[SI-LIST] Re: Guard Traces

  • From: "Oh, Dan" <doh@xxxxxxxxxx>
  • To: Lee Ritchey <leeritchey@xxxxxxxxxxxxx>, "weirsi@xxxxxxxxxx" <weirsi@xxxxxxxxxx>, "si-list@xxxxxxxxxxxxx" <si-list@xxxxxxxxxxxxx>
  • Date: Fri, 9 Mar 2012 09:08:52 -0800

Hi Ritchy, 
Are you indicating that my explanation is valid if we stay with the circuit 
concept?
Maybe I am missing radiation impact here which I do not have much experience so 
I will take your words. 
I will try to continue this thread offline since it may not be interested to 
others.
Best,
-Dan Oh

-----Original Message-----
From: Lee Ritchey [mailto:leeritchey@xxxxxxxxxxxxx] 
Sent: Friday, March 09, 2012 8:58 AM
To: Oh, Dan; weirsi@xxxxxxxxxx; si-list@xxxxxxxxxxxxx
Subject: Re: [SI-LIST] Re: Guard Traces

Dan, it's basic physics.  You need to cease looking at the problem in terms of 
current loops and start looking at it as fields and waves.


-----Original Message-----
>From: "Oh, Dan" <doh@xxxxxxxxxx>
>Sent: Mar 9, 2012 8:52 AM
>To: Lee Ritchey <leeritchey@xxxxxxxxxxxxx>, "weirsi@xxxxxxxxxx" 
><weirsi@xxxxxxxxxx>, "si-list@xxxxxxxxxxxxx" <si-list@xxxxxxxxxxxxx>
>Subject: [SI-LIST] Re: Guard Traces
>
>Hi Ritchey,
>Perhaps, you can elaborate why???
>The reason why we placed ground guards closer to the signal is to reduce the 
>current loop (assuming they are sufficiently stitched). This will reduce 
>coupling from other current loops (which is also small). Of course, this 
>depends on the height of trace which I assumed is fairly large compare to 
>spacing. Otherwise, it would not help as you indicated.
>So, the answer would be depends. But I want to make sure when the trace is 
>located high; then, the ground guard is a very effective way to mitigate this.
>May I am missing something here and need your lecture here.
>Best,
>-Dan Oh
>-----Original Message-----
>From: Lee Ritchey [mailto:leeritchey@xxxxxxxxxxxxx]
>Sent: Friday, March 09, 2012 8:46 AM
>To: Oh, Dan; weirsi@xxxxxxxxxx; si-list@xxxxxxxxxxxxx
>Subject: Re: [SI-LIST] Re: Guard Traces
>
>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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>>>>
>>
>>
>>--
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>>IPBLOX, LLC
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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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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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