]
\lfg 1045/1984 \ffg 1044/1984
Examples of Germ,an VDE standards VDE Ref.
Relating to
'iDE0565
Specification for RFI suppression devices Part 1 Capacitors Part 2 Chokes Part 3 Filters (up to 16 A) Part 4 Ceramic capaci tors Limits of RFI from RF apparatus and installations Part 1 ISNI Part 2 EDPjIT Interference suppression for radio and T\l receivers RFI from electrical utility plants and H\l systems Recommendation for RFI suppression RFI from appliances (frequencies below 10 kHz) Part 1 Household appliances Part 2 Fluorescent lighting Part 3 i\ppliances with motors Interference measuring apparatus Part 1 vVeighted indication Part 2 Disturbance analyser Part 3 Current probes RFI NIeasurement procedures Part 1 Interference voltage Part 2 Interference field strength Part 3 Interference on power leads RFI suppression for motor vehicles and engines
VDE0871
VDE0872 VDE0873 VDE0874 \lDE0875
\lDE0876
VDE0877
\rDE0879
259
260
A HANDBOOK FOR EMC TESTING AND MEASUREMENT
Table Al.4.2 Sun'lmary oj important test differences between USA (FCC) and Europe/ Gern'lany (VD E) FCC
VDE
Class A
For commercial/office products. Manufacturer does RFI test for self verification and labels product
VDE RFI test and FTZ certification is mandatory by law, primarily for systems or low volume products. FTZ number must be on product and user registers device location.
Class B
For residential products only. Requires FCC authorisation and FCC identifier number on device
Can test at VDE or self certify. Used for high volume and stand-alone products. German RFI declaration and ZZF registration required for self certification. A 2 dB margin is required when one unit is tested.
Class C
Class A equipment can be tested at installation site for self verification. The FCC does not perform this test
Test is conducted by local postal authority at place of installation. Class C is for large 'one-of-a-kind' systems installed in an industrial zone.
Radiated test distances
3 m for Class Band 30 m for Class A, preferred. Distances less than 30 m allowed if data are correlatable.
10m for Class B, 30 m for Class A (10 m, 470-1000 MHz).
Cond ucted test
450 kHz-30 MHz, Class A and B
10 kHz-30 MHz, Class B. 150 kHz30 MHz, Class A. For Class B tests, when a reading is wi thin 5 dB of the B limit, then rerun test with floating ground.
Magnetic field test
N/A
Performed per VDE0871 at 3 m from 10 kHz to 30 MHz. If EU'I' fails at 3 m then retest at 30 m for Class B.
Radiated test antennas
An tenna height is varied from 1 to 4 m for measurement distances up to and including 10m. For distances of 30 m the antenna is varied from 2 to 6 m
An tenna height is fixed at 3 m for up to 470 MHz and varied from 470-1000 MHz for Class A (30 m distance). Antenna height is varied from 1 to 4 m for Class B (10m distance).
Radiated test cables
Interface cables are configured to discover maximum emission
Interface cables are positioned at 1.5 m out from EUT and parallel with ground (see VDE0877).
RFI test equipment
EMI receivers are preferred but spectrum analysers are allowed. Spectrum analysers are widely used in the US
EMI receivers with CISPR are highly preferred. Spectrum analysers with QP and preselection are sometimes permitted. Spectrum analysers are rarely used in Germany
Appendix 1.5
US EMC regulations and standards Table Al.5.l FCC EM G~ regulations and information Part 15 Part 15j Part 18 Part 68 OST62 OST55 MP-4 Guly 87)
Standards for unlicensed or restricted radiation devices Covers the limits for computing devices Standards for industrial, scientific and medical equipment Specification for equipment connected to telephone network Understanding FCC EDP regulations Open-field sites for FCC testing FCC measurement procedures for EDP
Table Al.5.2 Examples of other US EMC standards SAE J-551 C [20]: SAE J -1113 SAEJ-1338 SAEJ-1407 SAEJ-1507
(1984).} (1981) (1982) (1986)
Vehicle ignition interference. This places limits of -2 dB flV/in/kHz (30-88 MHz) and 8 dB fl V /m/kHz (88-400 MHz) at 10m from the vehicle. Concerned with radiated susceptibility testing of whole vehicles (not aircraft), but could be applied to testing any large system
SAE ARP-937 [21]:
Jet engine EMI
SAE AIR-1147 [22]:
Precipitation static radio interference from jet engine charging
RTCA/DO-160B/C:
Environmental conditions and test procedures for airborne equipment
ANSI C63.2 [23J:
Specification for EMI and field strength instrumentation
Details of other US commercial specifications can be found in Appendix 1.9 in the compendium of EMC standards. [References relate to Chapter 2]
261
Appendix 1.6
Gerntan, North Anterican and Japanese EMC standards Table Al.6.1 Comparison oj VDE) FCC and VCC] test procedures VDE Horizontal antenna distance (m) Class A
FCC
VCCI
10 or 30 100 (H-field) 10 3 or 30 (H-field)
3, 10 or 30
3, 10 or 30
3, 10 or 30
3 or 10
Class A
1-4*
2-6
1--4
Class B
1-4*
1-4
1-4
Class B Vertical antenna height (m)
Floor-moun ted equipment is placed on nonconductive surface at 0.15m max height during test
Floor-mounted equipment is placed on floor
Floor-mounted equipment is placed as close as possible to metal plane
Table-top equipment is in normal operating posi tion on a noncond uctive test stand at 0.8 m height
T a ble-rnoun ted equipment is positioned 40 cm from ground that LISN is referenced to and at least 80 cm from nearest other ground
Portable equipment is placed on a non-metal stand 0.8 m above metal plane
Cable
For table-top equipment, cables are laid horizontally for 1.5 ill at table height, then allowed to drop vertically to floor. For floor-level equipment, cables are laid horizontally for 1.5 m at height where conductor leaves EUT but at minimum height of 0.1 m
Logic cables are moved to maximise emission profile
Eurr arrangement is close to conditions in which equipment is used
Cable terrnination
Each cable port has a cable attached to it
One of each type of peripheral is connected to the system
One of each type of peripheral is connected to the system
Equipment under test arrangement
* If the maximum dimension of the equipment under test is smaller than 10 % of the measurement distance, a fixed antenna height of either 1.5 rn or 3 m can be used. 262
GERMAN, NORTH AMERICAN AND JAPANESE EMC STANDARDS
263
Table Al.6.2 Canadian IT EMC limits
Limits of radiated radio noise emissions
Limits of conducted radio noise emissions
Class .,1
C?ass A
Frequency range
Limits at 30 m
Frequency range
Limits Narrowband
MHz ~ 30.000 > 88.000
dB (reference 1 J.1Vjm) 30 34
MHz ~ 0.450 > 1.600
dB (reference 1 J.1V) 60 73 70 83
> 216.000
~ ~ ~
88.000 216.000 1 000.000
~ ~
1.600 30.000
Broadband
37
Class B
Class B
Frequency range
Limits at 3 m
Frequency range
Limits Narrowband
MHz ~ 30.000 ~ 88.000 > 88.000 ~ 216.000 > 216.000 ~ 1 000.000
dB (reference 1 J.1Vjm) 40 44 46
MHz ~ 0.450
dB (reference 1 J.1V) 48 61
Regulations
SORj88/475 and CI08.8-M1983
~
30.000
Broadband
Appendix 1.7
Electrical safety and electroIIlagnetic radiation safety Table A1.7.1 Typical regulations on product safety Standard
Description
IEC950 } EN60950 CSA22.2 No950 UL1950
Safety of inforn1ation technology equipmnt including electrical business equipment
ULl14 } UL478 CSA22.2 No154 CSA22.2 No220
Older standards
IEC380 } VDE0806 CSA22.2 No143 BS6301 ZHl/618 (BG)* IEC601-1 VDE0750 BS5724 IEC204 VDEOl13 BS2771 IEC65 VDE0860 IEC335 VDE0700 BS3456
Safety of office equipment Telecom equipment Safety of video displays and computers
} }
Safety of medical prod ucts
Safety of industrial equipment
Safety of household appliances
*BG Injuries Insurance Institute (Germany)
264
ELECTRICAL SAFETY AND ELECTROMAGNErrIC RADIATION SAFETY
265
Table Ai.7.2 Examples of standards limiting exposure of hutnans to RF radiation
NCMDRH US [4J Public Law 90-602: ANSI (1982) [35J (under revision):
Limits for ionising and nonionising radiation
American National Standard C95.1 (Safety levels with respect to human exposure to radiofreq uency electromagnetic fields)
NRPB (1986) Consultative document [36J NRPB (1989) GS--11 [37J DEF-STAN 59/41 (part 3) [3J MoD (PE) Code of practice (1982): calls up STANAG 2345 DEF-STAN 05-74/1 (1989) [38J INIRIC (1984) Guidelines: reissued in 1988 [39J The EEC has introduced regulations regarding nonionising radiation limits under Articles 4 and 5 [40J [References relate to Chapter 2]
Table Ai.7.3 N RPB GSii recommendations: derived reference levels for exposure to electromagnetic fields atjrequencies below 300 GHz
Frequency
Hz
< 100 0.1-1 k 1--30 k 0.03-1 M 1--10 M 10---30 M
Root-mean-square values Electric field strength
Magnetic field strength
Magnetic flux density*
Vim 614,0001f (Hz) 614/f (kHz) 614/f (kHz) (kHz) 6 6141f (MHz) 61.4/f(MHz)
A/m 1630 163/f(kHz) 163/f(kHz) 4.89If(MHz) 4.89/f(MHz) 4.89/f(MHz)
mT
below 30 MHz: electric and magnetic fields are
Frequency
Hz 30-400 M 0.4-2 G 2-300 G
2 0.2/f(kHz) 0.2/f~kHz)
6.10- /f(MHz) 6.10- 3 /f(MHz) 6.10- 3 /f(MHz)
considered
Root-mean-square values Electric field strength
Magnetic field strength
Power density
Vim 61.4 97.1v!f (GHz) 137
A/m 0.163 0.258v!f (GHz) 0.364
W/m 10 25,[(GHz) 50
Frequencies above 30 MHz: electric field alternatives under far-field conditions
2
and power density are taken as equivalent
Appendix 1.8
Military EMC standards Table Al.B.l Classes of equipment for determining applicable test requirements Class
Description
I
Communication-electronic (CE) equipment: any item, including subassemblies and parts, serving functionally generating, transnlitting, conveying, acquiring, receiving, storing, processing or utilising information in the broadest sense. Subclasses are:
IA
Receivers using antennas
IB
Transmi tters using antennas
IC
Nonantenna CE equipment counters, oscilloscopes, signal generators, and other electronic devices working in conjunction with classes IA and IB)
ID
Electrical and electronic equipment and instruments which would affect mission success or if degraded or malfunctioned by internally generated interference or susceptibility to external fields and voltages
II
Noncommunication equipment. Specific subclasses are:
IIA
Noncommunication electronic equipment: equipment for which RF energy is intentionally generated for other than information or control purposes. Examples are medical diathermy equipment, induction heaters, RF power supplies and uninterruptible power units
lIB
Electrical equipment: electric motors, hand tools, office and kitchen equipment
IIC
Accessories for vehicles and engines: electrically and mechanically driven and engine electrical accessories such as gauges, fuel pumps, magnetos and generators Applicable only to accessories for use on items of Classes IlIA and IIIB.
