Re: [SI-LIST] : Trace Impedance Selection

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From: Vinu Arumugham (vinu@cisco.com)
Date: Wed May 03 2000 - 17:17:16 PDT


If the termination of the line is viewed as the parallel combination of the
terminating resistor and the impedance of free space, it looks like emissions
will be lower for lower impedance lines.

Vinu

Doug Brooks 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
> .
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