[SI-LIST] Re: Guard Traces

  • From: Rick Collins <gnuarm.2006@xxxxxxxxx>
  • To: doh@xxxxxxxxxx,"weirsi@xxxxxxxxxx" <weirsi@xxxxxxxxxx>, "si-list@xxxxxxxxxxxxx" <si-list@xxxxxxxxxxxxx>
  • Date: Fri, 09 Mar 2012 11:48:12 -0500

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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>
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