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


Istvan,
I agree with your assessment.  I have done IBIS simulations to find out
that shield traces do help in reducing crosstalk from high edge rate
signals, particularly in microstrip topology.  The higher amount of
crosstalk, when using extra trace spacing, may not have significant impact
on digital signals, but it can destroy a low-amplitude analog signal.
Hence, I use a mixed approach of using extra trace spacing and shield
traces, depending on the type of signal.

Regards, Ravinder
PCB Development and Design Department
IBM Corporation
Email: ajmani@xxxxxxxxxx
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                      Istvan Novak -                                            
                        
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                      <inovak@xxxxxxxxx         inovak@xxxxxxxxxxxxxxxxxxx      
                        
                      st.Sun.COM>              Subject:  [SI-LIST] Re: Remove 
Ground underneath         
                      Sent by:                  Differential signal is  
deserved or not?                
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Lee,

I think the source of confusion is that if a field solver is used to
extract the parameters of
the coupled traces, those extracted parameters will not directly show what
happens when you use
the shield traces properly connected.  You are correct in saying that the
coupling capacitance
and inductance is primarily dictated by the relative separation, and the
presence of trace(s)
in between has minimal (but not zero) affect on the coupling parameters.

Having said this, it is also true, as pointed out by others, that shield
traces CAN reduce the
crosstalk beyond what we would get just from spreading the traces to make
room for the shield
traces.  Below the half-wave resonance of shorted shield-trace segments,
there is a minimum of
about 6dB extra reduction of crosstalk.  You get this approximately 6dB
improvement on
microstrip, with the shield traces having the same geometry as the signal
traces.  If you have
a wider shield trace, the EXTRA crosstalk reduction is more.  In stripline,
the numbers are
different, but show the same trends.

This extra crosstalk reduction of shield traces can be simulated and
measured, but in terms of
simulations, we have to go beyond the field-solver excercise and have to
simulate the real
scenario: hook up a source and make sure that the shield trace is connected
to ground.

It is obvious that on a very wide bus, and using fast edges, the many
stitching vias create a
real routing problem, so spreading the traces a little bit further is a
better choice.  For a
few sensitive signals, though, shield traces can offer real benefits.

Regards
Istvan


             Delivered-To: si-list@xxxxxxxxxxxxx
             Date: Wed, 31 Jul 2002 07:56:50 -0700
             From: Ritchey Lee <leeritchey@xxxxxxxxxxxxx>
             MIME-Version: 1.0
             To: scott@xxxxxxxxxxxxx
             Cc: silist <si-list@xxxxxxxxxxxxx>
             Subject: [SI-LIST] Re: Remove Ground underneath Differential
signal is deserved or not?
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             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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Istvan Novak                         Sun Microsystems, Inc.
Istvan.Novak@xxxxxxx           Workgroup Servers, BDT Group,
                                     One Network Drive, Burlington, MA
01803
                                     Phone: (781) 442 0340

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