emc and the printed circuit board -...

EMC AND THE PRINTEDCIRCUIT BOARD

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W. Michael KingNorm Violette, Violette Engineering Corp.

Books of Related Interest from IEEE Press

PRINTED CIRCUIT BOARD DESIGN TECHNIQUES FOR EMC COMPLIANCEMark I. Montrose1996 Cloth 256 pp IEEE Product No. PC5595 ISBN 0-7803-1131-0

CAPACITIVE SENSORS: Design and ApplicationsLarry Baxter1997 Cloth 320 pp IEEE Product No. PC5594 ISBN 0-7803-1130-2

EMC AND THE PRINTEDCIRCUIT BOARD

Design, Theory, and LayoutMade Simple

Mark I. MontroseMontrose Compliance Services, Inc.

IEEE Electromagnetic Compatibility Society, Sponsor

IEEE Press Series on Electronics TechnologyRobert Herrick, Series Editor

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Printed in the United States of America

1 0 9 8 7 6 5 4 3 2 1

ISBN 0-7803-4703-X

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Library of Congress Cataloging-in-Publication Data

Montrose, Mark I.EMC and the printed circuit board : design, theory, and layout made simple / Mark

I. Montrose.p. cm. — (IEEE Press series on electronics technology)

"IEEE Electromagnetic Compatibility Society, sponsor."Includes bibliographical references and index.ISBN 0-7803-4703-X (alk. paper)1. Printed circuits—Design and construction. 2. Electromagnetic

compatibility. I. IEEE Electromagnetic Compatibility Society.II. Title. III. Series.TK7868.P7M65 1998621.3815'31—dc21 98-35408

CIP

To my family

Margaret,

Maralena,

and Matthew

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Contents

PREFACE xiii

ACKNOWLEDGMENTS xvii

CHAPTER 1 EMC FUNDAMENTALS 1

1.1 Fundamental Definitions 21.2 EMC Concerns for the Design Engineer 4

1.2.1 Regulations 41.2.2 RFI 41.2.3 Electrostatic discharge (ESD) 51.2.4 Power disturbances 51.2.5 Self-compatibility 6

1.3 The Electromagnetic Environment 61.4 The Need to Comply (A Brief History of EMI) 91.5 Potential EMI/RFI Emission Levels for Unprotected

Products 121.6 Methods of Noise Coupling 121.7 Nature of Interference 16

1.7.1 Frequency and Time (a la Fourier: timedomain <=> frequency domain) 17

1.7.2 Amplitude 181.7.3 Impedance 181.7.4 Dimensions 18

1.8 PCBs and Antennas 181.9 Causes of EMI—System Level 191.10 Summary for Control of Electromagnetic Radiation 20

References 21

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viii Contents

CHAPTER 2 EMC INSIDE THE PCB 23

2.1 EMC and the PCB 232.1.1 Wires and PCB-traces 252.1.2 Resistors 252.1.3 Capacitors 262.1.4 Inductors 262.1.5 Transformers 27

2.2 Theory of Electromagnetics (Made Simple) 282.3 Relationship Between Electric and Magnetic Sources

(Made Simple) 302.4 Maxwell Simplified—Further Still 34

2.5 Concept of Flux Cancellation (Flux Minimization) 37

2.6 Skin Effect and Lead Inductance 39

2.7 Common-Mode and Differential-Mode Currents 412.7.1 Differential-mode currents 422.7.2 Differential-mode radiation 422.7.3 Common-mode currents 442.7.4 Common-mode radiation 462.7.5 Conversion between differential and common mode 46

2.8 Velocity of Propagation 47

2.9 Critical Frequency (l/20) 49

2.10 Fundamental Principles and Concepts for Suppressionof RF Energy 492.10.1 Fundamental principles 492.10.2 Fundamental concepts 49

2.11 Summary 51

References 52

CHAPTER 3 COMPONENTS AND EMC 53

3.1 Edge Rate 53

3.2 Input Power Consumption 56

3.3 Clock Skew 583.3.1 Duty cycle skew 593.3.2 Output-to-output skew 593.3.3 Part-to-part skew 60

3.4 Component Packaging 60

3.5 Ground Bounce 65

3.6 Lead-to-Lead Capacitance 69

3.7 Grounded Heatsinks 70

3.8 Power Filtering for Clock Sources 74

3.9 Radiated Design Concerns for Integrated Circuits 763.10 Summary for Radiated Emission

Control—Component Level 78

References

Contents ix

CHAPTER 4 IMAGE PLANES 81

4.1 Overview 814.2 5/5 Rule 834.3 How Image Planes Work 84

4.3.1 Inductance 844.3.2 Partial inductance 854.3.3 Mutual partial inductance 864.3.4 Image plane implementation and concept 88

4.4 Ground and Signal Loops (Not Eddy Currents) 91

4.4.1 Loop area control 92

4.5 Aspect Ratio—Distance Between Ground Connections 954.6 Image Planes 974.7 Image Plane Violations 994.8 Layer Jumping—UseofVias1024.9 Split Planes 1044.10 Partitioning 106

