From: Bob Lewandowski (blewandowski@Vixel.com)
Date: Thu Mar 02 2000 - 11:36:31 PST
I believe Dave is correct. To elaborate a little further, with a sufficiently long, 'isolated' twisted pair you can create a "balun", a balanced-to-unbalanced transformer. If the length is sufficient
such that the common mode inductive reactance of the pair is high compared to the source impedance (>150 ohms for a 50 ohm source, and a 50 ohm characteristic impedance twisted pair), the output signal
is approximately "balanced" with respect to ground. Proximity to ground lowers the common mode inductance. It's also difficult to make a 50 ohm twisted pair. The insulation must be really thin and the
lines twisted tightly. This technique is usually done with magnet wire and can be found in rf mixers and similar devices. If you thread some ferrite beads or torroids over the twisted pair you can
increase the common mode inductance and improve the balance at lower frequencies. Most twisted pair, i.e. Cat 5 Ethernet UTP or similar sized wire, has a Z0 of ~100 ohms. With a typical 50 ohm source,
and 100 ohm line you have significant mismatch issues. You also have to watch for the problem of the twisted pair being 'too' long. At the signal frequency where it's length is 1/2 wave long the 'balun'
is totally unbalanced again.
A true differential pair has balanced source impedances with respect to ground and the signals are equal magnitude and 180 degrees out of phase, thus the effect of the adjacency of the 'world' (ground)
much has less effect.
David Instone wrote:
> =?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
> > INVENTEC CORPORATION
> > Tel: 886-3-3900000 Ext:2558
> > E-mail: Ting.Steve@inventec.com
> 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
> http://www.xyratex.com Tel: +44 (0)23-92-486363
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