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

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From: S. Weir (weirsp@a.crl.com)
Date: Thu Oct 28 1999 - 15:03:52 PDT


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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