RE: can't seem to post- RE: [SI-LIST] : the old high-frequency return current model

Eric Bogatin (eric@bogent.com)
Mon, 4 Oct 1999 11:40:31 -0500

Ray-

thanks a lot for your help.

I guess I will try to limit my notes to under 50k- maybe post other figures
on my web site in the future if there is a need for more info.

--eric

Eric Bogatin
BOGATIN ENTERPRISES
Training for Signal Integrity and Interconnect Design
26235 W. 110th Terr.
Olathe, KS 66061
v: 913-393-1305
f: 913-393-1306
pager: 888-775-1138
e: eric@bogent.com
web: www.bogatinenterprises.com

> -----Original Message-----
> From: Ray Anderson [mailto:Raymond.Anderson@Eng.Sun.COM]
> Sent: Wednesday, September 29, 1999 11:11 AM
> To: eric@bogent.com
> Subject: Re: can't seem to post- RE: [SI-LIST] : the old high-frequency
> return current model
>
>
> Eric-
>
> I replied to this last nite but I think I may have screwed up
> the address.
>
> What I said was: I've temporarily opened up the gate to
> allow messages up to 150K in size pass. Try reposting the problematic
> traffic.
>
> -Ray
>
>
>
> > Ray-
> >
> > sorry to keep bugging you about my lack of success in posting to the SI
> > list. I tried sending the note below on Sunday, and it never
> got posted. The
> > first time I sent it, I had 3 figures and the size was about 100k. I
> > realized when I didn't see it post that it was too large, and I
> cut it down
> > to two figures and reduced their size. The second time I sent
> the note, it
> > was 53k. I thought this was under your limit of 60k. However,
> its been over
> > 12 hours and I did not see the posting of it. Am I doing
> something wrong?
> > Please advise, and again, I am sorry to waste your time with
> this trivial
> > infrastructure problem.
> >
> > thanks for your help.
> >
> > --eric
> >
> >
> >
> > Eric Bogatin
> > BOGATIN ENTERPRISES
> > Training for Signal Integrity and Interconnect Design
> > 26235 W. 110th Terr.
> > Olathe, KS 66061
> > v: 913-393-1305
> > f: 913-393-1306
> > pager: 888-775-1138
> > e: eric@bogent.com
> > web: www.bogatinenterprises.com
> >
> >
> >
> > > -----Original Message-----
> > > From: Eric Bogatin [mailto:eric@bogent.com]
> > > Sent: Sunday, October 03, 1999 5:03 AM
> > > To: si-list@silab.eng.sun.com
> > > Cc: eric
> > > Subject: RE: [SI-LIST] : the old high-frequency return current model
> > >
> > > Brad, and others-
> > >
> > > The recent discussion on the impact on differential pairs in
> crossing a
> > > split in a plane has gotten me thinking about how we could do a quick
> > > simulation, in the absence of doing a 3D FDTD simulation of the actual
> > > currents and voltages. This was a very good suggestion, and I
> also await
> > > to
> > > see the film.
> > >
> > > Here's one approach that I tried and would like folk's
> opinions on. It has
> > > certainly opened my eyes a bit at how robust differential pairs are.
> > >
> > > Consider a coupled, microstrip differential pair that is 7
> inches long. At
> > > the three inch point, the plane on the back side is etched off for a
> > > length
> > > of 1 inch. This is an extreme case of a differential pair
> passing over a
> > > split in the ground plane.
> > >
> > > For the special case of lines 5 mils wide, 5 mil spaced, with
> a 2.9 mil
> > > thick dielectric to the return plane, the matrix elements of
> the two lines
> > > are: Z11 = 52.4 Ohms and Z12 = 5.2 Ohms. I got these from my
> trusty Ansoft
> > > Maxwell 2D Extractor, 2D field solver. This gives a diff Z0 of 2 x
> > > (52.4 -5.2) = 94.6 Ohms.
> > >
> > > In the region where the plane has been removed, the single
> sided Z0 is 160
> > > Ohms. It is interesting to note that the field pattern of the
> single ended
> > > coplanar lines is the same as the differentially driven coupled
> > > microstrip.
> > > The difference, of course, is that the presence of the return plane
> > > dramatically changes the specific value of the characteristic
> impedance of
> > > the differential pair vs the single ended. From
> > > the differential signal's perspective, it will see an impedance
> > > discontinuity of 95 Ohms in the microstrip area and then 160
> Ohms for the
> > > 1
> > > inch distance, and back to 95 Ohms. The common mode will see
> an impedance
> > > of 57 Ohms in the microstrip region and infinite in the
> coplanar region. I
> > > think this is how the gap will "block"
> > > common mode signals, as D.C. was alluding to. Basically, the
> common mode
> > > impedance is open in the gap region, but the differential
> mode impedance
> > > is
> > > only about 80% larger in the gap.
> > >
> > > To first order, this will be the main effect the signal sees
> in crossing
> > > the
> > > gap. We all agree that there will be mixing of the return paths in the
> > > plane
> > > within a few line widths of the traces, at the edge of the
> gap. This is
> > > probably on the order of 20 mils. I think this discontinuity will be
> > > small,
> > > compared to the 1,000 mil discontinuity of the 95 Ohms to 160 Ohms
> > > transition. If this is the case, we can simulate the common and
> > > differential
> > > mode signals with a tool like Hyperlynx to look at the
> magnitude of the
> > > signal degradation. A number of folks pointed out that yes,
> there will be
> > > a
> > > discontinuity, but if you keep the lengths short compared to
> a rise time,
> > > it
> > > might not affect the noise margin much.
> > >
> > > In order to use Hyperlynx to simulate this structure, we have
> to trick it
