[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 09:13:20 -0800 (GMT-08:00)

I am saying that the problem is about fields and waves which happen to induce 
current flow.  The signal is the field and wave travelling on the transmission 
line.  You won't get valid conclusions if you try to look only at the current.


-----Original Message-----
>From: "Oh, Dan" <doh@xxxxxxxxxx>
>Sent: Mar 9, 2012 9:08 AM
>To: Lee Ritchey <leeritchey@xxxxxxxxxxxxx>, "weirsi@xxxxxxxxxx" 
><weirsi@xxxxxxxxxx>, "si-list@xxxxxxxxxxxxx" <si-list@xxxxxxxxxxxxx>
>Subject: [SI-LIST] Re: Guard Traces
>
>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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>>>>>
>>>>>
>>>
>>>
>>>--
>>>Steve Weir
>>>IPBLOX, LLC
>>>150 N. Center St. #211
>>>Reno, NV  89501
>>>www.ipblox.com
>>>
>>>(775) 299-4236 Business
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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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>


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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