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

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From: David Instone (david_instone@uk.xyratex.com)
Date: Tue Oct 24 2000 - 01:44:52 PDT


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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