RE: [[SI-LIST] : Trace Impedance Selection]

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From: Ron Miller (rmiller@Brocade.COM)
Date: Thu May 04 2000 - 17:02:15 PDT


What are you going to measure it with?

All the microwave test equipment is at 50 ohms, as are
the cables, and waveguide.

Only IF and video, TV use 75 ohms and above.

So, you can use any impedance you want if you also are willing
to develop ca bles, test equipment and all the infastructur you need.

Good luck

Ron Miller

> -----Original Message-----
> From: Bill Owsley
> Sent: Thursday, May 04, 2000 10:41 AM
> To: si-list@silab.eng.sun.com
> Subject: Re: [[SI-LIST] : Trace Impedance Selection]
>
> Arbitary seems right; tho' I've tended to favor higher impedances for
> lower
> current reasons, considering a lower enough impedance to get the load
> capacitance charged in the time constraints. but I recently heard another
> reason? for picking a certain value - in this case 50 ohms. This matches
> the
> impedance of a small coax used to sample the signals for measurment. 8^)
>
>
> Doug Brooks <doug@eskimo.com> 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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