III
Vehicles, engine driven equipment
IlIA
rfactical vehicles: armoured and tracked vehicles, off-the-road cargo and personnel carriers, assault and landing craft, alnphibious vehicles, patrol boats, and all other vehicles intended for installation of tactical CE equipment.
IIIB
Engine generators: those supplying power to, or closely associated with CE equipment.
IIIC
Special-purpose vehicles and engine driven equipment: those intended for use in critical communication areas such as airfields, missile sites, ships' forward areas or in support of tactical operations.
IIID
Administrative vehicles of basically civilian character not intended for use in tactical areas or in critical areas covered by Class IIIC, and not intended for installation of communication equipment.
IV
Overhead power lines
266
MILITARY EMC STANDARDS
267
Table A1.8.2 MIL STn 461 test requirements applicable to equipment classes Equipmen t class rrest RE01 RE02 RE03
RE04
IA
IB
IC
ID
IIA
lIB
IIC
III A
III B
CE03 CE04 CE05 CE06 RSOI RS02 RS03 RS04 CSOI CS02 CS03 CS04 CS05 CS06 CS07 CS08
IV
• • •
•
RE06
CE02
III D
• • • • • • • • • • • • • • • •
RE05
CEOI
III C
• • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • •
•
• • • • • • • • •
Description of test method 30 Hz to 30 kHz, magnetic field 14kHz to 10GHz, electric field 10 kHz to 40 GHz, spurious and harmonics 20 kHz to 50 kHz, magnetic field 150 kHz to 1 GHz, vehicles and engine driven eq. overhead power line test 30 Hz to 20 kHz, power leads 30 Hz to 20 kHz, con trol and sig. leads 20 kHz to 50 MHz, power leads 20 kHz to 50 MHz, con trol and sig. leads 30 Hz to 50 MHz, inverse filter method 10kHz to 12.4GHz, an tenna terminal 30 Hz to 30 kHz, magnetic field mag. induction field 14kHz to 10GHz, electric field 14 kHz to 30 MHz 30 Hz to 50 kHz, power leads 50 kHz to 400 MHz, power leads 30Hz to 10GHz, intermod ulation 30Hz to 10GHz, rej. of undesirable sig. 30Hz to 10GHz, cross-modulation spike, power leads sq uelch circuits 30Hz to 10GHz, rej. of undesira ble sig.
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A HANDBOOK FOR EMC TESTING AND MEASUREMENT
Table Al.8.3 Equipment and subsystem classes and applicable parts of MIL STD 461BjC Class
Description
A
Equipments and subsystems which must operate compatibly when installed in critical areas, such as the following platforms or installations:
Al
Aircraft (including associated ·ground support equipment)
2
A2
Spacecraft and launch vehicles (including associated ground support equipment)
3
A3
Ground facilities
4
A4
Surface ships
5
A5
Submarines
6
B
Equipments and subsystems which support the Class A equipments and subsystems but which will not be physically located in critical ground areas. Examples are electronic shop maintenance and test equipment used in noncri tical areas;
7
C
Miscellaneous, general purpose equipments and subsystems not usually associated with a specific platform or installation. Specific items in this class are:
Cl
Tactical and special purpose vehicles and engine driven equipment
8
C2
Engine generators and associated components, uninterruptible power sets and mobile electric power equipment supplying power to or used in critical areas
9
C3
Commercial electrical or electromechanical equipment
Applicable part
and mobile, including tracked and wheeled vehicles)
10
Table Al.8.4 Test changes between MIL STD 461A and 461B
Table Al.8.5 [IK RRE 6405 classes of equipment
CE05 RE06 RE05 RE04 CE02 CE04 CS08 RS04 CE07
Class
Description
A
Receivers using antennas
B
rrransmitters using antennas
C
Nonantenna equipment, such as counters, test equipment, lasers, computers, power supplies, digital eq uipmen t, and other electronic devices working in conjunction with classes A and B
D
Electronic equipment and instruments which would affect mission success or if malfunctioned or degraded by EM interference, SUCfl as autopilots, flight instruments, and control devices.
CS09 UM03 UM04 UM05
Deleted from 461 B Deleted from 461 B Replaced with UM03 Deleted from 461 B Combined with CE01 Combined with CE03 Deleted from 461 B Deleted from 461 B Power leads, time dcnnain in trod uced 461B Structure (comm. mode) introduced 461B Tactical and special vehicles (replaced RE05), introduced 461B Engine generators and related, introduced 461 B Common elec. and mech. equipments, introduced 461 B
MILITARY
EMC~
STANDARDS
269
Table Al.8.6 RRE6405 list oj tests
Identification
Equipment class
EMC parameter Aspect
Frequency, Hz
RCE01 RCE02 RCE03 RCE04 RCE05 RCE06 RCE07 RCE08 RCE09 RCE10 RCEll RCE12 RCE13 RCEl4
A-D A-D A-D A-D A--D A-D A-D A,B A,B A,B A,B A,B A,B A-D
DC power leads, current, AF AC power leads, current, LF All leads, voltage, narrowband All leads, voltage, broadband Signal and control leads, current All leads, current, narrowband, HF All leads, current, broadband, I-IF Antenna terminals, narrowband, SHF Antenna terminals, broadband, SHF Antenna terminals, narrowband, Antenna terminals, broadband, Ant. term, narrowband, key down Ant. term, broadband, key down Transient emission
20 - 50k 10 k- 50 k 50 k-100 M 50 k-100 M 20 - 50k 50 k-lOO M 50 k-100 M 10 k-100 M 10 k-lOO M 0.1 G- 40G 0.1 G- IG 10k- 40G 0.1 G- 40G Spikes
RCS01 RCS02 RCS03 RCS04 RCS05 RCS06 RCS07 RCS08 RCS09 RCS10
A-D A-D A,C A,C A,C A,C A,C A,C A-D A-D
DC power leads DC and AC power leads Two-signal intermodulation, HF Two-signal intermodulation, SHF Rejection of undesired signals, HF Rejection of undesired signals, SHF Cross modulation, HF Cross modulation, SHF DC and AC power leads Post-detector rejection
20 - 50k 50 k-400 M 20 -100M 0.1 G- 40G 20 -100M 0.1 G- 40G 20 -100M 0.1 G- 40G Spikes 20 - 40G
RREOl RRE02 RRE03 RRE04 RRE05 RRE06 RRE07 RRE08 RRE09
A,B,C A,B,C A--D A-D A-D A-D B B A-D
Magnetic field, LF Magnetic field, HF Electric field, narrowband, HF Electric field, broadband, HF Electric field, narrowband, SHF Electric field, broadband, SHF Antenna, spurious, narrowband Antenna, spurious, broadband Transien t emission
30k 20 30k- 3M 14 k-lOO M 14 k-100 M 0.1 G- 40G 0.1 G- IG 0.1 G- 40G 0.1 G- 40G Spikes
RRS01 RRS02 RRS03 RRS04
A---D A--D A-D A-D
Magnetic field, LF Magnetic field, induction Electric field, HF Electric field, SHF
50k 20 Spikes 14 k-lOO M 0.1 G- 40G
RMP01 RMP02 RMP03 RMP04
B B A,B A
Occupied bandwidth, transmitter Frequency tolerance, transmitter Antenna pattern, far field Receiver, bandwidth
1.0M1.0 M0.1 G0.1 G-
40G 40 G 40G 40G
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A HANDBOOK FOR EMC TESTING AND MEASUREMENT
Table A1.B.7 RRE6405 EM C' grading by environlnental conditions Grade
Shipboard
Air service
Land service
L
Equipment used outside the metal hull and/or metal superstructure of surface ships, or above decks only in any ship which is of metallic construction, or in any ship which is not of metallic construction
Equipment for use generally in civil and military aircraft
Equipment sited not within 2 m of another equipment that emits, or is susceptible to, electromagnetic radiation
M
Not used
Not used
Equipment sited not within 15 m of another equipment
N
Equipment used on board submarines and within the metal hull and /or superstructure of surface ships
Not used
Equipment sited not within 100 m of another eq uipn1en t
Table A1.B.B DEF STAN 59-41 EMC' tests Test reference
Test description
Service* a pplicabili ty
DCEOI DCE02 DCE03
Signal/controlline~ current
Power line, current 20 Hz-ISO MHz 20 Hz-ISO MHz Exported transients, power lines
Land sea air Land sea air I-land aIr
DREOI DRE02 DRE03
E-field radiation at 1 m 14kHz-18GHz H-field radiation at 70 mm 20 Hz-50 kHz E-field radiation, installed antenna 1--76 MHz
Land sea air Land sea air Land
DCSOI DCS02 DCS03 DCS04 DCS05 DCS06 DCS07
Power line, voltage 20 Hz-50 kHz Power, control and signal leads 50 kHz-400 MHz Control/signal, current 20 Hz-50 kHz Imported transient susceptibility Externally generated transients Imported long transient susceptibility 28 V systems Imported short transient susceptibility
I-Jand sea air Land sea air Land sea air aIr sea Land Land
DRSOI DRS02
H-field at 50 mm 20 Hz-50 kHz E-field, 14kHz-18GHz
Land sea air I-Jand sea air
DMFSOI
Magnetostatic field
Land sea
* Check individual limits for actual frequency range DCE conducted en1ission DCS conducted susceptibility DRE DRS radiated susceptibility DMFS magnetostatic field susceptibility
radiated emission
Appendix 1.9
COlnpendiuln of EMC and related standards With limited space this compendium cannot be exhaustive of all standards and specifications related to EMC and electrical safety worldwide. Regulations and standards are changing frequently as the field develops rapidly owing to technical advances, economic and political factors. However, this list contains sufficient examples of EMC standards to provide a starting point for further exploration.
