re: Help/cross-talk guidelines

fabrizio zanella (fabrizio=zanella%Eng%EMCHOP1@fishbowl02.lss.emc.com)
Wed, 9 Apr 97 17:31:23 EDT

It's good to see increased activity on the SI list.
I think the 1st design, with the guard band, is definitely overkill. The
guard trace as configured is not going to reduce coupling among the video
signals, and will probably add noise if it's only grounded at the ends and
once in the middle. It must be grounded at several points along the path,
spacing dependent on signal frequency. For most cases in order to fit a
guard trace you have to space the two signals so far away from each other
that the coupling becomes negligible.
If you space the video signals 4-5 times H (H = distance to nearest ground
plane) from each other you have eliminated most of the coupling, and should
get ~ 60dB isolation.
Modeling the effects of the guard band trace is not particularly easy, and
as Andy Ingraham points out, the results may not be positive at high
frequencies.
Regards,
Fabrizio Zanella
_/ _/ email:
zanella_fabrizio@emc.com
_/_/_/_/ _/ _/ _/_/_/ _/ voice: (508) 435-1000 Ext.
4645
_/ _/_/ _/_/ _/ _/ Fax: (508) 435-8949
_/_/_/_/ _/ _/ _/ _/ _/_/_/
_/ _/ _/ _/ EMC Corporation
_/_/_/_/ _/ _/ _/_/_/ 171 South Street
Hopkinton, MA 01748-9103
-------------
Original Text
From: Doug Brooks <doug@eskimo.com>, on 4/8/97 10:33 AM:
To: <si-list@silab.Eng.Sun.COM>

We have recently done boards for two customers, both
with very tight guidelines. Both had long traces carying
video signals. These are not differential signals; return
path is the adjacent plane. Both set guidelines to control
cross-talk.

First customer separated traces with a guard band grounded
at both ends with a single via approx in center. Spacing was
based on the 3W rule (edge to edge trace separation approx
3*W). He used both series and parallel terminations and
emphasized matching trace lengths. Line lengths were up
to 20".

Second customer used no guard band. Trace separation was based
on H (heighth above plane). Then based on line length:
over 1" 2.5 H
over 5" 5 H
over 12" 10 H
He used series termination, and while line lengths are matched,
his emphasis was on trace separation.

Both boards work fine. My question is what is the difference
between these two sets of guidelines, and which do you all
prefer? We have a new customer where, to meet Zo, W will be
5 mils and H 12 mils. A spacing based on 3W (or more) is
achieveable. A spacing based on 10 H is not. Also, this
customer has no room for terminations! What would you all
recommend?

Doug Brooks
UltraCAD Design, Inc.