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

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From: Denomme, Paul S. (Paul.Denomme@viasystems.com)
Date: Thu Oct 28 1999 - 12:30:51 PDT


        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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