[SI-LIST] Re: Guard Traces

  • From: Doug Smith <doug@xxxxxxxxxx>
  • To: si-list@xxxxxxxxxxxxx
  • Date: Thu, 08 Mar 2012 20:22:39 -0800

Hi Lee,

Your statement is true for most cases, but for two layer boards (every 
household has several of these with processors on them) guard traces can 
stabilize impedance, improve grounding, and lower noise coupling. I 
agree with you for advanced electronics.

We should always state the assumptions made when citing design or other 
rules. I see this a lot in EMC books where an equation is given (such as 
slot radiation) but the (sometimes many) assumptions are not given and 
people use the equations when they don't apply. Shielding effectiveness 
is another case where the common equations do not apply in many cases 
(far field conditions are assumed when in fact they are not met). Under 
every equation in EMC and other books there should be a list of 
assumptions given.

Doug

On 3/8/12 10:52 AM, Lee Ritchey wrote:
> 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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