FreeLists / si-list / [SI-LIST] Re: Shielding clock traces on PCB's

[Doug Smith <doug@xxxxxxxxxx>]

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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