Book: EMC and the
Printed
Circuit Board, Design, Theory, and Layout Made Simple
Publisher: Wiley-IEEE Press, 1999
Author: Mark I. Montrose
This review comes somewhat belated for many of you. In reality,
I have to confess, that it was several months ago that I got a
copy of the book from IEEE Press. The author told me recently
that this book has been translated into several languages, including
Chinese and Japanese and is selling well.
This book was written for the general Printed Circuit Board (PCB)
designer who wants to understand the EMI issues that exist within
a PCB. The book is outlined in nine chapters; each chapter covers
an EMC issue in the PCB design process. The focus of the book
is mainly on EMC for PCB. Other EMC issues such as shielding are
covered only briefly. Each chapter contains a list of useful references.
Chapter 1 addresses EMC fundamentals. It really sets up the fundamental
concepts of EMC and its terminology. Such fundamental concepts
include the electromagnetic environment, the need for product
compliance in that environment, methods of noise coupling (i.e
coupling path mechanisms) of radiated and conductive nature, and
the nature of interference in a PCB. Chapter 2 is titled EMC Inside
the PCB. The chapter opens with the high frequency behavior of
passive components, including PCB traces (which develop reactance
elements of considerable magnitude at higher frequencies). The
chapter explains, in simple illustrative approaches, some of the
fundamental concepts on the theory of electromagnetics, and how
RF exists within a PCB. Proper steps used in the minimization
of inductive coupling are discussed for a PCB, since it is one
of the major sources of EMI in a PCB (the other major ones being
common mode and differential mode currents and ground loops).
Other frequency effects discussed are skin effects and lead inductance
(as it applies specifically to wires). The most important concepts
discussed in Chapter 2 are differential mode and common mode currents.
I found the explanation of these two concepts very useful for
PCB designers, including the conversion between differential and
common mode currents.
Chapter 3 is titled Components and EMC. It deals mostly with the
RF energy generated by digital signals during transients resulting
from switching currents within a PCB. These transients produce
RF energy that is directly proportional to the rise and fall times
of the switching currents. Table 3.1 of the chapter shows the
approximate rise/fall times of different types of components in
the logic family. The table also shows the principal harmonic
content and the typical EMI frequencies observed from these components.
The transient currents are responsible for creating ground noise,
which is mostly observed as differential mode noise and it is
then converted to common mode noise currents. Common mode current
is the main source of radiated RF energy. Another source of current
transients is the power supply transition currents to a gate's
input (I=CdV/dt) which can account for many milliamps. Other important
subjects discussed in Chapter 3 are: a) the effect of component
packaging in the loop area of the die which can contribute to
EMI, b) the current flowing in a loop area between components
which can also cause EMI, c) the common mode currents from cabling
which can cause the largest source of EMI (analytical approximate
expressions are given for the radiated EMI from "b"
and "c" current sources), d) ground bounce, e) crosstalk
from lead-to-lead capacitance, f) parasitic effects of heat sinks
and the proper grounding of heat sinks to avoid becoming un-intentional
antennas, and f) power filtering of clock sources (to minimize
noise and ground bounce). About four different filtering techniques
are discussed.
I find Chapter 4 (Image Planes) the most interesting chapter in
the book because it discusses the main reasons for EMI in a PCB.
The concept of image planes was originated from the effort needed
to provide the best return path for RF currents, considering that
there are possible a number of parallel return paths that the
return current could follow. The number one return path is referred
to as the image plane. A good introduction to the image plane
is started by the concept of partial inductance. The author then
proceeds to show how only when the RF return path is connected
to the power and ground pins of a component will a real image
plane exist. The author shows that the separation between the
trace and image plane is the main issue for reducing the common
mode current in the image plane (by reducing mutual partial inductance).
A related concept in Chapter 3 is the discussion of ground and
signal loops. The author explains how these ground loops can be
formed in a poorly designed PCB. Image planes prevent RF ground
loops from being developed because RF currents couple themselves
to their source trace without having to find alternate return
paths. The author shows in the chapter that the distance between
ground stitch locations to a 0V reference should not exceed l/20
to avoid potential RF ground loops.
Image plane violations are discussed in Chapter 4, as well as
the proper use of vias. The concept of split planes (i.e. splitting
ground planes for providing return paths of analog and digital
components in the PCB) is discussed in the chapter. The isolation/partitioning
of the actual components in the PCB is also discussed through
several approaches (isolation, bridging). Proper RF return paths
are also discussed for wiring interconnects. For those PCBs that
cannot avail themselves of multi-layer image /ground plane designs
(i.e. single and double sided boards), design approaches are discussed.