BS 5406 (1988): Limitation of disturbances in electricity supply networks caused by domestic and similar appliances equipped with electronic devices (IEC 555) BS 5602 (1978): Code of practice for the abatement of RFI from overhead power lines (CISPR 18) BS 5750 Series: Quality assurance related standards BS 5783 (1984): Code of practice for handling electrostatic devices BS 6021 Part 3 (1982): Specification for fixed capacitors used for RFI suppression: General requirements and methods of test (IEC 384-14) BS 6299 (1982): Measurement methods for passive suppression filters and components (CISPR 17) BS 6345 (1983): Methods of measurement of RFI terminal voltage of lighting equipment (CISPR 15) BS 6491 (1984): Part 1 ESD BS 6527 (1988): Specification for limits and measurements of RFI characteristics of information technology equipment (CISPR 22), (VDE 08781) BS 6651 (1985): Protection of structures against lightning BS 6656 (1986): The prevention of inadvertent ignition of flammable atmospheres by RF radiation (read with HSE GS21 HMSO 1983) BS 6657 (1986): Prevention of inadverten t ignition of EEDs by RF radiation (read with OB. Procs 42413 and 42202) BS 6667 (1985): Electromagnetic compatibility requirements for industrial process control Part 1 General introduction (IEC 801-1) Part 2 Susceptibility to ESD (IEC 801-2) Part 3 Susceptibility to radiated EM energy (IEC 801-3) Part 4 Electrical fast transient/burst require ments (IEC 801-4) Part 5 Surge immunity requirements (draft in 1990) BS 6839 (1987): Part 1 Specification for communication and interference limits and measurements for mains signalling equipment Part 2 - Specification for interfaces for mains signalling equipment BS 3G 100 (1980): Part 4 section 2: EMI at radio
1 UK British Standards (EMC) BS 613 (1977): Specifies components and filter units for EMI suppression BS 727 (1983): Specification for RFI measurIng apparatus (CISPR 16) BS 800 (1988): Specification of RFI limits and measurements for household appliances, portable tools and other electrical equipment (CISPR 14), (VDE 0875) BS 833 (1985): Specification of RFI limits and measurements for electrical ignition systems of internal combustion engines (CISPR 12) BS 905 (1985): Specification of EMC for sound and TV receivers and associated equipment Part 1: Specification of limits for RFI (CISPR 13), (VDE 0872) Part 2: Specification of limits for immunity (CISPR 20) BS 1597 (1985): Specification for limits and methods of measuring electromagnetic interference from marine equipment and installations BS 2316 Parts 1 and 2 (1981): General requirements and tests for radio frequency cables BS 4727 Part 1 Gp 9 (1976): Glossary of technical terms connected with radio interference technology (IEC 50 Ch. 902) BS 4809 (1981): Specification of RFI limits and measurements for RF heating equipment (CISPR 11), (similar to VDE 0871) BS 5049 (1981): Methods of measurement of RF noise from power supply apparatus working above 1 kV (CISPR 18) BS 5260 (1981): Code of practice for RFI suppression on marine installations BS 5394 (1988): Specification of limits and measurement methods for RFI characteristics of fluorescent lamps and luminaires (CISPR 15), (VDE 0875) 271
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A HANDBOOK FOR EMC TESTING AND MEASUREMENT
and audio frequency requirements for equipment for use in aircraft BS G229: Specification of environments for equipment for use in aircraft (DO 160) NWM 0320: UK National weights and measures standards on EMC for metrology equipment
2 UK British Stand·ards (electrical safety) BS 2754: Protection against electric shock BS 2771: Electrical safety of industrial machines BS 3192: Electrical safety of radio and TV transmitters BS 3456: Household safety BS 5458: Safety requirements for electrical indicating and recording equipmen t BS 5724: Safety of mechanical equipment BS 6301: Safety of telecommunications apparatus
3 UK NAMASjNPL docutnents NAMAS M3: Concise directory of approved calibration and testing laboratories NAMAS MI0: General criteria of competence for calibration and testing laboratories NAMAS MIl: Regulations to be met by calibration and testing laboratories NAMAS M 13: Regulation concerning the use of the NAMAS logo/mark NAMAS M16: The quality manual - guidance for preparation NAMAS NIS20: Uncertainty of measurement for NAMAS electrical product testing laboratories NAMAS B3003: Expression of uncertainty In electrical measurements NPL 0004/8K/NJ /6/90: NPL Points of contact NPL 007/J.5K/NJ/9/90: RF and mIcrowave measurement services
4 CISPR EMC standards CISPR 1: RFI measuring set 0.15-30 MHz CISPR 2: RFI measuring set 25-300 MHz CISPR 3: RFI measuring set 10-150 kHz CISPR 4: RFI measuring set 300 MHz-IGHz CISPR 5: Peak, quasipeak and average detectors CISPR 1-6: inclusive have been superseded by CISPR 16 CISPR 7: Recommendations of CISPR (including recommend a tion 19: Limits in large-scale production) CISPR 8: Reports and study questions CISPR 9: CISPR and national limits CISPR 10: Organisation, rules and procedures of CISPR (1981 and 1983) CISPR 11: ISM limits and measurements (1975, 1976)
CISPR 12: Ignition limits and measurements (1978,1985) CISPR 13: Sound and TV receivers limits and measurements (1975, 1983) CISPR 14: Household equip. limits and measurements (1985) CISPR 15: Fluorescent lamps and luminaires limi ts and measurements (1985) CISPR 16: RFI measuring sets specification and measurements (1977, 1980, 1983) CISPR 17: Filters and suppressors measurement (1981 ) CISPR 18: RFI of power lines and HV equipment Part 1 description (1982) CISPR 19: Microwave ovens measurements above 1 GHz (1983) CISPR 20: Immunity of sound and TV receivers (1985) CISPR 21: Interference to mobile radiocommunications (1985) CISPR 22: IT equipment limits and measurement (1985) CISPR 23: Derivation of limits for ISM
5 Gertnan VDE EMC and electrical safety standards 5.1 VDE EMC standards VDE 0565: Specification for RFI suppression devices Part 1 capacitors Part 2 chokes Part 3 filters (up to 16A) Part 4 ceramic capacitors VDE 0839: Teil 10 generators ESD VDE 0843: Part 2 ESD identical with IEC 801-2 VDE 0846: Teil 11 test generators ESD VDE 0871: Limits of RFI from RF apparatus and ins tallations Part 1 ISM Part 2 EDP VDE 0872: Interference suppression for radio and TV receivers VDE 0873: RFI from electrical utility plants and HV systems VDE 0874: Recommendation for RFI suppression VDE 0875: RFI from appliances (below 10 kHz) Part 1 household appliances Part 2 fl uorescent lighting Part 3 appliances with motors VDE 0876: Interference measuring apparatus Part 1 weigh ted indication Part 2 disturbance analyser Part 3 current probes VDE 0877: RFI measurement procedures Part 1 interference voltage Part 2 interference field strength Part 3 interference on power leads
COMPENDIUM OF EMC AND RELATED STANDARDS
VDE 0879: RFI suppressIon for motor vehicles and engines DIN 43 305 pt. 302: Methods of measurement on radio receivers for various classes of emission: Methods of measurement for checking the immunity from. interference fields of radio receivers. Beuth Verlag, Berlin
5.2 VDE safety standards VDE 0113: Safety of industrial equipment (IEC 204) VDE 0700: Safety of household appliances (IEC 335) VDE 0750: Safety of medical equipment (IEC 601 ) VDE 0806: Safety of office equipment (IEC 380) VDE 0806: Safety of household appliances (IEC 065) VDE 0848: Hazards by electromagnetic fields ZH 1/618 (BG): Safety of video displays, etc
6 European and international standards EMC standards and specifications in force in Europe and most countries in the world are listed in the following publication: 'Electromagnetic compatibility regulations and standards worldwide', Netderlands Normalisatie lnstitut (UDC 621.391.82(100))
273
(CISPR 14, BS 800, VDE 0875, FCC Pt15: near equivalent) EN 55015: Limits and measurement methods for RFI characteristics of fluorescent lamps and lurninaires (CISPR 13, BS 5394, VDE 0875) EN 55020: Immunity of sound and television broadcast receivers and associated equipment in the frequency range 1.5 to 30 MHz by current injection method (CISPR 20, BS 905 Pt2) EN 55022: Limits and methods of measurement of the RFI emission characteristics of information technology equipment (CISPR 22, BS 6527) EN 55101-4: RF immunity of information technology eq uipmen t (in draft) EN 60555-2: Disturbances in supply systems caused by household appliances and similar electrical equipment - harmonics (lEe 555, BS 5406) EN 60555-3: Disturbances in supply systems caused by household appliances and similar voltage fluctuations electrical eq uipment (IEC 555, BS 5406) EN 60950: Safety of information technology and business eq ui pmen t HD 481: Immunity to RF, ESD, transients and surges of industrial process control equipment. Basic standard. (IEC 801, BS 6667) HD 481.1 S 1: General introduction HD 481.2 S 1: Method of evaluating susceptibility to electrostatic discharge HD 481.3 S 1: Method of evaluating susceptibility to radiated electromagnetic energy
