[SI-LIST] Re: Guard Traces

  • From: Lee Ritchey <leeritchey@xxxxxxxxxxxxx>
  • To: doh@xxxxxxxxxx, "weirsi@xxxxxxxxxx" <weirsi@xxxxxxxxxx>, "si-list@xxxxxxxxxxxxx" <si-list@xxxxxxxxxxxxx>
  • Date: Fri, 9 Mar 2012 08:46:09 -0800 (GMT-08:00)

Inductive coupling is precisely what guard traces cannot block.  If they could 
we would not have motors and transformers or bandpass filters in microwave 
applications.


-----Original Message-----
>From: "Oh, Dan" <doh@xxxxxxxxxx>
>Sent: Mar 9, 2012 8:40 AM
>To: "weirsi@xxxxxxxxxx" <weirsi@xxxxxxxxxx>, "si-list@xxxxxxxxxxxxx" 
><si-list@xxxxxxxxxxxxx>
>Subject: [SI-LIST] Re: Guard Traces
>
>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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>>>
>
>
>--
>Steve Weir
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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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