RE: [SI-LIST] : How to measure differential pattern on test coupo n?

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From: Bnd (Ting.Steve@inventec.com)
Date: Wed Mar 01 2000 - 18:10:16 PST


Thanks, John.

But I think my second question 2 remains.

What is the difference between (+S G) and (+S -S) definition for twist
pair?
I think the ground line is just the return path of the signal line for the
twist pair. If the return current is equal but opposite to the signal
current, why cannot I describe (+S G) as (+S -S)?
The ground line for the twist pair is not like the ground plane, it's got
the impedance on it.
If I can describe (+S G) as (+S -S), why cannot the single end impedance
and differential impedance be equal for twist pair?

With best regards,

Steve Ting
EDA/CAE Engineer
INVENTEC CORPORATION
Tel: 886-3-3900000 Ext:2558
E-mail: Ting.Steve@inventec.com <mailto:Ting.Steve@inventec.com.tw>

        -----Original Message-----
        From: John, Hans-Joerg [mailto:Hans-Joerg.John@pdb.siemens.de]
        Sent: Wednesday, March 01, 2000 5:15 PM
        To: 'si-list@silab.eng.sun.com'
        Cc: 'Ting, Steve (???)'
        Subject: AW: [SI-LIST] : How to measure differential pattern
on test coupo n?

        Hello Steve,

        I had similar questions some time ago. Pls have a look at Howard
Johnson's
        page:

        http://www.sigcon.com/news/3_10.htm

        There you find an explanation for your second question

        Best regards

        Hans-Joerg John
        
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        -------------------------------
        Dr.-Ing. Hans-Joerg John email:
        hans-joerg.john@pdb.siemens.de
        Fujitsu Siemens Computers phone: +49
5251 820
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+49 5251
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> -----Ursprungliche Nachricht-----
> Von: Ting.Steve@inventec.com [SMTP:Ting.Steve@inventec.com]
> Gesendet am: Mittwoch, 1. Marz 2000 08:34
> An: si-list@silab.eng.sun.com
> Betreff: [SI-LIST] : How to measure differential pattern on
test
> coupon?
>
> 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
> impedance?
>
>
>
> 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.
>
>
> Regards,
> Steve Ting
> CAE engineer
> Inventec corp, Taiwan, ROC.
>

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