[SI-LIST] Re: Guard traces for differetial pairs

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
>
>
>  
>
>>[Original Message]
>>From: Tonglong Zhang <tlzhang@xxxxxxxxxxxx>
>>To: <si-list@xxxxxxxxxxxxx>
>>Date: 9/25/2003 4:09:32 PM
>>Subject: [SI-LIST] Guard traces for differetial pairs
>>
>>Gurus,
>>
>>Could you pls make comment on below statement on guard traces for
>>differential pairs?
>>
>>Use gnd traces as guard trace is not very effective if there is gnd =
>>plane
>>direct underneath the differential pairs. As the current is mostly going
>>through the plane (lower impedence path), there will be no (or little)
>>current in the guard traces. When there are not current in the guard =
>>traces,
>>the guard traces will only shield electrical field but not magnatic =
>>field.
>>So magnatic coupling will not be better with the gnd guard traces.
>>
>>Thanks
>>
>>
>>Tonglong Zhang=20
>>Staff Engineer=20
>>IC Packaging Engineering=20
>>Broadcom  Corporation=20
>>
>>
>>
>>
>>
>>
>>
>>
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>
>
>
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