Re: [SI-LIST] : Trace Impedance Selection

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From: Chris Padilla (cpad@cisco.com)
Date: Wed May 03 2000 - 16:56:26 PDT


Let's see:

Device/Space consideration is an obvious one but that's out.

Power consideration leads me to think of lower loss for a
wider trace but you killed that one as well.

Proper termination rules out reflections being a problem
and you already mentioned a constant Zo.

The question is probably not of a characteristic impedance
since that is based on pure geometry and you ruled out just
about everything one needs to make Zo something of value.

Hence, I agree with you to the point of your own argument:
"There IS nothing that determines what the characteristic
impedance of the trace should be." You've ruled out all
such cases.

----->Chris

At 01:31 PM, you wrote:
>OK, gurus....
>
>Lets say I have a trace on a board between two devices. There is nothing
>that determines what the characteristic impedance of the trace should be.
>That is, there are no device considerations, no space considerations, no
>power considerations, etc. that would favor one value of characteristic
>impedance over another. It is my opinion that as long as the characteristic
>impedance is constant over the length and properly terminated, the choice
>of impedance value in this situation is completely arbitrary.
>
>Does anyone (having accepted the assumptions above) have a different
>opinion? In particular, are there any EMI reasons that would favor one
>value of characteristic impedance over another? (But don't tell me the
>assumptions above are wrong! That's not the question!)
>
>Doug Brooks
>.
>************************************************************
>See our updated message re in-house seminars on our web page
>.
>Doug Brooks, President doug@eskimo.com
>UltraCAD Design, Inc. http://www.ultracad.com
>
>
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