6.1 European harmonised standards EN 29000: Quality assurance of calibration and testing laboratories EN 45001: General req uiremen ts for technical competence of testing laboratories EN 50065-1: Mains signalling on low-voltage electrical installations EN 50081-1: Generic RFI emission standard for any equipment in the domestic, commercial and light industrial electromagnetic environments (class 1 environments) EN 50082-1: Generic RF immunity standard for any equipment in the domestic, commercial and light industrial electromagnetic environments (class 1 environment) EN 55011: Limits and methods of measurement of RFI characteristics of ISM equipment (excluding diathermy apparatus) (CISPR 11, BS 4809, VDE 0871, FCC Pt18: near equivalent) EN 55013: Limits and measurement methods for RFI characteristics of sound and television broadcast receivers (CISPR 13, BS 905 Ptl, VDE 0872, FCC Pt15: near equivalent) EN 55014: Limits and methods of measurement of RFI characteristics of household electrical appliances, portable tools and similar equipment
6.2 Other European standards UN ECE Reg. 10: Control of RFI from motorcycles and vehicles with 3 or more wheels 72/245/EEC: Suppression of motor vehicle ignition (CISPR 12, BS 833) 71/320/EEC: Vehicle brakes UN ECE Reg. 13/05: Vehicle brakes EEC Directive 71/320/EEC (amended 85/647/ EEC): Braking devices EEC 451 7/79: Contains reference to ESD ECMA Standard 47: Limits and measurement methods for radio interference from EDP equipment. European Computer Manufacturers Association, Geneva, Switzerland ECMA TR/40: ESD testing of IT equipment ---technical report
6.3 lEe standards related to EMC IEC 50: International electro technical vocabulary, Chapter 161 on electromagnetic compatibility IEC 96: Part 0 Guide to the design and detailed specification of RF cables Part 1 - General requirements and measurement methods for RF cables .
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A HANDBOOK FOR EMC TESTING AND MEASUREMENT
IEC 106: Recommended methods of measurement of radiated and conducted interference from receivers induced by AM, FM and TV broadcast transmissions IEC 478: Part 3 - RFI tests for DC stabilised power supplies IEC 533: EMC of electrical and electronic equipment in ships IEC 555: Disturbances in supply systems caused by household appliances and similar equipment IEC 654: Operating conditions for industrial process measurement and control equipment IEC 801: EMC requirements for industrial process control instrumentation IEC 801-1: General introduction IEC 801-2: Susceptibility to ESD IEC 801-3: Susceptibility to radiated EM energy (draft) IEC 801-4: Electrical fast transients/burst req uirements IEC 801-5: Surge immunity requirements (draft) IEC 801-6: Immunity to conducted RF disturbances above 9 kHz (draft)
6.4 lEe standards related to electrical safety IEC 065: 0860) IEC 204: 0113) IEC 335: 0700) IEC 380: IEC 601:
Safety of household appliances (VDE Safety of industrial equipment (VDE Safety of household equipment (VDE Safety of office equipment (VDE 0806) Safety of medical products (VDE 0750)
6.5 ISO publications ISO 9000: Quality assurance aspects of testing ISO Guide 25: Technical competence of testing laboratories ISO/TC22/SC3/WG3 paper N 260: Road vehicles: Electrical in terference from electrostatic discharge INIRC/IRPA (1988): Guidelines on limits of exposure to radio frequency electromagnetic fields in the frequency range from 100 kHz to 300 GHz
7 US EMC regulations and standards FCC Code of Federal Regs. Title 47 volumes 1-5 (docket 20780) FCC Part 15: Electromagnetic compatibility ofRF devices Subpart A General Subpart B Administrative procedures Subpart C Radio receivers Subpart D General requirement for low-power communications devices
Subpart E Low-power communications devices (specific devices) Subpart F Field disturbance sensors Subpart G Auditory assistance devices Subpart H TV interface devices Subpart I Measurement procedures Subpart J Computing devices FCC Part 18: Electromagnetic compatibility of ISM equipment Subpart A General information Subpart B Applications and authorisations Subpart C Technical standards FCC Part 68: Electromagnetic compatibility of equipment to be connected with the telephone network FCC Doc. OCE44: Open-field site calibration (horizon tal polarisation) OST 62: Understanding the FCC EDP regulations OST 55: Open-field sites for FCC testing MP4 Uuly 1987): FCC measurement procedures for EDP SAE J 551 C : Vehicle ignition interference SAE J 1113 (1984): Susceptibility procedures for vehicle components (except aircraft) SAE J 1338 (1981): Open-field whole-vehicle radiated susceptibility testing SAE J 1407 (1982): TEM cells for automotive susceptibility tests SAE J 1507: Apparatus and test procedures for whole-vehicle susceptibility testing SAE ARP 936: ~1easurement of capacitors SAE ARP 937: Jet engine EMI SAE ARP 958: Test antenna calibration practice SAE Jl113 (1989) Part 5: Electromagnetic compatprocedure for vehicle ibility measurement, components: Susceptibility to ESD SAE J1338 SAE J1407 SAE J1507 SAE AIR 1147: Precipitation of static radio interference from jet engine charging SAE AIR 1499: Recommendations for commercial EMC susceptibility standards ANSI C16: EMC aspects of communication and electronic equipment ANSI C63: Measurement techniques for EMC C63.2: Specification for EMI and field strength ins trumen tation C63.4 (1988) EMC: Radio noise emissions from low voltage electrical and electronic equipment 10 kHz-1 GHz ANSI C68: EMC aspects of high-voltage testing techniques ANSI C95.1 (1982): Safety levels with respect to human exposure to RF EM fields (300 kHz100 GHz) ANSI April 1989 (draft 5): Guide for ESD test
COlvIPENI)IUM OF EMC AND RELA1'EI) STANDARDS
methodologies and cri teria for electronic equipment UL 1950: Safety of IT and electrical business equipment MDS-201-0004 (1979): EMC standard for rnedical devices. A voluntary code AAMI ( :' Heart pacemaker EMC voluntary standard FDA Infant Apena Monitor Standard Mandatory EMC regulation applied to these devices NCMDRH (US Public Law 90-602, 1968): Safe radiation limits from TVs, x-ray machines, microwave ovens, etc IEEE S302: Standard method for measuring EM field strength below 1000 MHz in radiowave propagation IEEE S299: Measuring shielding effectiveness of high-performance shielded enclosures IEEE S473 (1985): Recommended practice for an electromagnetic site survey (10 kHz-1 0 G Hz) IEEE S518: Guide on the installation of electrical equipment to minimise interference to controllers from external sour'ces IEEE ad hoc committee, 1990: Proposed new code of ethics, vol. no.3, p.5 US Safety Standard NFPA-STD-70-1971: Electric wiring and other devices
8 Exam.ples of other EMC regulations veCI (1986) Japan: 1'he control of netic emissions from electronic data and office machines (Voluntary Code establishes limits and test methods) SOR/88-475 (1988) Canada: Radio interference regulations amendment CSA C108.8-M1983 Canada: EMC procedures CSA 22.2 no.154: Safety of and electrical business equipment For further examples of worldwide regulations, see 1'able 2.1 in 2
9 US m.ilitary standards (EMC) MIL S1-'D 188: Military communications systems tactical standards MIL STD 188-125 (draft 1989): NEMP testing MIL STD 220: Insertion-loss measurements of components MIL STD 285: Attenuation measurement method for shielded enclosures MIL STD 331A: Contains MIL STD 454: General rpnlllll~""YY'lPY\t-C' equipment
275
MIL STD 46 interference characteristics requirements for MIL STD 462: characteristics, measurement of MIL STD 463: Electromagnetic interference characteristics, definitions systems of units MIL STD 469: EMC - radar engineering MIL STD 481A: Antenna: method of calibration, (para MIL STD 704: Electric power, aircraft characteristics and use MIL STD 826A: 'Transients: a 10 JiF '-'.....,I--J(A,'--'~ 'J~ MIL STD 831: Preparation of test reports MIL STD 833C: Involves aspects of ESD MIIJ STD 1310: and grounding methods on board for EMC and safety MIL STD 1337: General suppression and system req uirements for electric hand tools (ships) MIL STD 1344A: Method 3008 (1980): "--JJ"~~'~~'-A~'"J.;;;" effectiveness of multicontact connectors MIL STD 1377: Effectiveness of connector and weapon enclosure shielding and filters in precluding EM hazards to measurement of, MIL STD 1385: Preclusion of ordnance hazards in EM fields, general MIL STD 1512: initiated, req uirements MIL STD 1541: for space systems MIL STD 1542: EMC and ments for space systems facilities MIL STD 1857: ...