4.10.1 Functional subsystems 1064.10.2 Quiet areas 106

4.11 Isolation and Partitioning (Moating) 1074.11.1 Method 1: Isolation 1084.11.2 Method 2: Bridging 108

4.12 Interconnects and RF Return Currents 1124.13 Layout Concerns for Single- and Double-Sided Boards 114

4.13.1 Single-sided PCBs 1154.13.2 Double-sided PCBs 1164.13.3 Symmetrically placed components 1164.13.4 Asymmetrically placed components 118

4.14 Gridded Ground System 1194.15 Localized Ground Planes 120

4.15.1 Digital-to-analog partitioning 1224.16 Summary 123

References 124

CHAPTER 5 BYPASSING AND DECOUPLING 125

5.1 Review of Resonance 1265.1.1 Series resonance 1275.1.2 Parallel resonance 1285.1.3 Parallel C—Series RL resonance (antiresonant

circuit) 128

5.2 Physical Characteristics 1295.2.1 Impedance 1295.2.2 Energy storage 1315.2.3 Resonance 1325.2.4 Benefits of power and ground planes 134

x Contents

5.3 Capacitors in Parallel 1365.4 Power and Ground Plane Capacitance 138

5.4.1 Buried capacitance 1415.4.2 Calculating power and ground plane capacitance 142

5.5 Lead-Length Inductance 1435.6 Placement 144

5.6.1 Power planes 1445.6.2 Decoupling capacitors 144

5.7 Selection of a Decoupling Capacitor 1485.7.1 Calculating capacitor values (wave-shaping) 149

5.8 Selection of Bulk Capacitors 1525.9 Designing a Capacitor Internal to a Component's

Package 1555.10 Vias and Their Effects in Solid Power Planes 157

References 158

CHAPTER 6 TRANSMISSION LINES 159

6.1 Overview on Transmission Lines 1596.2 Transmission Line Basics 1626.3 Transmission Line Effects 1636.4 Creating Transmission Lines in a Multilayer PCB 1656.5 Relative Permittivity (Dielectric Constant) 166

6.5.1 How losses occur within a dielectric 1696.6 Routing Topologies 171

6.6.1 Microstrip topology 1716.6.2 Embedded microstrip topology 1726.6.3 Single stripline topology 1746.6.4 Dual stripline topology 1756.6.5 Differential microstrip and stripline 177

6.7 Routing Concerns 1786.8 Capacitive Loading 180

References 182

CHAPTER 7 SIGNAL INTEGRITY AND CROSSTALK 185

7.1 Need for Signal Integrity 1857.2 Reflections and Ringing 188

7.2.1 Identification of signal distortion 1917.2.2 Conditions that create ringing 192

7.3 Calculating Trace Lengths (Electrically LongTraces) 195

7.4 Loading Due to Discontinuities 2007.5 RF Current Distribution 202

Contents xi

7.6 Crosstalk 2037.6.1 Units of measurement—Crosstalk 2067.6.2 Design techniques to prevent crosstalk 207

7.7 The 3-W Rule 210

References 212

CHAPTER 8 TRACE TERMINATION 215

8.1 Transmission Line Effects 2168.2 Termination Methodologies 217

8.2.1 Source termination 2218.2.2 Series termination 2218.2.3 End termination 2268.2.4 Parallel termination 2278.2.5 Thevenin network 2308.2.6 RC network 2348.2.7 Diode network 236

8.3 Terminator Noise and Crosstalk 237

8.4 Effects of Multiple Terminations 239

8.5 Trace Routing 241

8.6 Bifurcated Lines 2438.7 Summary—Termination Methods 244

References 245

CHAPTER 9 GROUNDING 247

9.1 Reasons for Grounding—An Overview 247

9.2 Definitions 247

9.3 Fundamental Grounding Concepts 249

9.4 Safety Ground 2539.5 Signal Voltage Referencing Ground 254

9.6 Grounding Methods 2559.6.1 Single-point grounding 2569.6.2 Multipoint grounding 2599.6.3 Hybrid or selective grounding 2619.6.4 Grounding analog circuits 2619.6.5 Grounding digital circuits 262

9.7 Controlling Common-Impedance CouplingBetween Traces 2629.7.1 Lowering the common-impedance path 2629.7.2 Avoiding a common-impedance path 264

9.8 Controlling Common-Impedance Couplingin Power and Ground 266

9.9 Ground Loops 2689.10 Resonance in Multipoint Grounding 271

xii Contents

9.11 Field Transfer Coupling of Daughter Cardsto Card Cage 273

9.12 Grounding (I/O Connector) 277References 277

GLOSSARY 279

BIBLIOGRAPHY 287

APPENDIX

A The Decibel 291B Fourier Analysis 294C Conversion Tables 298D International EMC Requirements 302

INDEX 317

ABOUT THE AUTHOR 325

Preface

EMC and the Printed Circuit Board: Design, Theory, and Layout Made Simple is a com-panion book to Printed Circuit Board Design Techniques for EMC Compliance. When usedtogether, these two books cover all aspects of a PCB design as it relates to both time and fre-quency domain issues. One must be cognizant that if a PCB does not work as intended in thetime domain, frequency domain concerns become irrelevant, especially compliance to in-ternational EMC requirements. Time and frequency domain aspects must be considered to-gether.

