>From: "Jim Brown" <jim@audiosystemsgroup.com>
>Subject: Re: [TowerTalk] Subject: Re: Managing Balanced line transitions
>On Mon, 05 Feb 2007 08:16:44 -0600, Cecil Moore wrote:
> > A husky 1:1
> >choke with a few thousand ohms of choking impedance works well to
> >reduce the common-mode current. If the choke is placed at a balanced
> >to unbalanced junction, it is also functioning as an elementary balun.
>
>There are some serious misconceptions here.
>Transformer baluns and choke baluns are fundamentally different. A
>transformer balun MUST have low loss (all the transmitter power goes
>through the core, so loss converts it to heat),
Hmmmm, the misconception seems to be mostly yours. The differential
transmission
line current fields *cancel* in the core so, technically, they don't go
"through the core".
If the currents are perfectly balanced, only a negligible amount of flux is
induced inside
the core. The only appreciable amount of current-induced-fields inside the
core is from
the common-mode currents. This is true whether the transmission line used is
balanced
or unbalanced. So Transmission Line Transformers (baluns) are *NOT*
fundamentally
different from chokes. They both pass differential currents without
appreciable loss
and attempt to block common-mode currents with a resistive and/or an
inductive
impedance. What TLT's are fundamentally different from is ordinary
transformers and
what you say is true for ordinary primary/secondary transformers where the
energy
does indeed flow through the core. The TLT transmission line energy does
*NOT* flow in
the core because the differential fields are canceled in the core. (That
conceptual mistake
is easy to make and you may want to update the info on your web page.)
Consider two wires passing through a ferrite core. If they carry ideal
balanced currents,
one current will induce a field in the core and the other current will
induce a field in
the core that is 180 degrees out of phase with the first one. Thus, zero
flux will exist
in that core under ideal conditions. If there are no common-mode currents, a
very small
toroid can be used for kilowatt applications as it has absolutely no effect.
(But since
common-mode currents usually exist in a real-world system, don't try that at
home. :-)
--
73, Cecil, w5dxp.com
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