Tom,
You simply don't get it. The choke is NOT a filter. It is a high
impedance added to the common mode circuit. The common mode circuit is
acting as an ANTENNA, either for RX or TX. If for TX, the noise source
is inside the box, and the current will depend upon the voltage of the
source and behavior of the circuit as an antenna. If for RX (that is,
the noise is received on the wiring by simple antenna action and coupled
into the box via failure of the wiring to go the shielding enclosure)
the current will again depend upon the behavior of the circuit as an RX
antenna.
When we add a choke of sufficiently high resistive impedance to that
circuit, we reduce the current at frequencies where the choke is effective.
When preparing to publish my first research on this in 2003 (to the
AES), I found references in ancient applications notes from EU mfrs of
ferrites showing that they clearly understood this principle. Those
ferrite cores molded onto cables emerging from electronic equipment are
not filters, they are chokes. They do work in the frequency for which
they are designed, and the only capacitance in the circuit is their own
parallel capacitance that forms their resonant circuit -- it is the
capacitance from one end of the core to the other via the dielectric of
the core.
73, Jim K9YC
On Fri,1/30/2015 3:16 AM, Tom W8JI wrote:
Any filtering or decoupling system works by the ratio of series
impedance to shunt impedances.
A series choke is useless unless there is some value of shunt
impedance in the system. As a matter of fact, lack of established
shunt impedances is what can drive choke requirements to unrealistic
values. This is true in baluns, just like it is in line filters.
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