The intended audience for this book is the same as that for Printed Circuit BoardDesign Techniques for EMC Compliance: those involved in logic design and PCB layout;test engineers and technicians; those working in the areas of mechanical, manufacturing,production, and regulatory compliance; EMC consultants; and management responsiblefor overseeing a hardware engineering design team.

Regardless of the engineer's specialty, a design team must come up with a productthat not only can be manufactured in a reasonable time period, but will also minimize costduring design, test, integration, and production. Frequently, more emphasis is placed onfunctionality to meet a marketing specification than on the need to meet legally mandatedEMC and product safety requirements. If a product fails to meet compliance tests, re-design or rework may be required. This redesign significantly increases costs, which in-clude, but are not limited to engineering manpower (along with administrative overhead),new PCB layout and artwork, prototyping material, system integration and testing, pur-chase of new components for quick delivery (very expensive), new in-circuit test fixtures,and documentation. These costs are in addition to loss of market share, delayed shipment,loss of customer faith in the company (goodwill), drop in stock price, anxiety attacks, andmany other issues. Personal experience as a consultant has allowed me the opportunity towitness these events several times with small startup companies.

My main focus as a consultant is to assist and advise in the design of high-technol-ogy products at minimal cost. Implementing suppression techniques into the PCB designsaves money, enhances performance, increases reliability, and achieves first-time compli-ance with emissions and immunity requirements, in addition to having the product func-tion as desired.

Working in this industry has allowed me to participate in state-of-the-art designs aswe move into the future. Although my focus is on technology of the future, one cannot

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xiv Preface

forget that simple, low-technology products are being produced in ever increasing num-bers. Although the thrust of this book is toward high-end products, an understanding ofthe fundamental concept of EMC suppression techniques will allow any PCB being de-signed to pass EMC tests. The key words here are "fundamental concepts." When onedoes not understand fundamental concepts, compliance and functional disaster may await.

When management decides to bring in a consultant after production has started,having failed an EMI test, causing a stop-ship situation, it is too late for efficiency. Gener-ally, nothing can be done without major expenses being incurred. I have watched smallcompanies go bankrupt because they invested all their capital in a product for quick ship-ment and then had to redesign everything from scratch. Those who control the finances ofa company by mandating cost over compliance have frequently been spotted working at adifferent company every year. Accountants who do not understand what it takes to be ahardware or PCB designer engineer can doom a company to failure.

Sometimes, use of a single component (filter) costing $0.50 is too much for man-agement to accept on a $1000 product. Engineers may be able to implement a redesign toprevent use of this inexpensive filter. This redesign may cost the company tens of thou-sands of dollars (including new compliance tests) for a production build of a few hundredunits. Although the accountant may receive bonus pay for keeping the cost of the PCBdown, the Return-On-Investment (ROI) will never be achieved. I do not advocate addingcost to a design unless it is mandatory. High-technology products now require use of ad-ditional power and ground planes, filter components, and the like, all at a cost for bothfunctionality and compliance.

Detailed definitions of various terms are presented within specific chapters of thisbook. Before we proceed, an important distinction is in order. EMC stands for Electro-magnetic Compatibility. This means that electrical equipment must work within an in-tended environment. We can have EMI (Electromagnetic Interference) problems due toincompatibilities between equipment. EMC is achieved; EMI occurs. According to com-mon usage, EMC refers to the total discipline concerned with achieving electromagneti-cally compatible equipment and systems. EMI refers to the event or episode indicating anincompatibility, for example, a lack of EMC. EMI refers to all events experienced acrossthe frequency spectrum. Radio Frequency Interference (RFI) originally referred to thoseincompatibilities arising between radio sets. During the 1970s and 1980s, RFI was gener-ally not used because it failed to indicate the problems that can arise from Electromag-netic Pulse (EMP), lightning, Electrostatic Discharge (ESD), and so on. Over the past fewyears, however, RFI has been creeping back into our vocabulary. Caution should be usedwith the acronym RFI, however, for its meaning is unclear in the field of EMC.

The main differences between my two books on EMC and PCBs are as follows.Printed Circuit Board Design Techniques for EMC Compliance provides informa-

tion for those who have to get a product designed and shipped within a reasonable timeframe and within budget. It illustrates that a PCB may exhibit an EMI problem, it brieflyexplains why the problem occurs, and it shows how to solve the design flaw during lay-out. It does not go into detail on how and why EMI occurs, theoretically.

University textbooks are available (listed in the References sections) that cover allaspects of theoretical physics related to EMC. Numerous other publications present EMCconcepts in a brief manner, giving just enough detail to make one aware of theory withminimal mathematical analysis. Many managers and some engineers do not care about

Preface xv

why something happens. Printed Circuit Board Design Techniques for EMC Compliancehas compiled a track record of successful results.

