[SI-LIST] Re: Guard Traces

  • From: Lee Ritchey <leeritchey@xxxxxxxxxxxxx>
  • To: leeritchey@xxxxxxxxxxxxx, aaditya.kandibanda@xxxxxxxxx
  • Date: Tue, 13 Mar 2012 15:20:05 -0700 (GMT-07:00)

Wow!  That came out pretty bad.  Sorry.  What I meant to say is very little of 
the return current will be in the trace.  Most of it will be in the planes.
-----Original Message-----
>From: Lee Ritchey <leeritchey@eare in thethlink.net>
>Sent: Mar 13, 2012 3:10 PM
>To: aaditya.kandibanda@xxxxxxxxx
>Cc: fred@xxxxxxxxxxxxx, doh@xxxxxxxxxx, weirsi@xxxxxxxxxx, 
>si-list@xxxxxxxxxxxxx
>Subject: [SI-LIST] Re: Guard Traces
>
>Aaditya,e in the planes.  Very little will be in the adjacent trace no matter 
>how it is connected.  What the adjecent trace might do is resonate and 
>increase cross talk.  I would adise no putting in such structures.
>
>Most of the return current will b
>
>
>-----Original Message-----
>>From: Aaditya Kandibanda <aaditya.kandibanda@xxxxxxxxx>
>>Sent: Mar 11, 2012 8:40 PM
>>To: leeritchey@xxxxxxxxxxxxx
>>Cc: fred@xxxxxxxxxxxxx, doh@xxxxxxxxxx, weirsi@xxxxxxxxxx, 
>>si-list@xxxxxxxxxxxxx
>>Subject: [SI-LIST] Re: Guard Traces
>>
>>Hello Mr Ritchey,
>>If I have a trace which is stitched to ground at both ends along side of a
>>another trace, will there be any current loop formed which creates a EM
>>radiation between the two traces? what path will return current take?
>>
>>Thanks and regards
>>Aaditya
>>
>>On Sat, Mar 10, 2012 at 11:25 AM, Lee Ritchey <leeritchey@xxxxxxxxxxxxx>wrote:
>>
>>> 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
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>>> >>>
>>> >>> --
>>> >>> Steve Weir
>>> >>> IPBLOX, LLC
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>>> >
>>> >--
>>> >Steve Weir
>>> >IPBLOX, LLC
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>>> >
>>> >(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
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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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