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
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Topband Reflector
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