Re: [SI-LIST] : Re: Diff probes for tdr.

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From: Jim Freeman (freeman@broadcom.com)
Date: Tue Oct 24 2000 - 10:42:34 PDT


Hi David,
    I personally like the history of the thread being carried forward. The threads
are so many and diverse that it is hard to remember what is going on in each thread.
It takes 3 or 4 responses before the question becomes exciting and the history helps
to jibe the arguments.
    Also I hope you are not aware of the major interpretetion of the acronym SOB. It
is considered an epithet in the US.

Jim Freeman

David Instone wrote:

> Ron,
> I think I tend to disagree,
>
> 1. If you are making a TDR measurment on an unpowered unit which is
> connected only to the the TDR set then the only current source is from
> the TDR set.
>
> 2. I believe that one major TDR manufacturer does differential
> measurements by firing a pulse down one line of the pair (and at the
> same time noting the cross talk to the other) and then firing the pulse
> down the other pair and finally calculating the differential impedance.
> This is essentially two single mode measurements for which the ground
> (or reference plane/s) is most definitely required.
>
> Which brings me to a further point, If the lines are running between
> two return planes (eg Vcc and Gnd) or change return plane (ie swap from
> one side of the pcb to the other) then make sure that either all the
> decoupling caps are in place (best method) or place a short in all their
> places.
>
> Finally a plea to all, SOB! save our bandwidth. Please trim off previous
> messages more than two deep. This one was 5 levels deep before I
> trimmed it!
>
> Ron Miller wrote:
> >
> > Hi Jeff
> >
> > As you say there is current in the ground plane. There are many sources for
> > this current so for critical signals stay away from it. Measurement implys that
> > accuracy is important so decouple stray sources. That means do not connect the
> > ground unless needed. For differential signals ground is not needed.
> >
> > Ron
>
> --
> Regards
>
> Dave Instone. Compliance Engineer
> Test Systems, MP24/22
> Xyratex, Langstone Rd., Havant, Hampshire, P09 1SA, UK.
> Tel: +44 (0)23-92-496862 (direct line)
> Fax: +44 (0)23-92-496014
> http://www.xyratex.com Tel: +44 (0)23-92-496000
>
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