RE: [SI-LIST] : Comments from your SI seminar (SendII)`

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From: sweir (weirsp@a.crl.com)
Date: Fri Oct 29 1999 - 05:27:33 PDT


Paul,

Where is this article? I think the claim on matched lengths is going to
cause quite a bit of controversy.

For just the signal integrity, never mind the EMI:

If the risetime is known, then any mismatch in length can be equated to a
differential voltage offset during transitions. The offset appears in
opposite polarity for opposite edges, so a duty-cycle distortion
results. This shows up in both simulation and real lab measurements. If
the risetime is slow compared to the mismatch, then the effects will be
minimal. For the examples given this is definitely not the case.

180pS/inch @ 1.6 in = 298pS, none of the signal transition is
differential. The receiver will switch when the voltage on the shorter
trace satisfies the receiver hysteresis. This is hardly a good situation
for skew and jitter control.

180pS/inch @ 0.5in = 90pS for LVDS @ 350mV/300pS per lead, introduces an
effective offset of about -95mV in the longer lead for - to + transitions,
and +95mV in the longer lead for + to - transitions. This approximates the
maximum voltage required at the receiver to switch, 100mV. So, again, the
switching suffers skew and jitter because we have lost the benefits of
differential signaling for switching. Not a problem if a 200pS out of the
timing budget doesn't matter. If we look at eye patterns at 50MHz no one
will notice. However, at common transmission rates of 1-2.5Gbps, 200pS is
a lot to give up.

These are just the static effects. If we then look at the impairment
caused by the physical antenna paths such differences cause relative to
aggressors, this situation may deteriorate quite a bit more.

If you would like a more intuitive approach, consider that the advice(
routing around holes, etc ) is tantamount to claiming that you can make
equally accurate high-speed oscilloscope measurements with an extra 0.5
inches in the ground lead rather than maintaining coax all the way to the
signal source. Most of us know all too well such a notion is hogwash. Why
does the author claim our digital circuits are different?

Regards,

Steve.