EMC and the Printed Circuit Board: Design, Theory, and Layout Made Simple is acompanion book targeted at those designers who want to know how and why EMI occurswithin a PCB. These designers may not be directly responsible for the actual PCB layout,but they may have to oversee the end product. Engineers generally want to understandtechnical concepts. This book is written for ease of understanding a subject that is gener-ally not taught in universities or other educational environments, again using a minimalamount of math.

In the present book, we examine two sides of the coin—time domain (signal func-tionality and quality) and frequency domain (EMC). A signal that is present within thePCB may be viewed in both domains. No difference exists between the two; rather, onlythe way one examines a signal. Test instrumentation also differs. Chapter 2 illustratesusing simplified physics, with minimal mathematical analysis, how these two domainsexist simultaneously. Theory is presented in a format that is easy to comprehend in thelimited time one has to read and study a book on EMC and PCB, especially when workneeds to be done at the office.

The focus of this book is strictly on the PCB. Discussion of containment techniques(box shielding), internal and external cabling, power supply design, and other system-level subassemblies that use PCBs as subcomponents will not be discussed in depth.Again, excellent reference material is listed in the References on these aspects of EMCsystem-level design engineering.

The incentive for writing this book has come from my numerous seminar and work-shop students in the United States, Europe, and Asia. These students ask, "How and whydoes EMI get developed within a PCB?" Recognizing a need to fill a gap that currentlydoes not exist within the published literature in the public domain worldwide (at time ofwriting), I want to enlighten the reader to a field of engineering that is considered to be aBlack Magic art. Those who do not take electromagnetic compatibility seriously providejob security for EMC engineers. EMC engineers know various tricks of the trade on howto apply rework or a quick fix to a PCB to pass a particular test. These under-the-pressureenhancements implemented during compliance testing are identified as Band-Aid tech-niques. These PCBs could have been designed properly from the start. The concept ad-vanced is to change design habits and thinking from Band-Aids to low-cost suppressionlayout techniques during the design cycle.

Mark I. MontroseSanta Clara, California

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Acknowledgments

I want to acknowledge the following individuals who played a part in the review cycle ofthis book.

Bill Kimmel and Daryl Gerke, Kimmel & Gerke Associated, Ltd., both of whomperformed a thorough and technical review not only of this book, but also Printed CircuitBoard Design Techniques for EMC Compliance. As always, both gentlemen provided ex-cellent input to ensure accuracy.

Elya B. Joffe, K.T.M. Project Engineering, Ltd., Israel, who was a technique re-viewer scrutinizing everything for accuracy, in addition to providing numerous commentsin the interest of enhancing the reader's ability to understand complex concepts.

Bob Herrick, Department of Electrical Engineering, Purdue University, who as edi-tor of the IEEE Press Series on Electronic Technology, served as my direct interface withIEEE Press.

David Angst, TCAD, provided a sanity check on the time-domain portion of thisbook.

William Hubbard, Department of Electrical Engineering, Purdue University, for acomprehensive review on all subject matter.

Norm Violette, Violette Engineering Corporation, who performed a technical re-view and also provided insights into unique aspects of EMC theory with a different pointof view.

Hugh Denny, Georgia Technical Research Institute, who as the IEEE EMC Soci-ety's liaison to IEEE Press, and a technical reviewer, took me to task on various issuespresented to guarantee the accuracy of my discussions.

Hans Melberg, EMC Consultant, who gave me my first opportunity to documentmy knowledge of EMC and PCBs into written form at Wyse Technology. This documentinspired me to write my first book, Printed Circuit Board Design Techniques for EMCCompliance.

A special acknowledgment is given to W. Michael King, my mentor in the field ofEMC. Without his friendship, expertise, and the ability to teach me complex aspects ofEMC, along with providing a comprehensive technical review of the material, this bookcould not have been written.

xvii

xviii Acknowledgments

My most special acknowledgment is to my family. Margaret, my wife, and my twochildren, Maralena and Matthew. After surviving the time and effort it took me to writemy first book, my family stood by me again as I spent months to write another. Their un-derstanding and support are beyond belief.

Mark I. MontroseSanta Clara, California

EMC Fundamentals

This book seeks primarily to help engineers minimize harmful interference between com-ponents, circuits, and systems. These interferences include not only radiated and con-ducted radio frequency (RF) emissions, but also the influences of electrostatic discharge(ESD), electrical overstress (EOS), and radiated and conducted susceptibility (immunity).Meeting these requirements will satisfy legally mandated international and domestic regu-latory requirements and governmental regulations. A companion book, Printed CircuitBoard Design Techniques for EMC Compliance, presents design rules and layout con-cepts that assist in achieving an EMC-compliant product using suppression design tech-niques.

One of the engineer's goals is to meet design requirements in order to satisfy bothinternational and domestic regulations and voluntary industrial standards related to EMCcompliance.

The information presented in this book is intended for

Non-EMC engineers who design and layout printed circuit boards (PCBs).

EMC engineers and consultants who must solve design problems at the PCBlevel.

