[SI-LIST] Re: Guard Traces

  • From: "Oh, Dan" <doh@xxxxxxxxxx>
  • To: "weirsi@xxxxxxxxxx" <weirsi@xxxxxxxxxx>, "si-list@xxxxxxxxxxxxx" <si-list@xxxxxxxxxxxxx>
  • Date: Fri, 9 Mar 2012 08:40:49 -0800

Hi Steve and Doug, 
I also agree with you. You guys have one more vote. The guard trace is very 
effective to reduce the inductive coupling which cannot be easily reduced by 
spacing.
Best,
-Dan Oh
-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] On 
Behalf Of steve weir
Sent: Thursday, March 08, 2012 9:23 PM
To: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Re: Guard Traces

Doug, I agree that guard traces can help in certain situations.  In most, and 
particularly when inserted without doing the requisite engineering work, they 
can make a good mess of things.

Best Regards,


Steve.
On 3/8/2012 8:22 PM, Doug Smith wrote:
> 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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>>


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