MIL-B-5087: Bonding, electrical \.J",,-/vL~.'J.LJ. for aerospace systems MIL-W -5088: installed MIL-E-6051D: electromagnetic ~" OJ"LL~", MIL-E-55301 MIL-I-16910C MIL-I-6181D MIL-M-3851 O-ESD MIL-W-83575: design and AFSC DR 1-4: netic t-~ v'JAJ.l.
r>"1"Yl
t--J(A, ...
y-,.r-.
10 UK tnilitary standards DEF STAN 00-10: Part 2, section 4 DEF STAN 00--35: I)EF DEF STAN 07-55 and AvP Part 2 of service r/"'''''",,''"¥> " " Y \ ' ! T1
electro00-1, "",.Y\ TO
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A HANDBOOK FOR EMC TESTING AND MEASUREMENT
Part 4 natural environments (lightning) Part 5 ind uced environmen ts DEF STAN 08-4: Nuclear weapons explosion effects and hardening DEF STAN 08-5: Superseding AvP 32 'Design requirements for guided weapons' DEF STAN 59-41 (June 1988): Electromagnetic compatibility Part 1 general req uiremen ts Part 2 management and planning procedures Part 3 technical requirements (test methods and limits) Part 4 open-site testing Part 5 technical req uiremen ts for special EM C test equipment DEF STAN 61-5: Part 1 terminology and definitions Part 2 ground generating set characteristics Part 4 power supplies in warships Part 5 ground power supplies for aircraft servIcIng Part 6 28 V DC electrical systems in military vehicles NWS 2: EMC aspects of cabling in ship installations NWS 3 (1985): Electromagnetic compatibility of naval electrical equipments NWS 1000 (1986): Requirements for naval weapon equipment Part 1, chapter 5, section 10 Electro-
magnetic compatibility design guide for naval weapons platform (refers also to Part 2) NES 529: Nuclear hardening guide NES 1006 (1988): Supersedes NWS 6, Radio frequency environment and acceptance criteria for naval stores containing EEDs BR 2924: Radio hazards in naval service BR 8541: Safety req uirement for armament stores for naval use MVEE 595 (1975): Electromagnetic interference and susceptibili ty req uiremen ts for electrical equipments and systems in military vehicles (forms part ofDEF STAN 59-41) RRE 6405 (1974): Requirements for electromagnetic compatibility of electronic equipments Part 1 guide Part 2 req uirements Part 3 measurement techniq ues , AVP 118: Guide to electromagnetic compatibility in aircraft systems STANAG 1307: (Implement by NESI006) STANAG 4234: RF environmental conditions affecting the design of material for use by NATO forces MoD (PE) 1982: Intense radio frequency radia tion code of practice STANAG 2345: RF exposure limits for personnel NRPB (1986): Advice on RF safety NRPB GS11 (1988): Guidance on RF exposure
Appendix 2.1
Modulation rules Table A2.2 DEF STAN 59-41 modulation rules
Table A2.1 MIL STD 462 (N3) modulation rules
For communications equipment (a) AM receivers: modulate with 500/0 1 kHz sinusoid (b) FM receivers: When monitoring signal-tonoise ratio, modulate with 1 kHz signal and 10kHz deviation (c) SSE receivers: No modulation (d) Other equipments: As for AM receivers
Susceptibility frequency
Modulation type
Ship and land service use:
20 Hz-50kHz
Equipments with video channels: Use 90-100 % pulse modulation with duration of 2/ B Wand repetition rate of BW/1000, where BW == video bandwidth Digital equipment: Use pulse mod ulation with pulse duration and repetition rates equal to that used in equipment
CW
50 kHz- 1 G H:z
CW (a) (b) AM 100% square or sine at frequencies in the range 100 Hz-10kHz (c) AM 100% square 1 Hz
200 MHz-18 GHz
Pulsed CW, pulse length 1 j1s, PRF 1 kHz (for test DRS02 only)
For aircraft use:
20 Hz-50kHz 50kHz-2 MHz
CW (a) (b) AM 100% square at lkHzPRF
2MHz-30MHz
CW (a) (b) AM 100% square at 1 kHz PRF (c) AM 100% square at 1-3 kHz PRF
Nontuned equipment: AM at 50% with 1 kHz sinusoid
30 MHz--1 GHz
Radiated testing 1-18 GHz 150-225 MHz 580-610 MHz 0.79-18 GHz
277
CW
CW (a) (b) AM 100% square at 1kHzPRF (a)
CW
(b) A complex modulation of pulse (1 j1s) and AM (0.5Hz)
Appendix 3.1
NAMAS-accredited laboratories Lucas Aerospace Ltd. Power Systems Division Maylands Avenue Hemel Hempstead Herts HP2 4SP 01442 242233
As given in the EMC Awareness Campaign brochure (Reference 11 Chap. 13). British Telecom Product Evaluation Section (MC2) Materials and Component Centre 310 Bordesley Green Birmingham B9 5NF 0121 771 6007 BSI Testing Unit 5 Finway Road Hemel Hempstead 01442 233442
Matra Marconi Space UK Environmental Engineering and Test Lab Anchorage Road, Portsmouth Hampshire P03 5PU 01705 674554 National Weights and Measures Laboratory Stanton Avenue Teddington Middlesex TW 11 OJZ 0181 943 7242
HP2 6PT
Chomerics (UK) Ltd. Parkway Globe Park Industrial Estate Marlow, Bucks SL7 1YB 01628 486030
Thorn EMI Electronics Ltd. Sensors Division Manor Royal, Crawley West Sussex RHI0 2PZ 01293 528787 x4985
Cranage EMC Testing Stable Court Oakley Market Drayton Shropshire TF9 4AG 01630 658568
Other NAMAS-accredited test laboratories (listed in the NAMAS Concise Directory) follow. Apologies to those laboratories which may not have been included. Prospective users of an accredited EMC test laboratory should consult the most up-to-date NAMAS directory for details. Categories of permanent laboratory:
GEC Marconi Avionics EMC Test Centre Maxwell Building Donibristle Industrial Park Dunfermline KY11 5LB 01383 822131
0: Permanent laboratory where the calibration or testing facility is erected on a fixed location for a period expected to be greater than three years. I: Site calibration or testing performed by staff sent out on site by a permanent laboratory that is accredited by NAMAS.
GPT Ltd. Environmental Testing Group New Century Park PO Box 53, Coventry CV3 lHJ 01203 562046
AQL-EMC Ltd. 16 Cobham Road Ferndown Industrial Estate Poole Dorset BH21 7PE 01202 861175 British Aerospace (Dynamics) Ltd. EMC Test House, EME Department FPC065 PO Box 5 Filton, Bristol BS 12 7QW 0117 9693 866 x6549
ICL Winsford EMC Laboratory West Avenue Kidsgrove, Stoke on Trent Staffs ST7 1TL 01782 771000 x3772
278
Cat 0 and I
Cat 0
NAMAS-ACCREDITED LABORArrORIES
Chase EMC Ltd. St Leonards House St Leonards Road London SW147LY 0181 878 7747
Cat 0
ERA Technology Ltd. Electromagnetic Compatibility Division Cleeve Road Leatherhead Surrey KT22 7SA 01372 374151
Cat 0
GEC Avionics Ltd. Central Quality Department Airport Works Rochester Kent MEl2XX 01634 844400 ext 3540
Cat 0
GEC Ferranti Defence Systems Ltd. EMC Laboratory St Andrews Works Robertson Avenue Edinburgh EH11 1PX 0131346 3912
279
Cat 0
Hunting Communication Technology Ltd. Cat 0 Electromagnetic Assessment Group Royal Signals and Radar Establishment Pershore W orcestershire WR 10 2RW 01386 555522 Kingston Telecommunication Laboratories Cat 0 Newlands Science Park Inglemire Lane Hull Humberside HU67TQ 01482 801801
Appendix 3.2
Cotnpetent bodies As given in the EMC Awareness Campaign brochure (Reference 11, Chap. 13).
