Book Review

Dr. Bruce Archambeault, a Senior Technical Staff Member at IBM in Research Triangle Park, North Carolina, and a respected member of the EMC community, has written a book aimed at EMC considerations and design techniques for printed circuit boards. He’s had good help with this effort; he acknowledges the role of James Drewniak of the University of Missouri at Rolla as a contributor to this book. Where other books look at complete box or systems approach to EMC design, Dr. Archambeault attacks the problem starting with the board level issues, and then expands from there. With his doctoral research in the area of computational electromagnetics, one might think the book is a dry examination and theoretically based dissertation on PCB design. However, Dr. Archambeault does an excellent job of connecting electromagnetic laws to measured results and practical solutions. Avoiding what he calls the Rule-of-Thumb approach, he gives us practical advice and useful information for improved EMC results and signal integrity issues.
Laying the groundwork in his first two chapters, Dr. Archambeault starts with a very brief history of EMI/EMC and design strategies, including a six-point list of a very familiar design process. He covers sources of EMI, as well as inductance, shielding, and ground concepts, each of these to be covered in detail in the following chapters. He follows this with a chapter on EMC Fundamentals. This covers coupling mechanisms (inductive and capacitive), signal generation of harmonics, chassis resonance and shielding effects and other emissions sources, both intentional and unintentional.
From here, Dr. Archambeault has the foundation to tackle two often-misunderstood concepts: Inductance (chapter 3) and Ground (chapter 4). In covering inductance, he is not afraid to give us some difficult equations, including one for partial inductance, which takes a page, and a half (pg. 38-39). When reading through this chapter, a first glance may seem intimidating; however, there is a great deal of excellent and useful information in these pages. A chapter called The “Ground” Myth then follows this chapter. He gives us some history on the name “ground” and why it worked in its day (hint: think the first and very slow speed digital communication). He properly points out that what is often considered as ground is actually a signal return path, not a mythical “current sink” where you can dump noise. Armed with this understanding, discussions of single and multipoint returns or references, and the good and bad issues of each, is much more powerful. Discussions continue on heat sink connection, circuit to chassis connections, cable shield issues, and other referencing strategies.
Now Dr. Archambeault gives us a chapter dedicated to Return Current Design. Discussed herein are issues of splitting reference planes and the effects of “stitching capacitors” across these gaps, and frequency limitations to doing so. PCB layer stackup is introduced. Radiated emissions from various microstrip and board layout configurations are given from computer analysis. Mother-Daughter Card connection routing is shown, highlighting the best design strategies. One problem I found with chapter 5 is the misplacement of several figures. Often a reference to a figure would be two pages before the figure would appear.
In the next two chapters, the book introduces EMI controlling techniques. In chapter 6, intentional signals are handled, while in chapter 7, unintentional signals are discussed. Regarding the Fourier Analysis of a square wave and the results on the waveform of keeping the first ten harmonics, the problem is transformed from the time domain into the frequency domain. At this point, Dr. Archambeault makes the following statement: “For EMI applications, however, the current is the most important consideration. Current radiates, not voltage!” After introducing a formula for electric field, he points out that “…radiated electric field is dependant on only two things: the magnitude of the current and the size of the loop.” With these tools, he is able to demonstrate radiation from microstrip circuits, inductance in vias, common mode voltages across split planes, and other significant PCB issues.
This brings a natural transition to unintentional signals on a PCB found in chapter 7. Dr. Archambeault points out 90% of PCB emissions are from unintentional signals. In this regard, the most significant effect is from common mode signals. Issues of split ground planes are revisited, while external cables, signal crosstalk, “ground-guard” traces, and uses of solid planes are introduced. A brief discussion of trace routing is included, which could use more space for greater understanding.
In chapter 8, Decoupling Power/ Ground Planes, we transition from the theoretical to the laboratory verified. Dr. Archambeault displays several graphs of calculated voltage levels versus measured levels. These graphs show fairly good prediction between his models and actual values. Using this, he investigates using an array of capacitors across the circuit board. His results show, as would be expected, the inability to decouple noise at frequencies above board resonance. That is to say, the transfer function for a 10” x 12” circuit board between power and ground planes is not significantly affected by use of decoupling capacitors, no matter what value or how many, for frequencies of two times the resonance frequency and above. However, moving the power made a significant improvement and ground planes closer together, thus increasing their “buried capacitance”. This is an excellent chapter on the use of decoupling capacitors.
Chapter 9 covers EMC Filter Design. Dr Archambeault keeps the concepts simple and straightforward. He shows how to configure the components for internal versus external impedances, emissions versus immunity, and when to consider pi and T configurations. A model for a “non-ideal” capacitor is given, along with graphical representations of impedance versus frequency for capacitors and ferrite beads. A discussion of common-mode filters is included.
Chapter 10 covers the use of signal integrity tools for EMC analysis. Recalling that the currents in the traces and wires cause electric fields, Dr. Archambeault describes how models can be used for EMC predictions.
The topic of chapter 11 is Printed Circuit Board Layout. Compressed into 10 pages are some excellent guidelines for PC board stack-ups, trace routings and reference plane usage. Often overlooked is the use of a power plane for a reference plane. Since power planes may have significant high frequency energy on them, driven by the microprocessors, proper decoupling becomes very important. Also, issues of traces changing layers and leaving its reference plane are also discussed and should be avoided. Component placement and isolation are also touched upon.
A basic chapter on shielding enclosures is given in chapter 12. Issues of shielding effectiveness, use of gaskets, apertures, and case resonance are all discussed. A discussion of ground stitching around the PCB edge and the effects is helpful; however, an accompanying graph shows four traces all the same with a legend off to the side. This makes it more difficult to be sure what is being shown. The chapter closes with a brief talk on shielded cables and terminations.
Chapter 13 is appropriately titled What To Do When a Product Fails in the EMC Lab? The chapter includes several excellent troubleshooting tips and explanations of sources. Construction of a contact voltage probe is given with cautionary statements. The chapter ends with two case studies.
An appendix called Introduction to EMI/EMC Computational Modeling completes the book. For those interested in more information on modeling techniques and methods, this section will be valuable.
For those who are looking for sound design concepts in printed circuit board stackup and decoupling, this book will be helpful. Most concepts are from basic to intermediate in difficulty and use explanations of why things work the way they do. The layout allows the reader to find concepts in most chapters without needing excessive background from other chapters.
Patrick G. André is the Principle Consultant with André Consulting, Incorporated. He may be reached at pat@andreconsulting.com, http://andreconsulting.com. EMC


If you would like to contact the IEEE Webmaster
© Copyright 2004, IEEE. Terms & Conditions. Privacy & Security

return to contents
IEEE logo