Design engineers who want to understand fundamental concepts related to howelectromagnetic interference (EMI) exists within a PCB.

Those who want a comprehensive understanding of how PCB design and layouttechniques work within a PCB.

This book is applicable for use as a reference document throughout any design project.With these considerations in mind, the reader should understand that EMC and the

Printed Circuit Board is written for the engineer who never studied applied electromag-netics in school, requires a refresher course, or has minimal hands-on experience withhigh-speed, high-technology product designs. As we well know, technology is advancing

1

2 Chapter 1 EMC Fundamentals

at a rapid rate. Design techniques that worked several years ago are no longer effective intoday's products with high-speed digital design requirements. Because EMC may be in-sufficiently covered in engineering schools, training courses and seminars are now beingheld all over the country and internationally to provide this information.

Only a minimal amount of mathematical analysis is presented here because the in-tent of this book is to present a basic understanding and analysis of how a PCB createsRF energy, and the manner in which RF energy is propagated. The information presentedis therefore in a format that is easy both to understand and to implement.

Since World War II, controlling emissions from a product has been a necessity foracceptable performance of an electronic device in both the civilian and military environ-ment. It is more cost-effective to design a product with suppression at the source than to"build a better box." Containment measures are not always economically justified andmay degrade as the life cycle of the product is extended beyond the original design speci-fication. For example, the end user often removes covers from enclosures for ease of ac-cess to repair or upgrade. In many cases, sheet metal covers are never replaced, particu-larly those internal subassembly covers that act as partition shields. The same is true forblank metal panels or faceplates on the front of a system that contains a chassis or back-plane assembly. As a result, containment measures become compromised. Proper layoutof a PCB with suppression techniques also promotes EMC compliance with use of cablesand interconnects, whereas box shielding (containment) does not. In addition to EMCcompliance, signal functionality concerns exist. It does us no good if a product passesEMC tests and then fails to operate as designed.

This book provides details on why a variety of design techniques work for mostPCB layout applications. It is impossible to anticipate every possible application or designconcern. The concepts presented are fundamental in nature and are applicable to all elec-tronic products. While every design is different, the basics of product design rarelychange unless new components and materials become available.

Herein we discuss high-technology, high-speed designs that require new and ex-panded techniques for EMC suppression at the PCB level. Many traditional PCB tech-niques are not effective for proper signal functionality and compliance. Components havebecome faster and more complex. Use of custom gate array logic and application-specificintegrated circuits (ASICs) presents new and challenging opportunities. The designand layout of a PCB to suppress EMI at the source can be realized while maintainingsystemwide functionality.

Why worry about EMC compliance? After all, isn't speed the most important de-sign parameter? Legal requirements dictate the maximum permissible interference poten-tial of digital products. These requirements are based on experiences in the marketplacerelated to emission and immunity complaints. Often, suppression techniques on a PCBwill aid in improving signal quality and signal-to-noise performance.

1.1 FUNDAMENTAL DEFINITIONS

The following basic terms are used throughout this book.

Electromagnetic Compatibility (EMC). The capability of electrical and electronicsystems, equipment, and devices to operate in their intended electromagnetic envi-

Section 1.1 Fundamental Definitions 3

ronment within a defined margin of safety, and at design levels or performance,without suffering or causing unacceptable degradation as a result of electromagneticinterference. (ANSI C64.14-1992)

Electromagnetic Interference (EMI). The lack of EMC, since the essence ofinterference is the lack of compatibility. EMI is the process by which disrup-tive electromagnetic energy is transmitted from one electronic device to an-other via radiated or conducted paths (or both). In common usage, the termrefers particularly to RF signals, but EMI can occur in the frequency rangefrom "DC to daylight."

Radio Frequency (RF). A frequency range containing coherent electromagneticradiation of energy useful for communication purposes—roughly the range from10 kHz to 100 GHz. This energy may be transmitted as a byproduct of an electronicdevice's operation. RF is transmitted through two basic modes:

Radiated Emissions. The component of RF energy that is transmittedthrough a medium as an electromagnetic field. Although RF energy is usuallytransmitted through free space, other modes of field transmission may occur.Conducted Emissions. The component of RF energy that is transmittedthrough a medium as a propagating wave, generally through a wire or inter-connect cables. LCI (Line Conducted Interference) refers to RF energy in apower cord or AC mains input cable. Conducted signals do not propagate asfields but may propagate as conducted waves.

Susceptibility. A relative measure of a device or a system's propensity to be dis-rupted or damaged by EMI exposure to an incident field of signal. It is the lack ofimmunity.Immunity. A relative measure of a device or system's ability to withstand EMI ex-posure while maintaining a predefined performance level.

Electrostatic Discharge (ESD). A transfer of electric charge between bodiesof different electrostatic potential in proximity or through direct contact. Thisdefinition is observed as a high-voltage pulse that may cause damage or lossof functionality to susceptible devices. Although lightning qualifies as a high-voltage pulse, the term ESD is generally applied to events of lesser amperage,and more specifically to events that are triggered by human beings. However,for the purposes of discussion, lightning is included in the ESD category be-cause the protection techniques are very similar, though different in magnitude.Radiated Immunity A product's relative ability to withstand electromagneticenergy that arrives via free-space propagation.Conducted Immunity. A product's relative ability to withstand electromag-netic energy that penetrates it through external cables, power cords, and I/Ointerconnects.

