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

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From: David Instone (
Date: Thu Mar 02 2000 - 02:18:19 PST

=?big5?B?VGluZywgU3RldmUgKKRCvW69ZCk=?= wrote:
> 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
> Tel: 886-3-3900000 Ext:2558
> E-mail:

If your twisted pair is suspended in mid air then I believe you are
right. However, if it is close to, or even supported on, a conductive
surface then you have a G which will give you common mode impedances and
the differential measurement is required.


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 Tel: +44 (0)23-92-486363

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