Chase EMC Ltd. St Leonards House St Leonards Road London SW147LY 0181 878 7747
Assessmen t Services Ltd. Segensworth Road Titchfield Fareham Hampshire P0155RH 01329 443322
Dedicated Microcomputers Ltd. 1 Hilton Square Pendlebury Manchester M27 IDL 0161 794 4965
AQL-EMC Ltd. 16 Cobham Road Ferndown Industrial Estate Ferndown Poole Dorset BH21 7PE 01202 861175
EMC Projects Ltd. Holly Grove Farm Verwood Road Ashley Ringwood Hampshire BH24 2DB 01425 479979
BNR Europe Ltd. EMC Engineering Centre London Road, Harlow Essex CM179NA 01279 429531
ERA Technology Ltd. EMC Department Cleeve Road Leatherhead Surrey KT22 7SA 01372 374151
British Aerospace Defence Ltd. Dynamics Division Dept 319, FPC 047, PO Box 5 Filton Bristol BS 12 7RE 0117 969 3866
GEC Avionics Ltd. Central Quality Department Airport Works Rochester Kent ME12XX 01634 844400 x8097
British Telecom Research Laboratories EMC Engineering Group Martlesham Heath Ipswich Suffolk IP5 7RE 01473 642319
Hunting Communications Technology Ltd. Electromagnetic Assessment Group R.S.R.E. Pershore W orcestershire WR 10 2RW 01386 555522
Cambridge Consultants Ltd. Science Park Milton Road Cambridge CB4 4DW 01223 420024
IBM (UK) Ltd. EMC Laboratory PO Box 30 Spango Valley Greenock PA 16 OAH 01475 892000
CF Europe Ltd. Greencourts Business Park Styal Road Moss Nook Manchester M22 5LG 0161 436 8740
IBM (UK) Ltd. EMC Laboratory H ursley Park Winchester Hampshire S021 2]N 01962 844433 280
COMPETENT BODIES
Interference Technology International 41-42 Shrivenham Hundred Business Park Shrivenham Swindon SN6 8TZ 01793783137 JRS Associates 59 Titchfield Road Stubbington Fareham PO 14 2JF 01329 665549 Kingston Telecommunications Labs (KTL EMC) Newlands Science Park Inglemire Lane Hull Humberside HU6 7TG 01482 801801 MIRA Watling Street Nuneaton Warwickshire CV 10 OTU 01203 348541
Radio Frequency Investigation Ltd. Dunlop House Dunlop Ayrshire KA34BD 01560 83813
Radio Technology Laboratory Whyteleafe Hill Whyteleafe Surrey CR3 OYY 0181 660 8456
Serco Services Ltd. DMCS EMC Facility VI Building RSRE North Site Leigh Sinton Road Malvern Worcestershire WR 14 1LL 01684 592989
OFFER Electricity Meter Examining Service Hagley House Hagley Road Edgbaston Birmingham B 16 8QG
SGS EMC Services The Industrial Estate St Michael's Way Sunderland SR 1 3SD 0191 515 2666
Radio Frequency Investigations Ltd. Ewhurst Park Ramsdell Basingstoke Hampshire RG26 5RQ 01256 851193
TRL Technology Ltd. Alexandra Way Ashchurch Tewkesbury Gloucestershire GL20 8NB 01689 850438
281
Appendix 3.3
EMC consultancy and training Organisations offering consultancy and training in the UK in addition to competent bodies as given in the EMC Awareness Campaign brochure (Reference 11 Chap. 13). See also list of organisations identified as shadow UK Competen t Bodies and list of NAMAS accredited laboratories. Most offer consultancy, some also provide training.
Salford University Business Services Ltd. 0161 736 2843 Cherry Clough Consultants
01457 871605
*U niversity of Hertfordshire School of Engineering 01707 279176
Higher Degrees: MSc in Electromagnetic Compatibility University of Hull Department of Electronics Engineering 01482 465891
York Electronics Centre University of York 01904 432323
U niversi ty of York Department of Electronics
Paisley University
0141 848 3401
MSc in Electrical and Electromagnetic Engineering University of Wales Department of Electrical and Electronic Engineering Cardiff 01222874000
*Northern Ireland Technology Centre Queens University of Belfast 01232 245133 *University of Central England Birmingham Faculty of Engineering and Computer Technology 0121 331 5000 *Napier University Edinburgh Contact Alexander MacLeod
0131 444 2266
*Huddersfield University Manufacturing Advice Centre
01484 422288
lEE Professional' Development Stevenage 01438 313311 lEE Distance learning package, in conjunction with York Electronics Centre 01438 313311 Society of Environmental Engineers Buntingford 01763 71209
*The John Moore U niversi ty Liverpool School of Information, Science and Technology 0151 231 2052 *University of Plymouth School of Electrical Engineering
01904 432323
Elmac Chichester
01243 533361
Pera International Melton Mowbray 01664 501501 01752 232588 Schaffner EMC Reading 01734 770070
*Highbury College of Technology Department of Electrical and Electronic Engineering Portsmouth 01705 383131
Sira Communications (Training) Chislehurst 0181 467 2636
*Staffordshire U niversity . Stafford 01785 52331
Sira Test and Certification (Consultancy) Chislehurst 0181 467 2636
*university of Glamorgan The Technology Centre Pontypridd 01443 480480
Surrey Group Staines 01784 461393
*Denotes regional electronics centre offering EMC consultancy
Note: many other companies offer EMC traInIng and consulting; see current EM C trade press 282
Appendix 3.4
Useful publications on EMC 'UK Regulations' (SI 1992/2372) are available from HMSO and its agents.
CORP, M.B.: 'Zzaap! taming ESD, RFI and EMI'. (Academic) ISBN 0-12-189930-6
'EMC workbook'. DTI training aid available from Interference rr echnology International 0793 783137
DENNY, H.W.: 'Grounding for the control of EMI'. Don White Consultants ISBN 0-93226317-8
'EMC directive and amending directive'. Centre for European Business Information, Contact: Steve Terrell 071 828 6201 Also published in the Official Journal of the European Communities, ref: L139 23 May 1989
GEORGOPOULOS, C.J.: 'EMC In cables and interfaces'. ICT
European explanatory document on the EMC Directive, List of competent bodies, Guidance document on the content of the technical construction file, Product standards booklet on EMC DTI, 151 Buckingham Palace Road, London SW1W 9SS
SCHLICKE, H.M.: 'Electromagnetic compatibility: applied principles of cost effective con trol of electromagnetic interference and hazards'. (Marcel Dekker) ISBN 0-8247-1887-9
BSI 'EMC manual'
KEISER, B.: 'Principles of electromagnetic compatibility'. (Artech) ISBN 0-89006-206-4
VIOLETTE, WHITE and VIOLETTE: 'Electromagnetic compatibility handbook'. (Van Nostrand) ISBN 0-442-28903-0
0908 226888
Bibliography and information pack 1991, lEE technical information uni t, Coupland and Fountain
Texts that include rigorous analytical explanations of electromagnetic phenomena associated with EMC and which may be suitable for undergraduate or postgraduate study include:
WILLIAMS, T.: 'E~;[C for product designers'. (Bu tterworth-Heinemann)
PAUL, C.R.: 'Introduction to electromagnetic compatibility'. (Wiley, 1992)
MARSHMAN, C.: 'A guide to the EMC directive'. (York Electronics Centre, EPA Press)
GOEDBLOED, J.J.: 'Electromagnetic compatibility'. (Prentice-Hall, 1990)
ELLIS, N.: 'Electrical interference technology' (Woodhead Publishing) GOEDBLOED, J.: 'Electromagnetic ibility'. (Prentice Hall)
A text specifically concerned with explaining the European Community EMC Directive is
compat-
MARSHMAN, C.: 'Guide to the EMC Directive 89/336/EEC' (EPA Press) 1992
JACKSON, G.A.: 'The EMC Directive A third status report'. Proceedings of EMC 92 conference. ISBN 0 7008 0431 5
EMC in the 1990s is a rapidly developing field and no one book can be said to adequately cover it. Therefore a number of publications need to be available to any reader who seeks either a broad picture of the topic, or desires specific information on a detailed aspect of it. I t is hoped that the content and references given in this book enable most readers to acquire the information on EMC which they need. An additional way of acquiring information and learning new skills associated with EMC is via a distance learning programme such as that offered by the lEE. Brief details follow.
JACKSON, G.A.: 'The achievement of electromagnetic compatibility'. ISBN 0 7008 0400 5 'Safety and EMC' bimonthly newsletter. Contact for all these: Publication Sales, ERA Technology 0372 374151 OTT, H.W.: 'Noise reduction techniques in electrical systems'. (Wiley Interscience, 2nd edn.) 'U nderstanding, simulating and fixing ESD problems'. Don White Consultants ISBN 0-93 2263-27 -5 283
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A HANDBOOK FOR EMC TESTING AND MEASUREMENT
Electrom.agnetic com.patibility (with particular em.phasis on EC Directive 89j336jEEC) An lEE distance learning video course produced by the York Electronics Centre, University of York. The course consists of 12 videos, in four modules, total running time approximately 10 hours, with accompanying course notes. I t is intended to be a stand-alone training aid to be used either as an individual teaching package, or with tutored video instruction. The course identifies the range of disciplines involved in achieving electromagnetic compatibility and provides a sound basis for the application of design procedures to achieve EMC and in particular to ensure conformity with the EC Directive and the various CENELEC standards currently applicable. It has a modular structure to cater both for designers of electrical! electronic systems and those involved in EMC measuremen t and testing.
