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

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From: Jim Freeman (freeman@broadcom.com)
Date: Thu Oct 28 1999 - 13:17:50 PDT


Either you misread the article or the information is faulty

Jim Freeman

"Denomme, Paul S." 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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