At 08:04 AM 10/29/99 -0400, you wrote:
> It is also stated in this article that critically matched lengths is
>unnecessary, however you MUSt take into account your noise margins(parallel
>aggressors). It states that differential transmissions only need about 15mV
>difference to switch.
> Assuming no noise/ crosstalk and a 300pSec rise time in FR-4(180
>pSec/In), the pairs length could be mismatched by as much as 1.6
>inches(Tr/Td) and still function. So a 500 mil difference in line length
>would be acceptable. What he is really stating is that if you need to route
>one of the traces around a through hole, just do it.
>
> It only shows an example for ECL and LVDS Logic in the acrticle.
>
>Regards,
>
>Paul Denomme
>
>
> > -----Original Message-----
> > From: S. Weir [SMTP:weirsp@a.crl.com]
> > Sent: Thursday, October 28, 1999 6:04 PM
> > To: si-list@silab.eng.sun.com
> > Subject: RE: [SI-LIST] : Comments from your SI seminar (SendII)`
> >
> > Paul,
> >
> > Are you assuming:
> >
> > 1. The traces have been specified to remain at a constant separation?
> > 2. The trace pair will have a minimum separation from parallel aggressors?
> > 3. The traces will have matched lengths?
> >
> > I am not sure how such things can be assumed and make it to the physical
> > design. If the trace lengths don't match, the signal will have lots of
> > common mode for fast enough edges. If there is not enough separation from
> >
> > parallel aggressors, then aggressors can inject differential mode
> > noise. If the traces do not maintain a constant separation, the impedance
> >
> > will vary by about half the variation in the coupling coefficient.
> >
> > Regards,
> >
> >
> > Steve.
> >
> > At 03:30 PM 10/28/1999 -0400, you wrote:
> > > I have read an article recently that states that the use of
> > >specifying the differential impedance of two traces on a circuit board is
> > >unnecessary. The only thing you need to worry about is the individual
> > trace
> > >impedance. If you need a differential impedance for two lines to be 100
> > >ohms, just use two 50 ohm lines rather than using two signals whose
> > >differential impedance is 100 ohms. Also when connecting a 110 ohm
> > twisted
> > >pair to PCB you should just connect it to two 55 ohm traces to achieve
> > the
> > >110 ohm differential impedance. I have done enough research to draw my
> > own
> > >conclusions, but I would like to get the reaction from people in this
> > forum
> > >regarding this issue.
> > >
> > >Thank you,
> > >
> > >Paul Denomme
> > >Viasystems Inc.
> > >
> > >
> > >
> > > > -----Original Message-----
> > > > From: Doug Brooks [SMTP:doug@eskimo.com]
> > > > Sent: Thursday, October 28, 1999 12:59 PM
> > > > To: si-list@silab.eng.sun.com
> > > > Subject: RE: [SI-LIST] : Comments from your SI seminar (SendII)`
> > > >
> > > > >But a comment on our industry in general,
> > > > >
> > > > > I went to several courses at the PCB Design East, and each
> > course
> > > > >instructor had their own opinion on what they believe is the correct
> > way
> > > > of
> > > > >doing things.
> > > > >It is sad that our industry cannot take a concensus and come up with
> > the
> > > > >CORRECT way of doing things. Instead of using testing and empirical
> > data
> > > > to
> > > > >determine what is accurate, they bicker about why ones methods will
> > or
> > > > won't
> > > > >work.
> > > > >
> > > >
> > > >
> > > > As a seminar presenter at PCB East, and one who is also concerned
> > about
> > > > the
> > > > fact that students hear different things in different courses, I'd
> > like to
> > > > offer a few random comments here.
> > > >
> > > > First, people in our industry need a better understanding about
> > > > fundamental
> > > > electrical engineering!! And I am not just talking about those without
> > an
> > > > engineering degree, but also those with an engineering degree who (1)
> > > > didn't take certain kinds of classes related to such high speed issues
> > as
> > > > crosstalk, transmission lines, and stray trace/lead inductance, etc.
> > (2)
> > > > took them and didn't understand them, or (3) took them and forgot
> > them!!
> > > > And I am not criticizing them --- in my second job out of college my
> > > > company was designing state-of-the-art components for the
> > state-of-the-art
> > > > Illiac IV computer that were water cooled ECL devices running at the
> > > > remarkable speed of 3 MHZ! Things DO change.
> > > >
> > > > Second, it's nice to have rules of thumb, but it is better to
> > understand
> > > > where those rules of thumb came from and when they might (and might
> > not)
> > > > apply. I often get comments like "In so-and-so's class HE said ...".
> > My
> > > > response is to try to make the issue UNDERSTANDABLE for the student so
> > > > he/she can make up his/her OWN mind about what position seems more
> > > > reasonable. But that can be a challenge when the student has very
> > little
> > > > technical understanding.
> > > >
> > > > Thirdly, as has been pointed out, there aren't a lot of absolutes in
> > our
> > > > industry. If there were, we'd all understand and be teaching the same
> > > > (absolute) rules of thumb. While I am a strong supporter of studies
> > (and
> > > > have contributed to two of them --- the effects of vias on traces and
> > the
> > > > effects of 90 degree corners) this is not always the answer. Because
> > ...
> > > > each design has a unique environment. So, what works in one
> > environment
> > > > might not apply to another. Once again, my approach is usually to try
> > to
> > > > present to the student the ISSUES and the alternative opinions, so
> > they
> > > > can
> > > > recognize problems and (hopefully) potential solutions when they
> > arise. As
> > > > before, it is improved understanding that helps the designer (and the
> > > > engineer) solve problems, not rules of thumb or others' studies.
> > > >
> > > > Finally one last observation about studies. We lead a study on right
> > angle
> > > > corners where the measurements were taken by the respected people at
> > the
> > > > University of Missouri (Rolla). The results of that study were
> > > > independently confirmed by Mark Montrose with (a) a board of his own
> > > > design
> > > > and (b) another board from our study. These results have appeared in
> > at
> > > > least two publications. Nevertheless, take a position on right angle
> > > > corners in one of these e-mail forums and see how much discussion it
> > > > generates!! Some people's minds are made up, facts be darned!
> > > >
> > > > Doug Brooks
> > > >
> > > >
> > > > .
> > > > ****************************************************
> > > > Doug Brooks, President doug@eskimo.com
> > > > UltraCAD Design, Inc. http://www.ultracad.com
> > > >
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