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,
https://andreconsulting.com.
EMC
Book
Review