Containment. A process whereby RF energy is prevented from exiting an enclo-sure, generally by shielding a product within a metal enclosure (Faraday cage orGaussian structure) or by using a plastic housing with RF conductive paint. Recip-rocally, we can also speak of containment as preventing RF energy from enteringthe enclosure.Suppression. The process of reducing or eliminating RF energy that exists withoutrelying on a secondary method, such as a metal housing or chassis. Suppressionmay include shielding and filtering as well.


1.2 EMC CONCERNS FOR THE DESIGN ENGINEER

Within the field of EMC, multiple design concerns exist. Most items identified here arenot obvious. Past experience determines the amount of effort required to address these is-sues as they relate to EMC compliance along with signal functionality. Awareness of fivekey areas is mandatory for understanding why electromagnetic compatibility is required.With an understanding of these five areas, we can reduce difficult problems to simple ap-plications of design techniques and implementations. About 95% of all EMC issues en-countered are associated with the following. Each will be discussed separately [2].

1. Regulations2. RFI

3. Electrostatic discharge

4. Power disturbances5. Self-compatibility

1.2.1 Regulations

Part of the need for regulations stems from complaints regarding interference toelectronic products used in both residential and commercial applications and part from therequirement to protect vital communication services. Without regulations, the "electro-magnetic environment" in which we live would be crowded with interference and only afew electronic devices could survive and operate.

Regulations protect the radio spectrum and limit "spurious" radiation from both in-tended radiators (such as transmitters) and unintended radiators (most electronic equip-ment). Numerous consumer complaints developed basically over interference to televi-sion and radio reception. In addition, aeronautical communication systems started tobreak down; police and fire units were unable to use their radios for emergency purposes;and commercial and residential electronic products were failing in the field owing to thepresence of other electronic equipment located in the general vicinity. With these com-plaints, the Federal Communications Commission (FCC) developed a set of requirementsfor electronic equipment that would limit the amount of interference polluting the electro-magnetic environment. The FCC followed the lead of Germany's Verband DeutscherElectrotechniker (VDE), which implemented mandatory requirements shortly after WorldWar II. Other countries worldwide have followed the VDE and FCC in developing re-quirements for digital products.

Regulations control not only emissions but also susceptibility (or immunity). Euro-peans have taken the lead in mandating immunity tests; in North America, however, thesesame tests are only voluntary at the time of this writing.

1.2.2 RFI

Radio Frequency Interference (RFI) poses a threat to electronic systems due to theproliferation of radio transmitters that exist. Cellular phones, handheld radios, wireless re-mote control units, pagers, and the like are now quite widespread. It does not take a greatdeal of radiated power to cause harmful interference. Typical equipment failures occur in

Section 1.2 EMC Concerns for the Design Engineer 5

the electric field level range of 1 to 10 volts/meter. For example, a 1 watt radio transmitterat 1 meter distance from an electronic device has a field strength of approximately 5 V/m,depending on the frequency and antenna used for measurement purposes. Preventing RFIfrom corrupting a device has become legally mandatory for all products used within Eu-rope, North America, and many Asian countries.

1.2.3 Electrostatic Discharge (ESD)

ESD technology has progressed to the point where components have become ex-tremely dense along with small geometries (0.18 micron). The sensitivity of high-speed,multimillion transistor microprocessors is easily damaged by external ESD events. Theseevents can be caused by either direct or radiated means. Direct contact ESD events gener-ally cause permanent damage of the device or create a latent failure mode that will triggerpermanent damage sometime in the future. Radiated ESD events (caused, for instance, byfurniture moving in a room, reflected ESD energy off a structure, or a person walkingacross a carpet) can cause an upset in the device that may result in improper operationwithout leading to permanent damage to the system.

An ESD event is considered to be a broadband high-frequency problem with edgerates that are usually less than 1 nanosecond. This translates to a spectral bandwidth prob-lem that can approach 1 GHz. It is not uncommon to observe ESD in the sub-nanosecondtime period. This faster edge rate becomes a problem well into the gigahertz spectralbandwidth.

ESD is treated under the immunity requirements for compliance with the EU's (Eu-ropean Union's) EMC Directive. Most manufacturers worldwide recognize this problem.These manufacturers must design suppression and layout techniques into their products toguarantee that failure will not occur in the field.

1.2.4 Power Disturbances

With more and more electronic equipment being plugged into the power mains net-work, potential interference occurs. These problems include power-line disturbances,electrical fast transients (EFT), power sag and surges, voltage variations (high/low volt-age levels), lightning transients, and power-line harmonics. Older products and powersupplies were generally not affected by these disturbances. With newer, high-frequencyswitching power supplies, these disturbances are starting to become noticeable as theswitching components consume AC voltage generally on the crest of the waveform, notthe complete waveform.

