Ward Silver wrote:
>> Recall that common mode chokes work by adding a high resistive
>> impedance in series with the common mode circuit.
>
> Recall also that the common-mode circuit for coaxial line is on a
> separate conductor from those carrying the differential-mode signal,
> creating a "three-wire" system in which the common-mode currents are
> flowing on the outside of the coax shield, completely independently
> of the differential-mode currents flowing on the inside of the shield
> and the outer surface of the center conductor. That means remedies
> for the common-mode signal can be applied to the "third wire" without
> affecting the differential-mode signal.
>
> For parallel-conductor transmission line, though, there is no "third
> wire" - the common-mode signal flows equally on both wires of the
> line, along with the differential-mode signal. Anything that affects
> the common-mode signal is going to have an effect on the
> differential-mode signal, as well.
Unless the widget that's doing the affecting only "sees" the common mode
signal. If you were "sufficiently" far away from the pair, the
differential mode cancels.
We do this all the time to filter network cables by running them through
a toroid. The HF/VHF signals being carried in differential mode aren't
affected by the toroid, but the toroid blocks the common mode RF being
radiated by the network cable.
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