> > > into seeing the coplanar region as a coupled microstrip region, as it
> > > assumes perfect planes as return paths for all signal lines.
> We do this by
> > > using a 2.9
> > > thick dielectric for the differential sections and using a 50
> mil thick
> > > dielectric for the coplanar region. Using my trusty Ansoft 2D field
> > > solver,
> > > I found that for two coupled microstrips, as long as the
> dielectric to the
> > > return plane is thicker than 15 mils, the differential
> impedance saturates
> > > at 140 Ohms. The common mode impedance continues to rise,
> however, as the
> > > plane is lowered.
> > >
> > > This is a bit lower than the 160 Ohms of the single ended line with no
> > > return plane, and the dielectric 2.9 mils thick, since in the
> case of just
> > > thicker dielectric over the plane, there is field in the
> thick dielectric
> > > and higher capacitance, and hence, lower impedance. The difference is
> > > small,
> > > and I went ahead and used the two coupled microstrip, with 50
> mil thick
> > > dielectric, as my gap region. The common mode impedance was 320 Ohms-
> > > large
> > > compared to the 57 Ohms common mode impedance of the front of
> the line.
> > >
> > > The Hyperlynx circuit I set up has three diff pairs in
> series. The first
> > > section is 3 inches long, 95 Ohm. The next section is 1 inch long, 140
> > > Ohms
> > > and the third is 95 Ohm, 3 inches long. Each trace is defined
> by its cross
> > > section, taking advantage of the built in field solver to
> calculate the
> > > matrix elements. I have independently verified the Hyperlynx
> field solver
> > > to
> > > be within 1-2% of the Ansoft tool. I used a differential TDR as the
> > > source,
> > > with a 100 psec rise time. (The 1 inch gap is about 130 psec
> long) The end
> > > of the lines are differentially terminated with 95 ohms.
> > >
> > > Slide 1, in the attached file, is the near end response of
> one channel
> > > of the TDR and its far end response, TDT, and the
> differential signal at
> > > the
> > > far end. I compare the response of the differential pair with
> a 1 inch gap
> > > and a 0.1 inch gap. You see the impedance discontinuity of
> the gap in the
> > > TDR, as you expect, a little distortion in the TDT, and very
> little effect
> > > in the differential response. The impact from a 0.1 inch gap,
> maybe more
> > > realistic even with 100 psec rise times, is almost non existent. This
> > > suggests that the differential signals are not impacted much
> by the gap.
> > >
> > > However, as many have pointed out, the real problem with differential
> > > lines
> > > is when there is an asymmetry, as with driver skew. In Slide
> 2, I compare
> > > the same traces, with a delay of 50 psec in the second driver, but
> > > recorded at the far end. The two unmarked scope traces are the + and -
> > > signal lines. The green is the differential signal. The skew is 1/2 a
> > > rise time, and might be a common magnitude of delay. Again the
> > > differential
> > > signal is not degraded very much- at most, its rise time is
> slowed down a
> > > bit, but the signal quality is good. The big impact is the
> generation of
> > > common mode voltages (and currents) on the other side of the
> gap. I can't
> > > plot common mode voltage in Hyperlynx, but you can see it
> clearly in the
> > > ripples that move in step at the receiver side. Note that
> without a gap,
> > > you
> > > still get some common mode generated from the driver skew, its just
> > > cleaner.
> > >
> > > I looked at the near end waveforms, at the source, with and without a
> > > skew, and these show the generation of large common mode
> noise with the
> > > skew, and low values without the skew. I would have appended
> this figure,
> > > but this list allows a max posting size of 60k, and I
> couldn't fit it in
> > > this note.
> > >
> > > Conclusion: The gap will be an impedance discontinuity to the
> differential
> > > signals, and the common mode signals. The differential
> signals in passing
> > > over the gap are relatively clean, as they see a short, series,
> > > discontinuity. Unavoidable asymmetries in the lines and
> > > drivers will cause larger common mode voltages, since the common mode
> > > impedance discontinuity is larger. The reflected signals will scale
> > > with the TD of the gap. If the skew is ever comparable to the
> rise time,
> > > series
> > > termination might be needed to dampen the reflected common
> mode signals.
> > > The quickest way to get an intuitive and quantitative feel for the
> > > behavior of the differential and common mode signals is to look at the
> > > size and length of the differential and common mode impedance
> > > discontinuity a gap or other uncontrolled effect presents.
> > >
> > > The recent discussion on this topic has been very timely for
> me, as I am
> > > working on a paper for DesignCon with Mike Resso of HP and
> Steve Bird of
> > > Hadco, on differential impedance analysis with TDR. Based on
> the last few
> > > weeks' discussions, I am adding a test structure to our board
> that has a
> > > split plane.
> > >
> > > Comments are always welcome.
> > >
> > > --eric
> > >
> > > Eric Bogatin
> > > BOGATIN ENTERPRISES
> > > Training for Signal Integrity and Interconnect Design
> > > 26235 W. 110th Terr.
> > > Olathe, KS 66061
> > > v: 913-393-1305
> > > f: 913-393-1306
> > > pager: 888-775-1138
> > > e: eric@bogent.com
> > > web: www.bogatinenterprises.com
>
>