April 2014 • The PCB Design Magazine 49
brooks' bits
ELECTROMAGNETIC FIELDS, PART 3 continues
mutual coupling and the potential for cross-
talk goes down.
We intuitively know that one solution for
crosstalk is to separate the traces. By consider-
ing what the electromagnetic fields look like,
we can see why.
Finally, look at Figures 5 and 6. They illus-
trate 8 mil traces with a 50 mil separation. In
the microstrip case (Figure 5) the traces are 10
mils from the underlying plane. In the strip-
line case (Figure 6) the traces are 10 mils from
either plane. Now however, in the stripline
case, the electromagnetic fields are captured
between the two planes. No part of the field
is allowed to escape from between the planes.
The fields are also restrained from spreading
out as far as they do in the microstrip case.
Clearly, then, the stripline case will perform
better from both an EMI standpoint and a
crosstalk standpoint.
It is tempting to draw the conclusion from
Figures 5 and 6 that all critical traces should be
placed in a stripline environment, and/or that
good EMI performance can't be obtained in a
microstrip environment. Be careful about this
conclusion; it can be overstated. It is absolutely
possible to design traces that perform perfectly
well (from both an EMI and a crosstalk stand-
point) in a microstrip environment. People do
it all the time. Nevertheless, it is still true that
the fields are contained better in a stripline en-
vironment.
These illustrations provide further evidence
that by visualizing the electromagnetic fields
around conductors or traces we can get an in-
tuitive feel for how those traces will perform on
an actual board from a signal integrity stand-
point.
PCBDESIGN
Notes
1. See How Electromagnetic Fields Deter-
mine Impedance, Part 1, and Electromagnetic
Fields, Part 2: How They Impact Propagation
Speed.
Douglas Brooks, PhD, is the
founder of ultraCAD Design
and now retired from daily work.
he has written numerous articles
in several disciplines and has held
signal integrity seminars around
the world. Brooks has spent most of his career
in the electronics industry in positions of engi-
neering, marketing, management, and as CeO
of several companies. Prentice hall has recently
published his latest book, PCB Currents; how
They Flow, how They react, and a seven-hour
educational video series on signal integrity.
visit his website at www.ultracad.com.
Figure 5: Microstrip 8 mil traces, 50 mil
separation, 10 mil above plane.
Figure 6: stripline 8 mil traces, 50 mil separation,
20 mil between planes.
