From: John, Hans-Joerg (Hans-Joerg.John@pdb.siemens.de)
Date: Wed Mar 01 2000 - 01:15:06 PST
I had similar questions some time ago. Pls have a look at Howard Johnson's
There you find an explanation for your second question
Dr.-Ing. Hans-Joerg John email:
Fujitsu Siemens Computers phone: +49 5251 820
PCS SHV server DH4 fax: +49 5251
EMC & Signal Integrity Design Support
> -----Ursprüngliche Nachricht-----
> Von: Ting.Steve@inventec.com [SMTP:Ting.Steve@inventec.com]
> Gesendet am: Mittwoch, 1. März 2000 08:34
> An: email@example.com
> Betreff: [SI-LIST] : How to measure differential pattern on test
> Dear SI-List,
> Some Tektronics AE told me that it's not necessary to connect differential
> probes' ground to the ground pad on the test coupon (Microstrip or
> Stripline structure) when measuring differential pair on the coupon .
> Since the TDR will generate the same but opposite polarity signals (odd
> mode signals) to each of the channel, the virtual ground will be formed
> between the probes. Just keep the probes' grounds touched each other but
> don't have to connect them to the ground on the board - it will make no
> difference if you connect them to the ground on the board.
> But some practical measurement showed that they won't be the same for most
> cases. We obtained higher differential impedance from TDR reading if we
> don't connect the probes' grounds to board's ground. And some colleagues
> think it'll be more reasonable to connect the probes' ground to board's
> ground, because it'll be closer to real case - components will reference
> the board's ground. So what is the proper way to measure the differential
> And the other question I'd like to ask is one regarding the return path.
> What is the difference between theses two measurement for impedance of
> twist pair with (1). one TDR channel - connecting probe's signal to one
> line and probe's ground to the other; and (2). two TDR channels that drive
> odd mode signals - connecting each of the probe's signal to one of the
> lines and touch the probe's ground to each other (like the method
> mentioned above). According to what I learnt about the signal's return
> path, I expect them should be the same. Because in method (1), all the
> return signal must enter the probe's ground and back to TDR. Then since
> the return current is opposite to the TDR's signal in the twist pair, and
> the electric parameters are the same for each of the line of the twist
> pair. They will form the odd mode signals just like what I want to do in
> the method (2).
> However, the real measurement showed about 6 ohm difference between them
> (100 ohm - 116 ohm). The method (1) measured some lower impedance then
> method (2). Where am I wrong? Does it due to the phase between the twist
> pair not exact 180 degree out of phase?
> Any input would be highly appreciated.
> Steve Ting
> CAE engineer
> Inventec corp, Taiwan, ROC.
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