Re: [SI-LIST] : What's your favourite Screwy SI Concept?

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From: Lee Ritchey (leeritchey@earthlink.net)
Date: Tue Jan 11 2000 - 08:30:32 PST


As to "Guard or Ground" traces placed between signal lines to reduce crosstalk, sometimes it seems to work. Remember that the trace is an LC circuit,
grounded or not. As a result it can turn into a resonant element that can become a bandbass filter, actually increasing coupling at some frequencies.
bandpass filters are made this way in RF applications every day.

I've seen actual designs with these guard traces actuall function as unintended bandpass filters making the design unusable.

Harris, George wrote:

> Hi Kim,
>
> I too have been following the favorite concept thread with a chuckle. Here
> is what the tradeoffs are between the grounds or empty space between
> signals.
>
> 1. Stripline (trace between two reference planes):
> If the dielectric is thin compared to signal spacing, most of the
> field will couple to the reference planes, and added ground will have little
> effect. You can prove by spice simulation that an apparent improvement in
> crosstalk of about 10% can be obtained, but lab experiments we performed did
> not confirm it. The effect was too small to be measurable. Change in
> impedance was too small to be measurable as well.
>
> In order for this scheme to work on the module, you would have to
> ground the added trace every inch or so, otherwise crosstalk induced in the
> added trace might reflect from the shorted ends multiple times and possibly
> couple back into the signal lines. At least that is what one prominent
> proponent of this scheme used to recommend, however their application was
> for surface etch, where it might be more justifiable.
> (Sorry but I am bound to secrecy on that one -- NDA's, IP's and all that
> jazz).
> As a practical consideration, this is not implementable because the size of
> the pad/via combination would require spacing the etch so far apart, that
> there would not be any measurable crosstalk anyway.
>
> 2. Microstrip (surface etch):
>
> Just today I looked at the patents that Compaq obtained on using
> interleaved grounds to control impedance of microstrip etch on a 2-sided pc
> board (no reference planes). Under those conditions the interleaved ground
> traces are very useful. I recommend that you peruse the patents referenced
> in a recent si mail -- unfortunately I deleted my copy of the message. (US
> patents 5,764,489 and 5,986,893).
>
> 3. The 20-H rule.
>
> There was a message about it last year on the SI-list which actually
> explained what it is... It refers to having the ground planes overlap the
> power planes around the edges of the board by 20X the spacing between the
> two planes, e.g. , if dielectric thickness between the two planes is 12m,
> the overlap would be 240mils.
>
> I don't know of any proof that this trick works to reduce EMI or
> module vdd noise.
>
> -George
> George Harris
> Tel: 508-467-8893
> george.harris@compaq.com
> COMPAQ Computer Corporation
>
> -----Original Message-----
> From: Kim Helliwell [mailto:khelliwe@acuson.com]
> Sent: Monday, January 10, 2000 4:17 PM
> To: si-list@silab.eng.sun.com
> Subject: Re: [SI-LIST] : What's your favourite Screwy SI Concept?
>
> Chris Heard wrote:
> >
> > Here's another...
> >
> > Use a ground trace in between signal traces to reduce crosstalk....
> >
> > Chris
> >
>
> I'm curious why this is screwy. Is it because:
>
> 1. The separation of the signal traces alone (because of the ground trace)
> would be enough to reduce the crosstalk, so the ground trace is not
> needed, or
>
> 2. The ground trace actually contributes more crosstalk.
>
> I assume it's #1. What has been frustrating me with this whole discussion
> is the bald statement of the screwy concept, without some explanation
> why the concept is screwy. Those of us still learning signal integrity
> concepts are left hanging out to dry. I now know a bunch of things that
> are considered screwy enough by one person to have it posted here, but
> I'd like to know more. At least references so I can look them up would
> help; I'm not that lazy!
>
> I'm still wondering *what* the 20H rule is, let alone why it's screwy!
>
> OK, guess that's enough "emporer has no clothes" for me today!
>
> --
> Kim Helliwell
> Senior CAE Engineer
> Acuson Corporation
> Phone: 650 694 5030 FAX: 650 943 7260
>
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