[SI-LIST] Re: Shielding clock traces on PCB's
- From: Doug Smith <doug@xxxxxxxxxx>
- To: a.ingraham@xxxxxxxx
- Date: Sun, 07 Jan 2007 17:30:30 -0800
Hi Andrew,
In general I agree but there are assumptions made there that are not
explicit and if these don't hold the conclusion does not either. I
think the assumptions are:
- similar signals between the traces (such as logic signals)
- possibly multi-layer boards combined with the above
- frequencies involved such that the guard traces are a significant
portion of a wavelength
I have seen cases where a guard trace would have been one solution of
several possible.
The world is full of signals that are not small ones. For instance,
a board I have seen (years ago) had a small switching supply well into
the board from the edge connector. The supply was powered from -48
Volts. The -48 Volt trace to the supply ran parallel to a +5 Volt lead
for a few inches from the same edge connector. This was a two layer
board. Many modern products use them still such as DVD players - lots
of fast logic, two layer board, class B).
The inrush current into the switching supply and its input caps
magnetically induced a 7 Volt spike into the 5 Volt trace in a phase
as to add to 12 Volts being sent to some devices on the 5 Volt rail.
About every tenth board insertion, one or another 5 Volt device would
blow. A ground trace connected on both ends between the -48 and +5
leads would have prevented this (not a unique solution).
If the length of the guard trace (between its ground connections) is
somewhat less than 1/4 wavelength at the highest frequency in the
nearby traces, filtering should not be a problem.
Just an example I thought might be interesting.
Doug
Andrew Ingraham wrote:
>>Guard traces ar great ways to make accidental band pass filters. Why
>>would anyone advise using them to control cross talk?
>
>
> Because the word hasn't gotten around to most electrical engineers that it
> doesn't work like they think it should. So it continues to be done, and
> many engineers wouldn't think there is anything wrong with it.
>
> On first glance it seems like a reasonable thing to do, akin to putting a
> shield around a wire, turning it into coax. Reality is another matter.
>
> Andy
>
>
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