[SI-LIST] Re: Remove Ground underneath Differential signal is deserved or not?

  • From: Scott McMorrow <scott@xxxxxxxxxxxxx>
  • To: silist <si-list@xxxxxxxxxxxxx>
  • Date: Wed, 31 Jul 2002 14:07:08 -0700

I've been able to successifully achieve better than -70 dB of crosstalk 
isolation between multiple analog video signals using guard traces for 
over 60 inches of total trace length.  They do work quite nicely if vias 
are used to ground the trace over regular intervals.

Not only did I have these results in Spice simulations.  But also they 
were subsequently confirmed by VNA measurements on the traces, within 1 
dB of the simulated results.  Without the guard traces, the crosstalk 
was greater than 6 dB worse.  In the same project I also was 
successfully able to extract with a 2D field solver, simulate with 
Hspice and correlate to measurements, the crosstalk isolation between 
two traces seperated by a 1/2 oz copper plane.  Through simulation we 
were able to determine that by using 1 oz copper we could increase the 
crosstalk isolation by 10 dB.

We utilized guard traces for several hundred single-ended and several 
hundred differential traces on an analog video crosspoint switch. 
 Without the guard traces and stitch vias, we would not have been able 
to meet our end-to-end crosstalk isolation requirement of -60 dB.

Best regards,

Scott

-- 
Scott McMorrow
Teraspeed Consulting Group LLC
2926 SE Yamhill St.
Portland, OR 97214
(503) 239-5536
http://www.teraspeed.com


Ritchey Lee wrote:

>Scott,
>
>If you just space the two traces in question the same as would be required to
>make room for the guard traces, the cross talk will be the same as with the
>guard traces and you don't need to add those extra structures.  This is
>relatively easy to show with a field solver.
>
>In the bargain, there is no risk of creating unwanted resonant strucutres.
>
>Lee
>


>
>Scott McMorrow wrote:
>
>  
>
>>Lee,
>>
>>    
>>
>>>What I show in my class is that guard traces are always LC networks that
>>>resonate at some frequency and can and do turn into bandpass filters
>>>at some
>>>frequencies.  I demonstrate this will actual failed circuits.
>>>
>>>
>>>      
>>>
>>This is exactly why Mike Conn and myself recommend stitching guard
>>traces to ground at random intervals across the length.  The average
>>spacing of the via sites should be 1/10 of the wavelength of the fast
>>signal frequency or equivalent risetime that will be present on this
>>trace.  These stitch vias to ground will eliminate the problems which
>>you have alluded to.  They are backed by extremely good analytical and
>>experimental science.
>>
>>Best regards,
>>
>>Scott
>>
>>--
>>Scott McMorrow
>>Teraspeed Consulting Group LLC
>>2926 SE Yamhill St.
>>Portland, OR 97214
>>(503) 239-5536
>>http://www.teraspeed.com
>>
>>--
>>Scott McMorrow
>>Teraspeed Consulting Group LLC
>>2926 SE Yamhill St.
>>Portland, OR 97214
>>(503) 239-5536
>>http://www.teraspeed.com
>>
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>
>
>  
>



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