(Reprinted with Permission)
Dr. Howard Johnson is president of Signal Consulting Inc., a high-technology consulting firm specializing in solving high-speed digital design problems. He regularly presents technical workshops for digital engineers, including courses for Oxford University and UC-Berkeley.
He is the author of "High-Speed Digital Design: A Handbook of Black Magic" (Prentice-Hall, 1993) ISBN: 0-13-395724-1. He can be reached at (806) 556-0800; fax (206) 881-6149; e-mail <firstname.lastname@example.org>.
Signal Consulting The Art of High-Speed Digtial Design http://www.sigcon.com
on the big leather sofa, listening to
the others drone on. His tie was
tight. He wasn't often invited into
the corner office, and he knew why.
It's because I'm not like the others,
and because I can't keep covering
Finally, Ernie couldn't stand it
any longer. uot;Jim, you're the presi-
dent, and you've got to know," he
blurted. "There's some sort of
glitch on the main processor board.
It might be a crosstalk effect or a
noise problem. We just don't know.
We'll have to hold up shipments
until we get it sorted out."
Jim turned white, then red. The
veins began to stand out on his
forehead. He hesitated, and then
stood up and thundered, "So that's
what you guys are trying to tell
me?" He was used to problems, but
this was absolutely unbelievable.
"Let's see, gentlemen. We have
145 manufacturing people, 35 tech-
nicians, and a warehouse staff wait-
ing for this product. Our sales team
is trained, advertising is in the
works, and I have four press inter-
views starting tomorrow. The comp-
any burn rate is about $100,000
per day. If this problem takes a
week, we lose five hundred grand.
That's more than you're worth,
Ernie. Why didn't you tell me about
this before!! How could we get this
far and not know there was a prob-
Ernie's lips trembled. He tried to
make everyone understand. "Hey,
look, I did everything I could. The
simulator said everything was per-
fect. The timing was all checked
out. How was I supposed to know
we'd have crosstalk." How was I
supposed to know that fast signals
wouldn't go through that con-
|"They don't even teach crosstalk
and ringing in college. Those are
the subjects we're supposed to
learn through experience. That's
just the way it works."
It's cruel how our educational sys-
tem has failed us. Twenty years
ago, all computer designers went
through a common electrical engi-
neering curriculum. This included
basic analog circuits, transmission
I lines, and linear systems theory.
Trouble is, the computer hardware
of that era was so slow that few
people needed to know anything
I about analog circuits to make their
systems function. Take a look in the
first edition of Texas Instruments
logic catalog (if you can still get a
hold of one). The typical LS-TTL
logic gate had a rise/fall time of
about 20 ns. That's very slow by to-
day's standard. It doesn't take an
analog guru to plug together a
lot of LS-TTL logic.
As a result, colleges and univer-
sities felt they could safely drop the
old analog curriculum requirements,
in favor of newer, more modern
computer science classes. And,
there were even those who said that
analog design was becoming ir-
Not that I have anything against
computer science. It's a wonderful
discipline. Without it we wouldn't
have the processor sophistication
we now have, or have nearly as many
people trained to work with
complex computer architectures.
I can't blame the educators for
their decisions. A modern univer-
sity has a lot on its plate. There are
too many subjects to teach, and too
few hours available to teach them.
modern processors are
clocked at over 100 MHz. Plain
vanilla chip outputs sport sub-
nanosecond rise times. Analog ef-
fects are beginning to dominate
system design. Digital designers
without basic analog training
operate at a serious disadvantage.
In reaction to this trend, some col-
leges and universities are reintro-
ducing analog material in their
computer science curriculum. I par-
ticularly applaud Clayton Paul at
the University of Kentucky and
Henry Ott of Bell Labs for educat-
ing digital engineers about the
tough new world of electromagnetic
compatibility (EMC) standards.
Other courses of importance to
digital engineers include basic cir-
cuit theory, ELM waves, and trans-
mission lines. A well-rounded dig-
ital engineer should understand
enough circuit theory to recognize
the ways two adjacent digital sig-
nals will behave like a loosely cou-
pled, single-turn transformer. He
or she should know enough E&M
wave theory to understand why a
ground plane is better for high-
speed boards than a loose mesh of
ground traces. All digital engineers
should have a grasp of signal propa-
gation, superposition, and reflec-
tions on transmission lines.
A proper understanding of basic
signal integrity and EMC is crucial
to the continuing evolution of dig-
ital technology. Engineers with a
solid background in signal integrity
and EMC EE will greatly enhance their
chances for career success.
Dr. Howard Johnson is