Johnson and Graham's "High Speed Digital Design" (ISBN-0-13-395724-1)
talks about this in section 5.6 - Guard Traces - on pages 201-204 in my
copy.
The message is that running your traces over a solid ground plane is
the way to substantially reduces cross-talk. Adding guard traces
between acitve tracks already running over a ground plane roughly
halves the residual cross-talk, and you can halve this again by putting
lots of vias along the guard trace to tightly couple it to the ground
plane.
"Lots of vias" (actually "vias at frequent intervals") is referred back
to J.A.Coekin "High Speed Pulses Techniques" Pergamon Press, Oxford,
1975 pages 203-205 - a book that I've never seen nor heard of.
Hi, Bill:-
Okay, thanks, I've found some info in "High-Speed Circuit Board Signal
Integrity" (Thierauf) which suggests vias every 1/10 of a rise time
in the guard trace(s). An example shows crosstalk reduced by 4:1 for
the same *signal* trace spacing. References for double guard traces
("coplanar transmission line") are to (neither of which I have easy
access to at the moment):
Gopinath "Losses in Coplanar Waveguides" IEEE Trans. Microwave Theory
and Techniques, Vol. MTT-30, No. 7, July 1982, pp. 1101–1104.
and
Knorr "Analysis of Coupled Slots and Coplanar Strips on Dielectric
Substrate" IEEE Trans. Microwave Theory and Techniques, Vol. MTT-23,
No. 7, July 1975, p. 541.
Hope this helps. I've got a pair of text books specifically covering
microstrip, but they weren't a good investment - I could poke around in
them if you are desperate, but I don't like my chances of coming up
with anything useful.
If there's anything (simple) on the effect of the guard traces on the
characteristic impedance, I'd really appreciate having it. This isn't
picosecond stuff I'm working with.
Thanks,
Best regards,
Spehro Pefhany