[SI-LIST] Re: Guard Traces
- From: Gwen and Wolfgang <gwmaichen@xxxxxxxxxxxxxx>
- To: si-list@xxxxxxxxxxxxx
- Date: Wed, 07 Mar 2012 19:57:38 +0100
Hello,
first, the short answer: simply don't use guard traces here, they won't
help much with isolation of high speed signals, and they use up way too
much routing space (on all layers, because of the stitching vias
necessary). Much easier to just increase the distance between diff pairs
etc. and you get the same benefit. (in fact, I suspect the reason many
people believe in the magic power of guard traces in this setting - high
speed signaling - is because those traces _force_ you to have enough
space between signal lines...). Guard traces have their place in
low-current / low-leakage / high-voltage applications, but NOT in high
speed digital. At least that's my opinion.
If you do need grounded traces, stitch them to the ground planes with
vias spaced no further than a quarter wavelength from the signal trace
and at an interval of no less than a quarter wave length from each
other. The wavelength given of course by bandwidth connected with the
signal rise time, BW = 0.33 / Tr, or a bit more conservatively, the knee
frequency f_knee = 0.5 / Tr. No special termination needed at the ends
of those traces because they are already solidly grounded along their
full length.
Wolfgang
On 3/7/2012 7:31 PM, Aaditya Kandibanda wrote:
> Hello Everyone,
> I have few questions about the guard traces.
>
> 1. How do I terminate the guard traces?
> 2. While using the guard traces for differential pair, how shall I place
> them between the differential pair?
> 3. How should I place the vias for guard traces?
> 4. What are disadvantages of Guard traces?
>
> Thanks and regards
> Aaditya
>
>
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