46 The PCB Design Magazine • April 2014
In the Part 1 and Part 2 of this series
[1]
, I
talked about how helpful it can be to recog-
nize what the electromagnetic field looks like
around a conductor or trace and how that field
may change as we change the stackup or trace
parameters. Visualizing how the electromagnet-
ic field changes can give us insights as to how
parameters like impedance and propagation
speed might be influenced by the changes in
the stackup or trace parameters. In Part 3, we
will look at how changes in the electromagnetic
field relate to changes in coupling between trac-
es (crosstalk) or between a trace and the outside
world (EMI).
(Special note: The figures in these three parts
are largely taken from my new video lecture se-
ries just published by Prentice Hall, PCB Signal
Integrity LiveLessons, and in particular from Les-
son 1.4. Learn more about these LiveLessons at
www.ultracad.com or at the Informit website.)
Let's begin the analysis by looking at Figure
1. It shows the electromagnetic field around
an 8 mil microstrip trace that is 50 mils above
the plane. It is separated from other traces by
50 mils. Note how the electromagnetic field
extends way out into the air and over to other
traces. This poses a significant possibility of EMI
problems.
Douglas Brooks
ulTrACAD Design
BROOkS' BITS
Electromagnetic Fields, Part 3:
How They Impact Coupling
column
Figure 1: 8 mil trace, 50 mil height, 50 mil
separation.
