Re: [SI-LIST] : How to measure differential pattern on testcoupon?

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From: Ron Miller (rmiller@Brocade.COM)
Date: Thu Mar 02 2000 - 16:07:52 PST


Hi Bob

I would just like to add that BI-filar wire is available in 2,3,4 insulated
wires
in parallel just for this purpose. Don't remember the manufacturer right now
but a search on the web should find it soon enough.

Ron Miller

Bob Lewandowski wrote:

> 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.
>
> ---Bob Lewandowski
> Vixel Corp.
>
> 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.
>>
>> --
>> Regards
>>
>> 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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>

--
Ronald B. Miller  _\\|//_  Signal Integrity Engineer
(408)487-8017    (' 0-0 ') fax(408)487-8017
     ==========0000-(_)0000===========
Brocade Communications Systems, 1901 Guadalupe Parkway, San Jose, CA  95131
rmiller@brocade.com,  rbmiller@sjm.infi.net

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