[SI-LIST] Re: Guard Traces

  • From: <Wolfgang.Maichen@xxxxxxxxxxxx>
  • To: <rohit.mishra@xxxxxx>, <weirsi@xxxxxxxxxx>, <si-list@xxxxxxxxxxxxx>
  • Date: Tue, 20 Mar 2012 13:11:53 +0000

Hello Rohit,

while it is true that the single ended impedance Zo changes less than Zeven or 
Zodd when the traces are brought closer together, it still changes. "A little" 
is qualitatively very different from "none whatsoever"... The exact answer 
comes out from any 2D field solver.

For a guard traces stitched to the ground plane an alternative picture can be 
to simply regard the guard trace as an extension of the ground plane that 
partially wraps around the trace. Clearly this will change the trace impedance 
since you changed the ground structure, just like changing the distance between 
trace and ground would change the impedance.

Regards,

Wolfgang

PS: Just as a little aside, on a professional forum it is usually regarded as 
more appropriate to refrain from "texting style" spelling; while fine in the 
appropriate context this can easily come across as immature (and the poster 
risks being taken less serious) in such places. In this case, "you" rather than 
"u". Just my personal pet peeve! (and others may disagree)



-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] On 
Behalf Of Rohit MISHRA
Sent: Tuesday, March 20, 2012 1:39 PM
To: weirsi@xxxxxxxxxx; si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Re: Guard Traces

Steve : When you  introduce coupling of any kind you alter I vs V and therefore 
the impedance.

Agree but when u introduce coupling to some trace then that mutual coupling 
will also interfere coupling of that trace with its return path, right ? Like 
If I consider the case of capacitive coupling between traces then as the traces 
are brought together, both capacitances i.e. cap between trace1 and its return 
path(C11) and cap between trace1 and trace2(C12) should change. C11 will 
decrease as some of the fringe fields between trace1 and its return path are 
intercepted by the trace2, and thus C12 will increase so I see that the total 
loaded capacitance to trace1 i.e. C11 + C12, will not change very much to its 
unloaded value.

Now if we drive trace 2 with 0V and only trace1 is being driven, the impedance 
of trace1 will depend on its loaded capacitance which is almost same as 
unloaded value. I agree when u place trace2 very close( less than trace 1 width 
!) to trace1 then there will be some reduction in trace1 impedance but it 
should always be lesser than differential and common mode signal case.

Do u see there is something wrong in my argument here ?

Rohit


-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] On 
Behalf Of steve weir
Sent: Tuesday, March 20, 2012 4:24 PM
To: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Re: Guard Traces

On 3/20/2012 2:13 AM, Rohit MISHRA wrote:
> Thanks Wolfgang,
>
> Your mail made me think that if there is some problem in my understanding 
> about impedance of line(trace).
>
> I understand that a uncoupled line always have some characteristic impedance 
> and now If I bring other trace closer to uncoupled line, their fringe 
> electric and magnetic fields will overlap and the coupling between them will 
> increase. But capacitive&  inductive coupling between traces doesn't always 
> mean that my first trace will see different impedance. It's potential at 
> second trace that decides if total field seen by first trace has changed, 
> right ?
Wrong.  Impedance is just the transfer function of V/I.  When you
introduce coupling of any kind you alter I vs V and therefore the
impedance.  The only issue is by how much at a given frequency.
>
> My understanding is that when my second trace is at ground potential( like in 
> guard trace), my first trace should still see almost same field which it was 
> seeing when it was uncoupled so its impedance shouldn't change.
>
> In case when second trace is driven with differential or common mode signal( 
> same potential as first trace ) then I can understand that total field seen 
> by first trace will change and hence change in its impedance.
>
> Do u see any problem in my point ?
>
> Rohit
>
>
>
>
> -----Original Message-----
> From: Wolfgang.Maichen@xxxxxxxxxxxx [mailto:Wolfgang.Maichen@xxxxxxxxxxxx]
> Sent: Tuesday, March 20, 2012 1:33 PM
> To: Rohit MISHRA; fred@xxxxxxxxxxxxx; si-list@xxxxxxxxxxxxx
> Subject: RE: [SI-LIST] Re: Guard Traces
>
> In theory a guard trace will ALWAYS affect the line impedance (it lowers the 
> impedance). The question is just how much. The farther it gets away, the less 
> effect it has, but there is no distance where the effect is exactly zero. As 
> a rough estimate, once the guard trace is more than about 3 line widths away 
> the effect will be negligible. But don't take my word for it, a 2D field 
> solver (e.g. TNT/MMTL) can give you precise answers. In any case, a guard 
> trace at that distance will neither help not hurt with crosstalk or signal 
> integrity.
>
> Wolfgang
>
>
>
> -----Original Message-----
> From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] On 
> Behalf Of Rohit MISHRA
> Sent: Tuesday, March 20, 2012 8:53 AM
> To: fred@xxxxxxxxxxxxx; si-list@xxxxxxxxxxxxx
> Subject: [SI-LIST] Re: Guard Traces
>
> Fred,
>
> Today, I was going through your below mail and in this mail, you said that as 
> the guard trace gets closer, the impedance of the line changes. I want to 
> know that when u say closer, do u mean to say guard trace closer than the 
> line and ground plane separation.
>
> My understanding says that if separation between guard trace and the line is 
> more than line and plane separation than ground potential at guard trace 
> should not affect current through the line and hence no change in line 
> impedance.
>
>
> Rgds,
> Rohit
>
>
> -----Original Message-----
> From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] On 
> Behalf Of Fred Balistreri
> Sent: Saturday, March 10, 2012 6:12 AM
> 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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>
> --
> Steve Weir
> IPBLOX, LLC
> 150 N. Center St. #211
> Reno, NV  89501
> www.ipblox.com
>
> (775) 299-4236 Business
> (866) 675-4630 Toll-free
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