Topband: Zo of an individual CAT5 twisted pair

[Jim Brown]

On 8/13/2013 2:00 PM, Tom W8JI wrote:
> Balanced lines, by system definition, are where a line has equal and 
> opposite voltages from each conductor to "ground" or to space around 
> the conductors and between the conductors, along with equal and 
> opposite currents in each conductor.

Actually, the balance of a circuit is defined by its impedances to the 
reference plane.  If the impedances are equal, it is balanced. The best 
analysis I have seen of balanced circuits is by Bill Whitlock of Jensen 
Transformers, who correctly recognizes a balanced interface as a 
Wheatstone bridge. As recognition of a large body of work, including 
this, Bill was elected a Fellow of the Audio Engineering Society. And as 
a result of this work, IEC Standards for the measurement of common mode 
rejection ratio (CMRR) were changed. To appreciate the significance of 
this, the people who write these standards include representatives from 
many countries and companies around the world, and they are VERY 
reluctant to change, so it means that his analysis convinced a lot of 
very good engineers.

It is the balance of the impedances, as well as the ratio between the 
circuit impedance and the common mode source and load impedances  that 
gives the balanced interface its inherent noise rejection, and 
determines the magnitude of the rejection. And it is the balance of the 
circuit that sets the degree of balance of the voltages and the current 
in any circuit, not the other way round.

Another point -- for interference rejection to occur, these impedance 
relationships must be satisfied at all frequencies where rejection is 
desired. For example, an audio circuit may be balanced at audio 
frequencies, but not at RF. Such a circuit would strongly reject audio 
frequency fields, but would not reject RF fields.

Whitlock's original paper was presented at the San Francisco AES 
Convention in October 1994 (it followed Neil Muncy's paper on SCIN and 
the Pin One Problem) (I was there) and was published in the June 1995 
Journal of the AES. That issue is available from the AES Website for 
about $10, it appears that he will mail you a copy if you ask. The 
essence of his analysis is in various tutorials on the Jensen 
Transformer website. http://www.jensen-transformers.com/apps_wp.html

The paper you want is "Balanced Lines in Audio - Fact, Fiction and 
Transformers"

 >A twisted pair transformer, even in transformer mode, often
 >has significant capacitance between conductors. Because of
 >the capacitance increase, it no longer is independent from
 >primary to secondary for voltages.

I've never heard of a "twisted pair transformer." Perhaps you could 
provide a description?

ANY magnetic transformer has stray capacitance between the windings, as 
well as stray capacitance between turns. The amount of that stray will 
depend on spacing, winding style, conductor size and shape, and the 
dielectric. Bifilar windings, where one winding is a primary and the 
other a secondary, has a lot more stray capacitance between primary and 
secondary than the same windings at widely spaced parts of a magnetic 
core. And even then there will be capacitance between primary and 
secondary if the core is a ferrite because ferrites are a dielectric. :)

But what I really like about your post is that it strongly reinforces my 
primary point, to which I think you were responding -- that the word 
"balun" is used to describe so many different physical circuit elements, 
and arrangements of circuit elements, that it clouds the issue.  And 
that the only good way to understand how things work, to discuss how 
things work, and how well they work, is to use words that accurately 
describe them.

73, Jim K9YC