[SI-LIST] : Guard trace question

Gary Sanders (gsanders@csidaq.com)
Fri, 24 Sep 1999 15:04:38 -0700

I want to use guard traces on a multilayer PCB to provide almost
complete isolation, simulating the results as if coax were used instead.
I want to bury two signal layers between ground plane layers.
It seems to me that superposition must apply to EM fields in this
situation. I want to find the spacing and trace width of the guard
traces to provide about 100 dB of total isolation. I know that the
additional isolation provided by a guard trace is 6-12dB (depending on
whether ground stitching is provided). This assumes that the guard trace
is the same width as the signal trace, and is centered between the
aggressor and the victim traces, spaced by the width of the signal
trace.

Here's the specific question- is the isolation provided by a very wide
guard trace the superposition of the isolation of several normal width
guard traces placed side-by-side? Example--- assume 5 mil signal traces,
a 5 mil layer thickness of FR4, and a spacing between them of 20 mils
(between centers). The isolation calculated by formula without any guard
trace is 24.6dB. A stitched-ground guard trace would increase this
isolation to 36.6dB.

NOW- if we instead used a 25 mil wide guard trace, centered such that it
is 25 mils from the other traces, what is the isolation? I calculate
48dB without the guard trace. If we assume superposition, then the 25
mil guard trace is like having five 5 mil guard traces, each providing
12dB of isolation; so we have a total of 60dB isolation. This added to
that of the total spacing is 48dB + 60dB = 108dB of isolation. SO- is
this correct? Can I assume that the wide guard trace is the
superposition of several narrow guard traces? Note that this is a big
improvement on using a normal width guard trace, since the spacing
between the aggressor and victim would have to be 1" to get 104dB of
isolation (with a 5mil guard trace); and the wide guard trace approach
gives the same isolation with a spacing of 80 mils. I appreciate
anyone's thoughts.

--
Regards,
Gary L. Sanders, Staff Analog Engineer, gsanders@csidaq.com
L3 Communications, Inc.   Celerity Systems   www.csidaq.com
Cupertino, CA         dir 408-861-7325     fax 408-873-1397
      Ultra Fast Acquisition & Data Generation Systems
           "The fog is wine. The sun is my gold."

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