John Smith wrote:
On 9/3/2010 10:01 PM, dave wrote:

...
Common mode rejection of noise happens in a truly balanced configuration.

That is the worst suggestive BS I have ever seen, no antenna knows the
difference between "RF noise" and a desirable RF signal ...

Regards,
JS

The noise is in-phase and cancels when the transmission line is
terminated in a "differential" input. The desired signal is + on one leg
and - on the other. The desired signal remains after differential summation.

John, lo:

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

Any external signal sources tend to induce only a common mode signal on
the line and the balanced impedances to ground minimizes differential
pickup due to stray electric fields. The conductors are sometimes
twisted together to ensure that each conductor is equally exposed to any
external magnetic fields that could induce unwanted noise.

Some balanced lines also have electromagnetic shielding to reduce the
amount of noise introduced.

A balanced line allows a differential receiver to reduce the noise on a
connection by rejecting common-mode interference. The lines have the
same impedance to ground, so the interfering fields or currents induce
the same voltage in both wires. Since the receiver responds only to the
difference between the wires, it is not influenced by the induced noise
voltage. If twisted pair becomes unbalanced, for example due to
insulation failure, noise will be induced. Examples of twisted pairs
include Cat-3 Ethernet cables or telephone wires.

Compared to unbalanced circuits, balanced lines reduce the amount of
noise per distance, allowing a longer cable run to be practical. This is
because electromagnetic interference will affect both signals the same
way. Similarities between the two signals are automatically removed at
the end of the transmission path when one signal is subtracted from the
other.
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