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

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From: Chris Heard (chris_heard@cereva.com)
Date: Mon Jan 10 2000 - 14:35:08 PST


Kim,

Usually by the time you space two lines far enough apart to fit another in
between, the edge-to-edge spacing between the lines approaches 2-2.5x the
signal-to-ground plane distance, which results is practically zero crosstalk
anyway.

There has been some work done at Syracuse U on the need to have a pth to
ground on that trace quite frequently (every 1" for a 1ns Tr), which adds
vias and steals routing density.

I've seen cases on busy boards where high speed clock lines with alot of
vias use more than one signal layer of a pcb. The ground return path for
these signals is suspect if a ground trace is not run right along side of
it. This is the only case where a nearby ground trace actually provides a
continuous return path if the signal path is jumping from one layer to
another.

Hope that helps,
Chris

-----Original Message-----
From: owner-si-list@silab.eng.sun.com
[mailto:owner-si-list@silab.eng.sun.com]On Behalf Of Kim Helliwell
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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