The chapter ends with a brief discussion of localized ground planes.
Chapter 5 is titled Bypassing and Decoupling. Bypass capacitors
are used in a PCB to prevent the transfer of energy from one circuit
to another, maintaining the constant power distribution among
all the PCB components (specially those power hungry). Decoupling
assures low impedance power supply during switching operations.
Low impedance allows high frequency noise to get diverted away
from signal traces, but low frequency noise remains mostly unaffected.
Three types of capacitors are covered in the chapter: a) decoupling
which removes RF energy injected into the power distribution network
from high frequency components in switching power supplies. Decoupling
capacitors also provide localized source of DC power for devices
and components, eliminating the peak current surges that could
occur, b) bypass capacitors are used to divert common mode current
from PCB components and cabling, creating a shunt to noise energy
from entering susceptible PCB circuits, and c) bulk capacitors
are used to maintain a constant dc voltage and current to components
during switching operations (low drop out voltage). Because capacitors
are not truly passive devices as the circuit frequencies increase
(i.e. they become reactive devices with inductive and resistive
effects), Chapter 5 goes through all the complexities that arise
from this fact in the actual PCB design process. The work is also
extended to power and ground plane capacitance. Practical aspects
discussed in the chapter are the proper placement of capacitors
for the different applications and the proper selection of decoupling
capacitors (i.e. the calculation of capacitor values for decoupling
applications). The chapter ends with the concept of designing
buried capacitance in IC packages.
Chapter 6 on Transmission Lines is written to provide the reader
with knowledge and understanding that high speed digital design
is really the process of learning how to reduce propagation delays,
manage reflections and crosstalk (i.e. signal integrity), and
reduce signal losses in PCB where all the interconnects behave
as transmission lines. Only the PCB "transmission lines"
are discussed. The chapter overviews the different types of transmission
lines created during the PCB layout process (microstrip, embedded,
single stripline, dual stripline, differential microstrip and
stripline) including the appropriate formulas (these are really
quasi-static approximations at lower frequencies) for impedance,
capacitance, and propagation delays. I found the concept of capacitive
loading discussed in the chapter to be the most important section.
Chapter 7 is titled Signal Integrity and Crosstalk. When components
operate at higher frequencies, signal edge rates become faster
in order to accommodate smaller clock pulse intervals. The behavioral
characteristics of source and load devices, the conductor impedance
(including the "loading" caused by the components' input
impedance), the physical and electrical parameters of the PCB
materials, parasitic capacitance and inductance of circuit elements,
fast edge rates, and the trace lengths, all play a role in the
characteristics, quality and reliability of the signals being
transmitted (and this is known as signal integrity). For example,
if the trace is electrically long (edge transition time of the
signal is less than the time it takes for the signal to travel
from source to load and return from load to source) signal integrity
issues arise. Transmission line effects such as reflections, overshoot,
undershoot, and crosstalk can distort the quality of the transmitted
signals. Sources of noise that may affect the signal integrity
are: reflections, ground bounce, crosstalk, thermal effects, ground
effects, power supply variations, etc. The chapter addresses several
of the most important aspects of signal integrity; such as: a)
reflection and ringing (identification of the ringing problem,
conditions that create the ringing problem, and its theory), b)
calculating the correct trace lengths during the PCB layout (avoiding
electrically long traces) with several worked examples, c) the
effect of loading the transmission line, and d) the concept of
crosstalk (coupling of signals among traces). The chapter ends
with some design techniques to avoid crosstalk.
Chapter 8 is titled Trace Terminations. Trace terminations are
important for guaranteeing signal integrity in some components.
The need to terminate a PCB trace is based on several design criteria,
of which the most important are electrically long traces. The
author states that when the trace is electrically long or when
the length exceeds one-sixth of the electrical length of the edge
rate, the trace requires termination. Even if the trace is short,
termination is required if the load is capacitive or highly inductive
to prevent ringing. The book covers resistive series and parallel
terminations with examples. These terminations can match trace
impedance, reduce ringing and reflections and can also slow the
edge rate of the clock signals. The book discusses advantages
and disadvantages of a series and parallel terminations. The chapter
also discusses alternative destination methods such as Thevenin
network and RC network terminations (including advantages and
disadvantages).
The last chapter in the book is Grounding. This chapter is typical
of similar chapters on grounding from other EMC books. It covers
such topics as fundamental grounding concepts, ground loops, safety
ground, grounding methods (single point ground, multipoint ground,
hybrid), common impedance coupling (and how to avoid it). The
chapter ends with such concepts as resonance in multipoint grounding
and field transfer coupling of daughter cards to card cage.
EMC
Book
Review