[SI-LIST] Re: Guard Traces

  • From: Lee Ritchey <leeritchey@xxxxxxxxxxxxx>
  • To: fred@xxxxxxxxxxxxx, doh@xxxxxxxxxx, weirsi@xxxxxxxxxx, si-list@xxxxxxxxxxxxx
  • Date: Sat, 10 Mar 2012 11:25:48 -0800 (GMT-08:00)

In the analog case cited, the isolation is for capacative coupling into low 
speed, high impedance signals rather than low impedance, high speed 
transmission lines.  Big difference.  Guard traces are never a good solution 
for transmission lines no matter how often you "stitch" them to this thing 
called "ground".  They are always resonant structures.


-----Original Message-----
>From: Fred Balistreri <fred@xxxxxxxxxxxxx>
>Sent: Mar 9, 2012 4:41 PM
>To: doh@xxxxxxxxxx, weirsi@xxxxxxxxxx, si-list@xxxxxxxxxxxxx
>Subject: [SI-LIST] Re: Guard Traces
>
>As the guard trace gets closer the impedance of the line changes. So you'll
>have to use a
>Field solver to predict.  If you can get it close and meet Zo then you're
>ok.   If it's high
>Speed then stiching vias to the gnd plane may be required.  Richie's point
>is that you usually
>can get the noise immunity you're looking for by simply using proper spacing
>and it may take up less geometry with less fuss.  
>
>Now there are cases where guard traces are useful. If one doesn't have
>planes for example or
>some analog applications where sensitive lines need to be carefully isolated
>from digital, switching supply or other such sources.  
>
>Bottom line depends on the application and skill set of the
>designer/engineer. 
>
>Cheers
>
>Fred
>
>-----Original Message-----
>From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] On
>Behalf Of Oh, Dan
>Sent: Friday, March 09, 2012 4:14 PM
>To: weirsi@xxxxxxxxxx; si-list@xxxxxxxxxxxxx
>Subject: [SI-LIST] Re: Guard Traces
>
>
>I agree completely with Steve.
>If the spacing between signal and guard trace is sufficiently narrow than
>dielectric height, it will help. It is all about assumption and we should
>not simply discount the guard trace for all cases. Cheers,
>
>-Dan Oh
>
>-----Original Message-----
>From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] On
>Behalf Of steve weir
>Sent: Friday, March 09, 2012 3:21 PM
>To: si-list@xxxxxxxxxxxxx
>Subject: [SI-LIST] Re: Guard Traces
>
>Rick, the percentage coupling between adjacent lines depends on the
>geometry.  For boards with modest dielectric thickness above solid planes
>the percentage coupling is small.  As the dielectric gets thicker, the
>percentage coupling between adjacent lines goes up.
>
>Steve.
>
>On 3/9/2012 8:48 AM, Rick Collins wrote:
>> I hate posted to this group because of all the Out Of Office replies,
>> but I can't let this go.
>>
>> It is exactly the inductive coupling that a guard trace has no impact
>> on, right?  The concept of the guard trace is that it is closer to the 
>> victim trace than the aggressor is and so the electrostatic potential 
>> of the guard is the dominant influence.  But the inductive coupling is 
>> not significantly affected.  I supposed there could be some return 
>> current induced into the guard trace, but this would be a small effect 
>> relative to the ground and power planes.
>>
>> Do I have this all wrong?  Does a guard trace reduce inductive
>> coupling via the induced ground current?
>>
>> Rick
>>
>>
>> At 11:40 AM 3/9/2012, Oh, Dan wrote:
>>> 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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>>>
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>
>
>--
>Steve Weir
>IPBLOX, LLC
>150 N. Center St. #211
>Reno, NV  89501
>www.ipblox.com
>
>(775) 299-4236 Business
>(866) 675-4630 Toll-free
>(707) 780-1951 Fax
>
>All contents Copyright (c)2012 IPBLOX, LLC.  All Rights Reserved. This
>e-mail may contain confidential material. If you are not the intended
>recipient, please destroy all records and notify the sender.
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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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