Analog and digital devices respond differently to power-line disturbances. Digitalcircuits are affected by spikes on the power system (EFT and lightning), as well as failuredue to excessively high or low voltage levels. Analog devices generally operate on volt-age levels, which may be degraded by a disturbance changing the reference level of thesystem's power source.

Power-line harmonics have become a major concern, especially in Europe. Nonlin-ear loads (switching power supplies) consume AC mains power at the peak of the cyclerather than over the entire sine wave. This varying load generates harmonics and wave-form distortions that affect the power distribution network. For example, it is common tosee 230 VAC, 150 Hz (third harmonic), or 250 Hz (fifth harmonic) present in a power


system that is intended to operate at 50 Hz consuming various levels of input current atthese higher frequencies.

1.2.5 Self-Compatibility

A commonly overlooked issue is self-compatibility. A digital partition or circuit caninterfere with analog devices, create crosstalk between traces and wires, or a fan motormay cause an upset with digital circuits. While most of these concerns are known to thesystem designer, these failures are not recognized as an EMI event. Recognition of thisconcern, along with design implementations that prevent internal system failures from oc-curring, will result in a less expensive and more robust system.

1.3 THE ELECTROMAGNETIC ENVIRONMENT

A product must operate within a particular environment compatible with other electronicequipment. To understand the need for compatibility in an environment where productsmust operate, we now examine this environment.

Any periodic signal (clock) generates a wide spectrum of RF energy when viewedin the frequency domain. Figure 1.1 illustrates a spectral plot of a nonsinusoidal oscillatorin the frequency range between 30 and 200 MHz. In studying this plot, we observe notonly the fundamental frequency of the oscillator (1.8432 MHz), but also all the harmonicscreated across the 170 MHz window. A low-frequency oscillator was chosen to illustratethis wide harmonic spectrum. The spectral bandwidth of the oscillator is determined bythe "edge rate" of the oscillator, not the "clock rate." A detailed discussion of why the"edge rate" of the digital pulse signal is of more concern than operating "frequency" ispresented in Chapter 3.

Using this same oscillator waveform, we examine a very narrow frequency range.Figure 1.2 shows that both even and odd harmonics of the primary oscillator in the fre-quency range of 88-108 MHz are present.

The FM radio band (88-108 MHz) is allocated to a specific range of pre-assignedfrequencies. Many digital products produce unintentional radiated RF energy within thisfrequency spectrum, especially lower order harmonics. In Fig. 1.3, we observe two traces.The upper trace displays FM radio signals. For this example, the spectrum analyzer wasconfigured to make the FM radio signal appear similar to the signature characteristics ofour clock signal. To help differentiate between the clock harmonics and the FM radio sig-nals, a 10-dB displacement is observed in Fig. 1.3 with the FM signals shown 10 dBhigher above the oscillator. The lower trace is a narrow-band view of the oscillator in thesame frequency range. Notice that the signals measured are harmonics from the 1.8432-MHz oscillator. With this situation, potential interference between the oscillator and FMsignal may exist. This scenario can be applied to any communications system, such as be-tween a nonintentional radiator (digital device) and aeronautical communications, or anemergency services broadcast.

To illustrate the effects of a design change, the lower trace in Fig. 1.4 represents acompliant product. Changing just one component, moving a single trace, or using an alter-nate manufacturer of a logic family for the same function (74Fxx in place of a 74LSxx)

Section 1.3 The Electromagnetic Environment 7

Figure 1.1 Oscillator and related harmonics (30-200 MHz).

now makes a compliant product noncompliant. This plot should enlighten those skepticswho believe that an alternate device that is identical in form, fit, and function can be eas-ily substituted. Although the component may be functionally 100% compatible, its effectson changing the overall EMC characteristics may be radically different. The edge rate ofthe source driver may differ between vendors, although functionality remains the same.Not all components are the same, EMI considered. Chapter 3 provides details on how andwhy different components with the same function can cause functionality and EMI con-cerns.

Although difficult to observe in Fig. 1.4, we are able to distinguish the effects of asimple change to the circuit, especially in the middle frequency range of the plot.

Designing products that will pass legally required EMI tests is not as difficult as onemight expect. Engineers often strive to design elegant products, but elegance sometimesmust give way to product safety, manufacturing, cost, and, of course, EMC. Such abstractproblems can be challenging, particularly if the engineer is unfamiliar with compliance ormanufacturing requirements. We must remove the mystery from the "Hidden Schematic"syndrome.

When an EMI problem occurs, the engineer should approach the situation logically.A simple EMI model has three elements:


Figure 1.2 Oscillator and harmonics within a narrow frequency range (88-108 MHz).

1. There must be a source of energy.2. There must be a receptor that is upset by this energy when the intensity of the

electromagnetic interference is above a tolerable limit.3. There must be a coupling path between the source and receptor for the un-

wanted energy transfer.

