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

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From: Fred Balistreri (fred@apsimtech.com)
Date: Mon Jan 10 2000 - 14:01:05 PST


The concept of using ground traces for shielding is not screwy. However the application of
such may not make sense for purely digital signals. When trying to acheive 90db of isolation
between channels then it makes sense IF implimented correctly. The reasons you give below
are both true. Number 1 tends to be true for digital apps. Number 2 occurs when the shielding
is not properly done, more EMI could result from number 2 as well.

The 20H rule has been explained. However the whys of it have not.

Best regards,
Kim Helliwell wrote:

> 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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--
Fred Balistreri
fred@apsimtech.com

http://www.apsimtech.com

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