I'm not disagreeing with anything you said, Jim, and I don't believe Tom is
either. We're just making the point that a common mode choke's effectiveness
in blocking common mode currents depends on the impedance to GROUND of the
termination of the conductors. I used a parallel line in my example merely
to illustrate the point, since current flow in e.g., coax cable. is more
complicated. Obviously a parallel line not terminated in anything will have
no common mode current because there's no place for the current to go.
Introducing choke coupling and stray capacitance, Q, resonances, etc., into
the discussion, while obviously important in real life, tend to obscure the
basics.
Jim W8ZR
> -----Original Message-----
> From: Jim Brown [mailto:jim@audiosystemsgroup.com]
> Sent: Friday, January 30, 2015 5:27 PM
> To: Jim Garland
> Subject: Re: Topband: Elimination of Treadmill RFI on 160 meters
>
> On Fri,1/30/2015 2:49 PM, Jim Garland wrote:
> > Guys, I'm not following all this, possibly because I'm confused by Jim's
> > distinction between "filters" and "chokes," because in both cases
they're
> > just lossy inductors, although used in different ways and for different
> > purposes.
> >
> > 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.
>
> But we're not talking about parallel wire lines, Jim. We're talking
> about unbalanced cables that carry power, audio, control signals, and
> RF. And those cables are terminated to equipment at both ends. It's not
> the capacitance to "ground," it's that common mode circuit analyzed as
> an antenna.
>
> > 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.
>
> A common mode choke has no direct effect on differential current. It CAN
> reduce differential current only to the extent that the cable it is
> applied to is imperfect. For one such imperfection, see
>
> http://k9yc.com/AES-SCIN-ASGWeb.pdf
>
> Bill Whitlock has shown that something similar can happen with power
> cables, depending on their construction.
>
> I repeat, a common mode choke is NOT A FILTER.
>
> 73, Jim K9YC
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