From: Denomme, Paul S. ([email protected])
Date: Fri Oct 29 1999 - 05:04:45 PDT

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:[email protected]]
> Sent: Thursday, October 28, 1999 6:04 PM
> To: [email protected]
>
> 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:[email protected]]
> > > Sent: Thursday, October 28, 1999 12:59 PM
> > > To: [email protected]
> > > 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
> > > 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 [email protected]
> > >
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