Help/cross-talk guidelines

Doug Brooks (doug@eskimo.com)
Tue, 08 Apr 1997 10:33:50 -0400

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.