[SI-LIST] Re: Remove Ground underneath Differential signal is deserved or not?
- From: jeff_latourrette@xxxxxxxxxxx
- To: inovak@xxxxxxxxxxxxxxxxxxx, leeritchey@xxxxxxxxxxxxx
- Date: Wed, 31 Jul 2002 12:20:21 -0600
All:
I have to agree with Istvan's numbers-I've run analysis of cross-talk
(coupling) between microstrip lines (1/4 wavelength at 6 GHz) with and without
a via grounded guard trace using Agilent Momentum (2.5D solver) out to 10 GHz.
My finding was around 6 dB improvement for tightly coupled lines (line-to-line
spacing=around a board thickness).
I also saw the improvement drop to just 3 dB for loosely coupled lines
(line-to-line spacing>2 board thickness), however this behavior should be much
more dependent on the relative spacing of boundaries (walls) set-up for the
problem. If in fact a correct result, this seems to support the finding that
if you have decent spacing to begin with, guard traces usually don't add much
benefit. I expect that the accuracy of measuring very loose coupling decreases
substantially as spacing is increased, so that experimentally, this could be
hard to verify conclusively.
Anyone else have similar or different findings ??
Regards,
Jeff LaT.
-----Original Message-----
From: Istvan Novak - Board Design Technology
[mailto:inovak@xxxxxxxxxxxxxxxxxxx]
Sent: Wednesday, July 31, 2002 8:54 AM
To: leeritchey@xxxxxxxxxxxxx
Cc: scott@xxxxxxxxxxxxx; si-list@xxxxxxxxxxxxx;
inovak@xxxxxxxxxxxxxxxxxxx
Subject: [SI-LIST] Re: Remove Ground underneath Differential signal is
deserved or not?
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>
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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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