Module 1:
Video 1 EMC explanatory introduction. Covers the general nature of EMI, definitions, EMI propagation mechanisms, areas for concern e.g. ignition hazards, overview of EMC testing, design for EMC and future trends Module 2: Video 1 European Community Directive on EMC. Describes EC Directive including scope, essen tial protection req uiremen ts, compliance, certification and marking requirements, obligations of administra-
tion, legislation, flowchart of actions required by manufacturers and suppliers and relevant standards Video 2 Relevant standards Video 3 Achieving compliance Module 3:
Video 1 Introduction to EMC measurements RF measurement fundamentals Video 2 EM waves Radiation mechanisms Measurements and measurement systems Video 3 Screened rooms Open-field test sites Video 4 Practical measurements Conducted emission measurements Radiated emission measurements Radiated immunity Conducted immunity Electrostatic discharge Module 4: Video 1 Shielding and crosstalk Video 2 EMC on a PCB Video 3 Conducted interference techniques Video 4 CAD for EMC
suppresslon
In addition, three videos on EMC are available as part of the DTI awareness campaign available from: Technical Video Sales (New Electronics) 01322 222222. The videos are managers and and 'Rou tes to for these videos book.
titled: 'EMC an introduction for senior engineers', 'EMC design' compliance (testing)'. The scripts were written by the author of this
Index rod 45 size field averaging 169, 21 7 spot size 81 TEM cell 45, 46, 48, 119, 123, 126 transmission line 48, 199 tuned dipole 45, 48, 89, 90, 91, III wideband 96 Asymmetric TEM cell 126 Automatic EMC testing 151 emission testing 152 in the future 152 susceptibility testing 152
Absorbing current clamp 67,69 Accuracy in testing 42, 168, 214, 216, 21 7, 218 .Achieving product EMC 238 PC/IT example 238 Amplifiers, see Power amplifiers Analysers, see Spectrum analysers Anechoic screened chambers, see Screened rooms ANSI 16,23,133,170,190,206,261,265,274 Antenna aperture 74 balun 91, 170, 217 bandwidth 84 basics 72 beamwidth 79 biconic dipole 45, 48, 94, III bounded wave 45, 119, 199 calibration uncertainty 21 7 'cavitenna' 158 conical logarithmic spiral 45,48,98, 113 Crawford cell 45, 48, 123 dipole, diode detection 48 dipole, electrically short 92 dipole, log-periodic 96 dipole, tuned half-wave III effective length of 83 E field generator 115 electrically short dipole 48 74 factor, transmitting & fibre, optically coupled 45, 48 for EMC 72, 73 110 for radiated susceptibility free field 45, 48, Ill, 200 gain 73 GTEM cell 45, 48,110,126 horn 45, 48, 100 horn fed dish 48 input impedance 84, 169 log-periodic 45, 48, 96, 112 long wire 45, 48, 118 loop, active 106 loop, calibration of 106 loop, large 45 loop, passive 105 loop, small 45 magnetic field 105 monocone 48 monopole 45, 48, 86, 88 mutual impedance coupling 75 parallel plate line 45, 48, 119, 199 parallel plate line in screened room 122 phase centre 75 polarisation 83, 21 7 radhaz monitor 48 reflector 45, 103, 114 ridged horn 48, 102
Balanced hardening 233 Balun antenna, see Antenna, balun Balun losses 170 Biconic dipoles 94, III bow tie 94 commercial 94 use of 94 wire approximation 94 BPM (Deutsche Bundespost) 21 British Standards (EMC) 18, 133, 239, 254, 271 emission standards 18 for civil aircraft 18 immunity standards 18 BS727 33, 50, 93, 96 BS833 33, 254 BS905 coupling capacitor 58 BS905 LISN 50 British Standards (power disturbances) BS2914206 BS5406206 BS6662206 British Standards (product safety) 264, 272 Broadband/narrowband signals 250 Bulk current injection (BCI) 171, 175 Capacitor coupling RF 52, 56, 58 clamps 45, 59 distributed 45, 55, 59 feedthrough 44,48, 51, 55 for coupling to AC power circuits 54, 56 for coupling to I/O and control lines 55, 59 for use in BS6667 (IEC 801 pt 4) 53 for use in BS6667 (IEC 801 pt 5) 53,57 wideband 44 CENELEC EMC standards 18, 239, 254, 255, 256, 257 Circulator 151 CISPR £1\;11 meter detectors 21, 133 CISPR standards 16, 17,24,68,91,92, 133, 136, 154, 160,166,169,170,171,215,219,239,254 list of 258, 272 EMC standards 15,254 Civil and
285
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A HANDBOOK FOR EMC TESTING AND MEASUREMENT
Canadian E1\1 C standards 25, 275 limits 263 comparing EMC tests 19, 260, 262 compendium of E1\'1 C and related standards 271 derivation of commercial standards 17 derivation of military standards 15 ESD and transient standards 27 European commercial standards 20, 254, 255, 256, 257, 273 examples of EMC standards 16 FCC requirements 23 generation of CENELEC E1\:1 C standards 18 German EMC standards 21, 259, 272, 273 Japanese EMC standards 24, 275 other US commercial standards (not FCC) 24 product-specific UK military standards 33, 34 range of standards in use 15 service specific standards (military) 31 UK/European commercial standards 18, 273 UK military standards 31, 268, 269, 270, 275 UK standards-commercial equipment 18,271 USA commercial standards 23,261,274 USA military standards 28, 275 USA military standards (other than 461/2/3 and 6051D) 31 Clamp-RF current 67,69 Coaxial couplers 150 Compatibility matrix 227 Competent body list of 280 report from 246 Component burnout 196, 197, 232 Component upset levels 232 Cond ucted emission 48 Conduction and induction couplers 44, 48 Conformance test plan 40 Consultants (EMC) 223, 241 Consultants, list of 282 Control plan, see EMC control plan Convolution 183 Couplers, coaxial, see Coaxial couplers Couplers, directional, see Directional couplers Couplers, distributed capacitance 55, 59 Couplers, inductive 61, 67, 69 CoupIers, waveguide, see waveguide couplers Coupling capacitor, RF 52, 57, 58 Coupling, low-level swept, see Low-level swept coupling Coupling models (simple) 229 Coupling, radiative 45 Coupling to victims 5 Crosstalk, capacitive and inductive 230 Current clamp 67, 69 Current probes cable 44, 48, 61, 62, 172 injection 65 measurement method principle of 62 surface 44, 48, 66, 68 transfer function 63 Definitions of EMC 1 DEF STAN 00-35 203
DEF STAN 05-74/1265 DEF STAN 59-41 7, 14, 33, 34, 49, 60, 64, 65, 154, 171, 175,265,277 list of tests 270 Design (for EMC) 223 approach to 242 for contractual assistance 224 for formal compliance 224 handbooks 225 hardening 235 process 225 example of 243 techniques 236 Detectors, see Receivers, EMI Determining EMC requirement 241 DI-EMCS-8020 40 Diode detection dipole 48 J)ipole aperture model (coupling to cables) 231 electrically short 92 log-periodic 96, 112 non-resonant 93 Roberts 92 tuned 89, III tuned, commercial 91 tuned, practical 90 tuned, radiated emission testing 91 Direct connection devices 48 Directional couplers 148 Distributed capacitance couplers 55 DO 160 114,204 DTI 18, 218, 239, 252 EC 89/336 20, 41, 224, 246, 252 ECMA 190,240,273 ECSA 190 EEC 20, 21, 224, 246, 252, 253, 254, 259, 265, 273 EED 27 E-field generator 115 E-field levelling loop 117, 142 E-field sensors 48 EIA 23, 189, 241 EMAS 227 EMC computer models 31 control plan 224 defini tions of 1 design, see Design for EM C early problems 7 early problems with military equipment 9 hardening requirement 228 hardening techniques 235 historical background 7 information about 238 customer sources 238 industry sources 240 notified bodies 239 other sources 239 professional bodies 240 regulatory authorities 239 suppliers 240 legislation, see UK EMC legislation philosophy of 12
INDEX requirernent, see Determining EMC requirement sensor groups 44 serious problems with 10 standards and specifications 14, 224 compendium of271 strategy 242 system specification 224 technical disciplines 10 test plan 227 training 241 sources of 282 useful publications 283 EMI coupling to victims 5 receivers, intended and unintended 6 sources, broadcast 4, 5 sources, continuous 3 sources, intended 4 sources, man-made 3 sources, natural 3 sources of 1 sources, transient 3 sources, unintended 4 Emission suppression requirement 233 EN (European Norm) 19,21,41, 133, 136, 151,206, 218,239,254,255,256,257,273 Equipment case hardening requirement 233 ESD 27, 140, 179, 185,204 air discharge test 189 charged device model 187 con tact discharge test 189 direct injection 44, 48 event 185 field induced model 188 human body model 187 IEC80 1-2 new ESD test 190 indirect injection 45 latent defects 188 nurn ber of discharges required 191 probe 187 types of 187 types of test 188 vol tage test levels 191 waveform 186 EUROCARE lightning standard 204 European El\1C standards existing and possible future 256 existing with eq uivalents 254, 255 proposed product specific 257
