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

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From: Ravinder Ajmani/San Jose/IBM (ajmani@us.ibm.com)
Date: Tue Oct 24 2000 - 08:57:56 PDT


The correspondence on this subject has provided me excellent information on
TDR measurements. I have been using a Tek 11801C oscilloscope with TDR
sampling head. I have measured same pair of differential traces with and
without the probe ground attached to the board, and I have not observed any
difference in the measured impedance. What is critical is that the ground
return of the two probes should be tied together very close to the probe
tips.

Regards, Ravinder
PCB Development and Design Department
IBM Corporation - Storage Systems Division
Email: ajmani@us.ibm.com
***************************************************************************
Always do right. This will gratify some people and astonish the rest.
.... Mark Twain

david_instone@uk.xyratex.com (David Instone)@silab.eng.sun.com on
10/24/2000 01:44:52 AM

Please respond to david_instone@uk.xyratex.com (David Instone)

Sent by: owner-si-list@silab.eng.sun.com

To: Ron Miller <rmiller@Brocade.COM>
cc: si-list@silab.eng.sun.com
Subject: Re: [SI-LIST] : Re: Diff probes for tdr.

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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