Consider common mode noise on a simple parallel wire transmission line.
Identical in-phase noise currents would flow on each of the parallel
wires.
A common mode choke around the line would insert a high impedance equally
onto both wires.The choke's effectiveness at suppressing the common mode
currents would depend on the shunt impedance to ground of the two wires.
The
shunt impedance between the two wires e.g., the impedance of the
transmission line, is immaterial since there is no common mode voltage
difference between them.
On the other hand, the choke's ability to shield differential (as opposed
to
common mode) currents depends a great deal on the differential shunt
impedance. The lower the shunt impedance, the more effective the choke.
Seems to me this is quite apparent if one draws out the circuit and
includes
both the impedances to ground and the differential impedance between the
two
wires. But, like I said, maybe I'm not following something.
Differential current suppression is a pretty complex function in a bifilar
or trifilar choke, because mutual coupling between conductors enters the
equation.
Bifilar winding of power leads is a two edged sword. While it reduces core
flux from power line currents, it also reduces differential suppression in a
similar fashion. In a tightly coupled choke, the windings simply behave like
a transmission line.
It takes a stand-alone choke in each conductor to add significant
differential mode impedances, or a poorly coupled common core choke. You
will not obtain that with a bifilar or trifilar winding.
All of these problems, regardless of strong opinions, translate to ratios of
shunt impedances and series impedances. They are nothing more than
"pi-attenuators", or "pi-filters".
If impedances loading a choke on either or both ends are very high,
attenuation is significantly reduced. We can see this with choke baluns. If
a choke balun is placed where the antenna sources very high common mode
voltages (high impedance), choking impedance often has to reach impractical
levels. The same is true for the load side of the choke.
As you say ZR, it is all about all of the impedances. It is a system, and
looking at one part in isolation is pointless.
I keep bringing up a CATV system I had to cure for BCI. An apartment complex
was constructed right over the radial system of a 5 kW AM station. The cable
TV company tried triple shielded cables, and regular common mode chokes.
They also tried CATV high pass filters. None of it worked, the system was
mess.
All I did was have the special cables removed, and standard cables
installed. All filters and chokes were removed. The cable entering each
building entered alongside power mains, and the shield was bonded to the
mains ground. The cable followed the power distribution to each apartment.
Inside the apartment, the CATV wall plate was commoned with the safety
ground of the wall outlet for the TV set though a suitable rating capacitor.
Nearly every apartment cleaned up completely without using a single choke or
filter.
This was because there was no differential RF allowed between the CATV and
the mains. The common mode of the TV, since it was very high at 1520 kHz,
prevented significant common mode to the set. A CM choke simply would not
have done anything, because the TV's were already electrically small and had
a pretty high CM impedance.
I have minimal CM choking here. I have no RFI issues, even beaming the four
square through the house (just 75-150 feet away). This is because I don't
allow differential between wires and cables by use of outlet strips that tie
the MATV antenna and other things to the power feeds at each hub. When I do
need a choke, it isn't an astronomical value or difficult design. Just a
snap on will often be enough, because the shunting impedances are kept low.
What works for ingress also works for egress.
73 Tom
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