JTRC Uapan) 24
FAA 204 FCC (Federal Comrrlunications Commission) 16,18,21, 22,41, 136, 160, 169, 170, 171,215,218,261, 262, 274 FCC pt 15j 16,23,24,89,91,92, 154,239,254,260, 261,262,274 FCC requirements 23 Feedthrough capacitor 44, 48, 51 Ferrite-cored loop 48 Ferrite wand 48 Fibre-optic transmitter 173 Filters 136 Fourier transforms 180
Lightning 179, 201, 228 discharge, definition of 202 effects on equipment 204 aerospace equipment 204 ground equipment 204 environment 201 LISN 44,48 BS3G100 50 BS727 50, 52 BS905 50, 53 DEF STAN 59-41 51 developmen t of 49 direct injection 50
287
Frequency meters 142 FTZ (Central telecommunications office, Germany) 18,22,218,260 German decrees and standards (EMC) 259 GS11 265 GTEM 45, 48, 110, 126 Hardening requirement, see EMC hardening req uiremen t Hardening techniques, see EM C hardening techniques HERO 27 High-impedance voltage probes 56 HIRF 114, 154 Historical background 7 HPM 154 Hybrid ring 150 IEC 1, 14, 16,27,206,239,264,273,274 IEC 555 28 IEC 80127,53,55,57,59,110,114,119,121,187,188, 206, 224, 254 IEC 801-2 ESD test (new) 190 lEE 240, 241, 282 IEEE 16,23, 106,206,239,240,241,275 IEMCAP 31,226 Immunity first? 243 Impulse generators 137 Impulse response 182 Induction windings 45, 48, 70 Inductive clamp 45, 67, 69 Inductively coupled devices 61 Information about EMC, see EMC, information about INIRC 27,265 Injection current probes 65, 175 Instrumentation for emission testing 130 for susceptibility testing 142 Intersystem and intrasystem EMI 7, 223 Intrasystem EMC 223, 226, 227 Inverted 'V' coupling model 231 IPP-1 226 IRAC 23 ISM (industrial, scientific and medical) 16, 17, 24, 25, 274 ISO 218, 274 Isolator 151
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A HANDBOOK FOR EMC TESTING AND MEASUREMENT
for testing commercial equipment 50 5 microhenry type 49 50 microhenry type 49 specification of 49 Losses in baluns 170, 217 in cables and connectors 170, 21 7 Low level swept coupling 172 Magnetic field probes 65 Magnetic field susceptibility tests 10 Magnetic induction tests 70, 107 Marconi 7 Mathematical modelling 11, 38, 39,44, 226, 227 Matrix, see Compatibility matrix Measurement devices for conducted EMI 48 MIL-B-5087B 203 MIL-E-6051 D 31 MIL STD 461/2/3 7, 17,28,33,35,40,41,49,51,52, 56,60,64, 70,86,95, 105, 118, 119, 136, 154, 184, 206, 277 MIL STD 461B 30, 268 MIL STD 461C (NEMP) 31, 197 MIL STD 461 equipment classes 266 equipment and subsystem classes 268 test changes between 461 Band 461 C 268 test requirements applicable to classes 267 MIL STD 83141 MIL STD 1541 17,61 Modelling, see Mathematical modelling Modulation rules 277 Modulators 147 arbitrary waveform generators 148 built in to equipment 147 req uiremen ts for 147 Monopole active 88 passive 86 MVEE 33 NAMAS 21,209,210,215,217,218,219,240,245, 246, 247 accredited laboratories, list of 278 advantages of laboratory accreditation 220 document list 272 requirements for laboratory accreditation 219, 220 Narrowband/broadband signals 250 NASA 23, 157,203 NATO 212 NBS (NIST) 23,93, 163, 164, 167 NCMDRH 23, 265, 275 Near field/far field boundary 76, 217 NEC model 39, 227 NEMA 189 NEMP 110,154, 179, 184, 192,204,228 bounded wave simulator 199 exoatmospheric pulse 193 free field simulator 200 induced currents 194 testing 195 components 196 equipment 197 system 198
types 193 NES 1006 33 Notified bodies 239 NPL 93, 167,221 documents 272 NRPB 27,265 NTIA 23 NVLAP 23 NW0320 National Weights and Measures Laboratory 19 NWS 3 33 NWS 1000 33 Open range test site 20,42,92, 165, 166, 167,246 Open range testing 165 site calibration 167 repeatability 168 antenna impedance changes with height 169 antenna size field averaging 169, 217 balun and VSWR losses 170,217 cable and connector losses 170, 217 differences in commercial antennas 170 EUT cables and peripherals 170 non-uniform field strength/range relationship 170 reflections from objects 168 weatherproof covers 168 testing proced ure 165 Ordnance Board 27 Oscilloscopes, digital 138 Parabolic reflector 114 Pick up on wires and cables 230, 231 Polarisation of antennas 83 Power amplifiers 144 frequency range 145 gain 145 gain compression 146 harmonic distortion 146 intermodulation distortion 146 output protection 147 power ou tpu t 145 specifying 145 TWT 147 Power disturbances 205 immunity standards 206 importance of transien ts 205 Power meter RF 141 Preselectors, see Spectrum analysers Product 25, 26 radiation hazards 26, 27, 265 regulations typical 264 safety mark \lDE and TUV 26 of electrical devices 25, 264, 272, 274 Product specific UK military standards 33, 34 Prod uction uncertain ty limits 219 Protection devices for amplifiers 148 Publications on EMC 283 Quasi peak detectors 20, 133 ANSI 20, 133 CISPR 20, 133 Radiated emission antennas 48
INDEX Radiated imrnunity field strengths 114 req uiremen ts for civil aircraft 114 req uiremen ts for commercial products 114 requirements for military 115 Radiated susceptibility testing 110 antennas used 11 0 standards requiring 110 Radiation hazards 26 limits for exposure 27 Radiative coupling (EMC antennas) 45 RAM 34, 42, 161 Receivers, EMI 130 commercially available examples 134 design of 130 detectors 133 Al\l/FM 134 average 134 peak 133 quasipeak 20, 133 slideback peak 133 n1easuremen t uncertainty 21 7 selectivity and sensitivity 131 Regulatory authorities 18,22,218,239 Repeatability in testing 41, 168, 21 7 Routes to compliance 244 RRE 6405 33, 268, 269, 270 RSRE 33 RTCA 23,261 SAE 16,23, 190,203,204,239,241,261,274 Scope of EMC activity 7 Screened rooms/chambers 154 anechoic screened chambers 159 full RAM solutions 160 partial RAM solutions 159 attenuation of 155, 156, 157 elliptical chamber 160 enclosed chamber testing 154 mode-stirred 163 novel facilities 164 reflections in 158 standard shielded enclosures 155 tapered anechoic 162 Self certification 246 SEMCAP 226 Signal bandwidth 250 Signal modulators, see Modulators Signal sources tracking generators 143 sweepers 143 synthesisers 142 Spectrum analysers 134 operation 135 preselectors and filters 136 types 134 Spiral induction windings 45, 48, 70 Spot size, antenna 81 Staff support for EMC 235 Standards and specifications for EMC 14 civil and military 15 contents of 14 the need for 14 the need to meet 14
Strip lines, see Antenna, parallel plate line Surface current probes 66, 68 Susceptibility hardening case study 231 System hardening (flow diagram) 233 System level requirements 228 System specification for EMC, see EMC system specification Technical construction file 244 circumstances requiring 245 contents of 245 or testing 246 'Technical disciplines in EMC 10 chemical knowledge 11 electrical engineering 10 legal aspects 11 mathematical modelling 11 physics 11 practical skills 12 quality assurance 12 systems engineering 11 Tempest 106, 154 Test differences between FCC and VDE 260 FCC, VDE and VCCI 262 Test facility cost of 248 in house 246 turnkey 248 Test plan, see EMC test plan Testing 38 accuracy 42 automatic 151 conformance 39 conformance test plan 40 development 38 preconformance 39 regimes 154 repeatability 41, 217 to verify modelling 38 Time domain 181, 182, 183 Time domain manipulation 184 Training, see EMC training Transformers audio 45,60 directly connected 60 high-voltage 60, 61 injection 48 Transient injection 60, 62, 140 Transients power, see Power disturbances Transient recorder, digital 140 Transient testing 179, 228, 246 transient types 179 Triboelectric series 186 Two box EMI problem 230 TUV safety mark 26 UK EMC legislation 252 UL (USA Underwriters Laboratories) 26, 264, 275 U ncertC\inty analysis 209 . combining random and systematic uncertainties 214 control factors 211, 216
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A HANDBOOK FOR EMC TESTING AND MEASUREMENT
coupling factors 211, 216 definition of terms 209 estimates of for EMC 216 estimates of total uncertainty 218 in EMC measurement 214 measurement factors 210 standard deviation 212 student's 't' distribution 213 sys tematic uncertain ty 213 random variables 211 uncertainty 212
VCCI 24, 154, 171,239,262,275 VDE standards 17, 21, 24, 105, 106, 133, 215, 239, 259, 260, 262, 264, 272 Voltage probes 44, 48, 56, 61 Voltmeters AFjRF 141 VSWR 121,144,168,170,215,217 Wave impedance in TEM cell 124 Wavefield impedance 76 Waveguide coupler 149
