[SI-LIST] Re: Guard traces for differential pairs
- From: Duane Takahashi <duanet@xxxxxxxxxxxxxxxxxxxxxx>
- To: si-list@xxxxxxxxxxxxx
- Date: Mon, 29 Sep 2003 08:56:04 -0700
It also depends on the type of crosstalk: electric or magnetic (
capacitive or inductive ). If it's magnetic, guard traces won't help.
If it's capacitive, guards will help but will leave you with the
'magnetic remainder' of the xtalk.
I've seen high impedance lines, more than 60 ohms, exhibit inductive
xtalk. My guess is guard traces would not have helped in this case.
Regards,
Duane
> Sogo,
>
> It is true that guard traces can INCREASE crosstalk, but
> this can be avoided (and crosstalk in fact can be REDUCED)
> if the via stitching is frequent enough. Between each
> stitching, there is a half-wavelength resonator, with a
> lowest resonance frequency of fres=1/(2*tpd), where
> tpd is the propagation delay between two stitching vias.
> As long as the bandwidth of the signal on the structure
> is safely below this resonance frequency, there should be no
> problem with signal integrity.
>
> Regards,
>
> Istvan Novak
> SUN Microsystems
>
> ----- Original Message -----
> From: "sghsu55" <sghsu55@xxxxxxxxxxxx>
> To: <scott@xxxxxxxxxxxxx>; <leeritchey@xxxxxxxxxxxxx>
> Cc: <si-list@xxxxxxxxxxxxx>
> Sent: Sunday, September 28, 2003 9:14 PM
> Subject: [SI-LIST] Re: Guard traces for differetial pairs
>
>
>
>>Scott,
>> I don't totally agree with you. I have watched
>>this problems for several years. My conclusion is
>>"guard trace can certainly improve coupling
>>coefficient, however, mostly, guard trace may cause
>>serious crosstalk".
>>
>>---------------------------------- (victim)
>>
>>
>>
>>|-------------|----------|---------| (guard trace with
>>stitching vias)
>>
>>
>>---------------------------------- (aggressor)
>>
>> Consider the above case in multi-layer PCB, the
>>guard trace with stitching vias can certainly reduce
>>the coupling coefficient between victim and aggressor
>>in frequency domain. It is well known that the
>>guard trace behaves a quarter-wavelength band pass
>>filter. However, in time domain, the situation is
>>not so easy. The coupling coefficient between guard
>>trace and aggressor is siginicantly higher than
>>the one between victim and aggressor. The impedance
>>mismatch of stitching vias will result in more
>>serious corsstalk from guard trace to victim. Totally,
>>the case with graud trace induced more nosie
>>in victim. The guard trace will help in reducing total
>>noise in 2-layer PCB. In my opinion, the reason is
>>the ratio of the distance between signal and guard
>>trace and thickness between signal and reference
>>plan.
>>
>>Sogo Hsu, Ph. D.
>>Simulation Center, Foxconn
>>
>> --- Scott McMorrow <scott@xxxxxxxxxxxxx> ao�XTR��!G>
>>Lee is right and wrong.
>>
>>> Some traces guard or shield neighboring traces ...
>>>some don't.
>>>
>>>There are no absolutes about what we might call a
>>>guard or shield trace.
>>>Lee is correct in advising due caution in their use.
>>> Many designs have
>>>failed because of mis-applied guard traces, causing
>>>untold crosstalk and
>>>emi problems. If a design can be made to work with
>>>just spacing, then
>>>just do-it. Don't mess with guard traces unless you
>>>are prepared to
>>>understand a bit of microwave resonance theory.
>>>
>>>However, in skilled hands, a shield trace can
>>>provide additional
>>>crosstalk isolation without adding space to a
>>>design. It is possible to
>>>achieve an additional 3 to 10 dB of isolation when a
>>>correctly designed
>>>shield is used. Mike Conn and I have discussesd
>>>this in the past on the
>>>reflector. The key is to use enough ground vias to
>>>"nail the shield
>>>down." We've previously discussed the control of
>>>resonance (Lee's band
>>>pass filter effect.) by the judicious use of these
>>>vias. We've even
>>>discussed the necessity for irregular spacing of the
>>>"stitch" vias, in
>>>order to inhibit the formation of periodic resonance
>>>patterns.
>>>
>>>Like any tool, guard traces or shields can be used
>>>correctly or they can
>>>be abused. If you don't understand how they work
>>>and the potential
>>>pitfalls, just don't even think about using them.
>>>But if you need to
>>>achieve extremely high isolation and significant
>>>reductions in EMI in
>>>your design, then a guard trace might be a possible
>>>solution.
>>>
>>>Regards,
>>>
>>>scott
>>>
>>>--
>>>Scott McMorrow
>>>Electromagnetic Field Wrangler
>>>Teraspeed Consulting Group LLC
>>>2926 SE Yamhill St.
>>>Portland, OR 97214
>>>(503) 239-5536
>>>http://www.teraspeed.com
>>>
>>>
>>>
>>>
>>>Lee Ritchey wrote:
>>>
>>>
>>>>Tonglong,
>>>>
>>>>Guard traces don't "guard". If you have the bad
>>>
>>>luck to get just the right
>>>
>>>>length, they become a band pass filter. They
>>>
>>>should not be used. Handle
>>>
>>>>cross talk by setting proper spacings.
>>>>
>>>>Lee
>>
>>
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--
Duane Takahashi phone: 408-235-4200
Greenfield Networks fax: 408-727-7790
3260 Jay St. email: duanet@xxxxxxxxxxxxxxxxxxxxxx
Santa Clara, CA 95054
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