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

Thanks Xiaoning very much for reply.
May I extrapolate a little. Considering that the guard trace, supposed to have
very small impedance to the ground, has so small impact from EMI point of view
one may conclude that floating copper has even less. Can someone comment on it?

Thanks,
Dorin




"Ye, Xiaoning" wrote:

> Droin,
>
> There is little improvement on EMI as well (Note: for microstrip case).
> Since I used 3-D tool, I was able to calculated the far field at 3 meters
> and the results support that.
>
> I think the reason is that for microstrip case, even we have the guard
> trace, the space above the trace (which is air) is still un-contained and
> there are still plenty of flux lines coming out of trace. This is the same
> reason that guard trace helps little to xtalk. I got fancy E/H flux line
> pictures from my solver so that I can have a better look at the physics.
> This also probably can explain why Jeff observed larger guard trace effects
> for very close coupled lines: if we look at the field plot of coupled lines,
> the field across the agressor and the victum is more intensive for closer
> coupled lines, and therefore putting a guard trace in between shorts more
> flux lines to ground (instead of going to the victim). Nevertheless, since
> we need to put shorting vias to stitch the guard trace to ground and the via
> diameter is usually ~20 mils, the spacing between aggressor and victim is
> pretty far away already and I expect that we will not observe the 6dB
> improvement that Jeff sees in his simulation.
>
> Xiaoning
> Intel Corp.
>
> -----Original Message-----
> From: Dorin [mailto:dorin.oprea@xxxxxxxxxxx]
> Sent: Thursday, August 01, 2002 6:01 AM
> To: xiaoning.ye@xxxxxxxxx
> Cc: 'jeff_latourrette@xxxxxxxxxxx'; si-list@xxxxxxxxxxxxx
> Subject: Re: [SI-LIST] Re: Remove Ground underneath Differential signal
> isdeserved or not?
>
> Hi Xiaoning,
>
> It appears to be no gain from cross talk point of view, but what about
> emission
> when the guard trace is used for EMI shielding on micro strip. Can you
> comment
> it?
>
> Thanks,
> Dorin
>
> "Ye, Xiaoning" wrote:
>
> > Jeff and all,
> >
> > I did some simulation (3-D fullwave) quite a while ago and my results show
> > that guard trace helps little for microstrip line.  It seems there are
> > distinct oppinions on the conclusion, but first I would like to make sure
> > that all of us are making apple-to-apple comparisons.  Suppose we have the
> > following three configurations (each line represents a trace):
> >
> > Configuration A:   _ _       (no guard trace, agressor and victum closely
> > spaced)
> > Configuration B:   _ ___ _   (guard trace in between, and stitched to
> > ground)
> > Configuration C:   _     _   (no guard trace, but the aggressor and the
> > victum keep the same spacing as Config B)
> >
> > My results show that:
> > * there is significant reduction of xtalk from A to B
> > * there is only minor (~2dB) improvement from C to B (even when the guard
> > trace is completely shorted to the ground by a vertical plane).  On the
> > other hand, if the guard trace is not stitched to ground by vias by close
> > spacing, Config B can be worse than Config C.  For example, if the stitch
> > spacing is 1", then we will see a resonance at around 3 GHz (which
> > correspond to half wavelength of 1").
> >
> > In summary, the guard trace appears to reduce xtalk because by "squeezing"
> > guard trace between the aggressor and the victum, the spacing between
> > aggressor and victum becomes larger. It is the "larger spacing", not the
> > "extra shielding" that does the work.
> >
> > If any of you have the results showing that Config B is much better than
> > Config C (for microstrip line), can you share with me a little bit more
> > detail of your configuration?
> >
> > Thanks,
> >
> > Xiaoning Ye
> > Intel Corp.
> >
> > -----Original Message-----
> > From: jeff_latourrette@xxxxxxxxxxx [mailto:jeff_latourrette@xxxxxxxxxxx]
> > Sent: Wednesday, July 31, 2002 11:20 AM
> > To: inovak@xxxxxxxxxxxxxxxxxxx; leeritchey@xxxxxxxxxxxxx
> > Cc: scott@xxxxxxxxxxxxx; si-list@xxxxxxxxxxxxx
> > Subject: [SI-LIST] Re: Remove Ground underneath Differential signal is
> > deserved or not?
> >
> > 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>
> >         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?
> >         Content-Transfer-Encoding: 8bit
> >         X-archive-position: 3574
> >         X-ecartis-version: Ecartis v1.0.0
> >         X-original-sender: leeritchey@xxxxxxxxxxxxx
> >         X-list: si-list
> >
> >         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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