In article ,
glen herrmannsfeldt wrote:

Rich Seifert wrote:
(snip)

You can use a transformer, but wired as a *balun* (coils in series with
the lines instead of across them); this provides the required impedance
transformation while still passing DC.

So that is how they do it.

TV baluns, from 300 ohm balanced to 75 ohm coax, don't do that.

http://en.wikipedia.org/wiki/Balun

Does that restrict which impedance transformation you can make?


The impedance transformation is determined by the winding ratio,
regardless of whether the coils are in series or parallel with the
lines.

Putting the balun in series provides better low-frequency response (down
to DC), but of course this configuration does not provide any electrical
isolation. It functions very much like a common-mode choke, with an
impedance change.


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