For interference to exist, all three elements have to be present. If one of the three el-ements is removed, there can be no interference. It therefore becomes the engineer's taskto determine which is the easiest element to remove. Generally, designing a PCB thateliminates most sources of RF interference is the most cost-effective approach (calledsuppression). The source of interference is the active element producing the originalwaveform. What is required is to design the PCB to keep the RF energy created to onlythose sections of the board which require this energy. The second and third elements tendto be addressed with containment techniques. Figure 1.5 illustrates the relationship be-tween these three elements and presents a list of products associated with each element.

With respect to PCBs, we observe the following.

Noise sources are clock generation circuits, component radiation within a plasticpackage, incorrect trace routing, electrically long trace lengths, poor impedancecontrol, internal cable interconnects, and the like.

Section 1.4 The Need to Comply (A Brief History of EMI) 9

Figure 1.3 Oscillator's harmonics and FM radio signals superimposed. Note: FM radiostations are plotted 10 dB above oscillator for clarity.

The propagation path is the medium that carries the RF energy, such as freespace or interconnect cabling (common impedance coupling).

Receptors can be components on the PCB that easily accept harmful radiated in-terference from I/O cables and transfer this harmful energy to circuits and de-vices susceptible to disruption.

1.4 THE NEED TO COMPLY (A BRIEF HISTORY OF EMI)

In North America, interference to communication systems became a concern in the 1930s,whereupon the United States Congress enacted the Communications Act of 1934. TheFederal Communications Commission was created to oversee enforcement and adminis-tration of this act. Harmful effects were being observed with the technology of this timeperiod, enough to cause the U.S. government to take action.

EMI was also recognized as a problem during World War II with vacuum tubes.The terminology used was Radio Frequency Interference (RFI). During this period, spec-trum signatures of communication transmitters and receivers were developed along withradar systems. Because of the size and expense of these devices, the military owned the


Figure 1.4 Effects of a single change to a circuit related to RF emissions.

majority of high-technology electronic systems. Research and information on EMC waskept from the general public under the guise of national security.

Following the Korean War, most EMC work was not classified unless it dealt withthe specifics of a particular tactical or strategic system such as the Minuteman rocket, B-52 bombers, and similar military and espionage equipment. Conferences on EMI began tobe held in the mid-1950s where unclassified information was presented. The first confer-ence on EMI (RFI) was held in 1956 sponsored by the IEEE (Institute of Electrical andElectronic Engineers) and the IRE (Institute of Radio Engineers). During this time frame,the Army Signal Corps of Engineers and the United States Air Force created strong ongo-ing programs dealing with EMI, RFI, and related areas of EMC.

In the 1960s, NASA (National Aeronautical and Space Administration) beganstepped-up EMI control programs for their launch vehicles and space system projects.Governmental agencies and private corporations became involved with combating EMI inequipment such as security systems, church organs, HI-FI amplifiers, and the like. All ofthese devices were analog-based systems.

As digital logic devices were developed, EMI became a greater concern. In approxi-mately 1970, research was started to characterize EMI in consumer electronics, which in-cluded TV sets, common AM/FM radios, medical devices, audio and video recorders, andsimilar products. Very few of these products were digital but were becoming so. Analog

Section 1.4 The Need to Comply (A Brief History of EMI) 11

NATURAL RADIATION BIOLOGICALTerrestial Far field Man

Atmospheric Plane wave AnimalTribo-electric Near field Plants

Extraterrestial Capacitive crosstalk MANMADESun Inductive crosstalk CommunicationsCosmic Forward crosstalk Broadcast receiversRadio stars Backward crosstalk Navigation receivers

MANMADE CONDUCTION Radar receiversCommunications Power distribution 2-way radio receivers

Broadcast Signal distribution IndustrialNavigation Ground loops ControllersRadar Amplifiers2-way radio Medical

Industrial Biomedical sensorsArc welders Ordinance

Ultra cleaners EED'sRF induction heaters Dynamic capsFluorescent lights Computing devices

Medical Line receiversCAT scanners Power suppliesDiathermy Disk drives

Home Video amplifiersShaversHV bug killersMicrowave oven

Computing devices

Figure 1.5 Items associated with the three elements of the EMI environment.

systems are more susceptible to problems than digital equipment because the threshold ofsusceptibility in a switch or control circuit is higher than that of a high gain amplifier.

In the late 1970s, problems associated with EMC compatibility became an issue forproducts that were beginning to be used within the commercial marketplace. These prod-ucts include home entertainment systems (TVs, VCRs, camcorders), personal computers,communication equipment, household appliances with digital logic, intelligent transporta-tion systems, sophisticated commercial avionics, control systems, audio and video dis-plays, and numerous other applications. During this time period, the general public be-came aware of EMC and the threats associated with it.

After the general public became involved with EMI associated with digital equip-ment used within residential areas, the Federal Communications Commission (FCC) inthe mid- to late 1970s began to promulgate an emissions standard for personal computersand similar equipment. Since personal computers comprised such a huge market, com-mercial entities became involved in the field of EMC. This was due in part to military andspace funding being tapered off during this time period. Now almost all electronic equip-ment is digital, whether or not it needs to be.

emc and the printed circuit board -...

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