[SI-LIST] Re: Guard Traces

  • From: Lee Ritchey <leeritchey@xxxxxxxxxxxxx>
  • To: dennis.han@xxxxxxxxx, si-list@xxxxxxxxxxxxx
  • Date: Thu, 8 Mar 2012 10:52:36 -0800 (GMT-08:00)

With all the feedback on this forum that guard traces are a bad idea, why is 
this discussion still going on?  Are there still people who cling to the notion 
that guard traces are useful, even in the face of mounting evidence that they 
don't help?


-----Original Message-----
>From: Dennis <dennis.han@xxxxxxxxx>
>Sent: Mar 7, 2012 12:59 PM
>To: si-list@xxxxxxxxxxxxx
>Subject: [SI-LIST] Re: Guard Traces
>
>In addition to what Wolfgang has stated, if the guard traces are too close to 
>the signal traces, and most people who believe in ground traces put them too 
>close (ground fill also falls in this category), the impedance of the signal 
>traces is lowered and there are less crosstalk and radiation because of it if 
>the now mismatched terminations don't cause other problems.
>
>
>
>
>
>--- In si-list@xxxxxxxxxxxxxxx, Gwen and Wolfgang <gwmaichen@...> wrote:
>>
>> 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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I just used the energy it took to get mad and wrote some blues.  Count Basie

Lee W. Ritchey
Speeding Edge
P.O. Box 2194
Glen Ellen, CA
707-568-3983
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