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

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From: Tony Sweeney (sween@lsil.com)
Date: Fri May 05 2000 - 09:31:21 PDT


Hi Ron,

Let me ask the master a simple question.

1. I know you can measure impedance with a TDR but you must know some
limitation here.

Regards,

Tony

Ron Miller wrote:
>
> 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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-- 
Anthony C. Sweeney
Field Applications Engineer
LSI Logic Corporation
7585 Ronson Road
San Diego, CA 92111
Ph: 858-467-6980
Fax: 858-496-0548
Pager: 888-961-7562

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