RE: [SI-LIST] : Trace Impedance Selection

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From: Chris Rokusek (crokusek@innoveda.com)
Date: Wed May 03 2000 - 18:11:35 PDT


Doug,

If your question is...

Given an unconstrained choice of Source Z, TLine Z, and Destination Z such
that the choices cause reciever to switch on incident, are there EMI reasons
that favor high or low Z?

Then my attempt at an answer...

Higher Z circuits involve higher voltages and higher near E-fields. They
are more likely to couple and radiate through an E-Field mechanism such as
capacitive coupling to a heatsink which is driven against the ground plane
or an attached cable to form an effective antenna. Also, Higher Z circuits
are more susceptible to induced B-fields (a small induced current will
result in large noise V).

Lower Z circuits involve higher currents and higher B-fields. So the
radiation problem here will likely be a B-field related mechanism such as
(surface layer) signal currents on return plane creating common mode
voltages (due to finite ground size) across the return planes and then
driving attached cables.

So you can get burned either way.

Regards,

Chris Rokusek
Innoveda

> -----Original Message-----
> From: owner-si-list@silab.eng.sun.com
> [mailto:owner-si-list@silab.eng.sun.com]On Behalf Of Doug Brooks
> Sent: Wednesday, May 03, 2000 1:32 PM
> To: si-list@silab.eng.sun.com
> Subject: [SI-LIST] : Trace Impedance Selection
>
>
> 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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