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

From: Ulrich Mussler ([email protected])
Date: Thu Mar 02 2000 - 03:36:20 PST

Hello, Steve,

i want to give you simple explanation of how I think the facts are:

If you drive a twisted pair cable with one source, signal on one wire,
ground to the other, you will get a system (S+ G) with an wire-to-ground
impedance. As you can expect, the return current on the ground wire is the
same as on the driven wire. Although the return current is in opposite
direction and of same magnitude as on the driven line thsi is not the case
as a symmetrical line.

In the symmetrical or differential case, each wire is driven with it's own
source, where output signal is same in magnitude but shifted by 180° in
phase. In the modelling you have a common ground, where both sources are
connected to. So it's obviously that each driven system causes a return
current, which would flow in the ground wire (which is in twisted-pair
system not present). Because of the opposite directions of the return
currents in the result it's cancelled.

Another fact is the coupling between the driven lines themselves which
causes a quasi-parallel-circuit of impedances from wire-wire- and
wire-ground-impedances. In the end the lower the coupling impedance between
the two twisted-pair wires is, the more coupling occurs and the lower is
wire-impedance itself. On the other hand, if coupling impedance is very
high (up to infinity) and no coupling orrcurs the wire impedance references
only to present ground, what is the case in a coaxial cable.

Summarized it should be seen, that a differential driven twisted pair acts
as an 3-wire-system, while in the single-driven only 2-wires are present,
and the relationships between wires and ground are quite different.

I hope this helps.

Best tregards,

Ulrich Mußler
RIA-BTR produktions-GmbH

-----Ursprüngliche Nachricht-----
Von: =?big5?B?VGluZywgU3RldmUgKKRCvW69ZCk=?= [SMTP:[email protected]]
Gesendet am: Donnerstag, 2. März 2000 03:10
An: '[email protected]'
Betreff: RE: [SI-LIST] : How to measure differential pattern on test
coupo n?

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: [email protected] <mailto:[email protected]>

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