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.
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 beteeen the two
wires. But, like I said, maybe I'm not following something.
73,
Jim W8ZR
> -----Original Message-----
> From: Topband [mailto:topband-bounces@contesting.com] On Behalf Of Jim
Brown
> Sent: Friday, January 30, 2015 2:27 PM
> To: topband@contesting.com
> Subject: Re: Topband: Elimination of Treadmill RFI on 160 meters
>
> 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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