What is the purpose of a magnetic "Choke" added to electronic wiring?

I am installing a Sony XM Satellite radio. It came with two snap-on magnetic chokes, which the instructions say are required by the FCC. One goes on the antenna wire and one goes on the power cord, right up next to the radio unit. They are exposed because the radio is freestanding, and they look like s**t. What is their purpose? Can I safely leave them off?

Their purpose is to trap common mode interference from being emitted or absorbed by the wires. Common mode interference is noise that is in phase on all the conductors and is prevalent in digital electronics which use square waves, which are inherently noisy since they have an infinite number of harmonics. You can leave them off, but you might find that the device doesn’t function optimally or it interferes with other radio and TV receivers nearby.

Bugger!

Good explanation, better than mine would have been.

To expand on what Q.E.D. said: Some appliances are susceptible to radio frequency interference (RFI). It is often found that common-mode RFI enters the appliance via an electrical cable. One way to correct this problem is to install a low-pass (LP) filter on each conductor just before it enters the appliance; the filter will attenuate RF noise while passing lower-frequency signals. This can be done many ways, but probably the easiest way is to install a series inductor for each conductor. (A choke is another name for an inductor.) This inductor, when combined with the input resistance-to-ground of the appliance, constitutes a first-order LP filter. A nifty way to insert an inductor is to wind each conductor around a ferrite core a few times. Alas, the “snap-on magnetic choke.”

Note also that it helps in the opposite direction, also (as stated by Q.E.D.).

It is called a “choke” because it “chokes” or stops the transmission of high frequency current through the wire (or the part of it that is common to all conductors within a wire bundle).

Whatever else may go wrong if you leave them off, I can’t see how it would be dangerous or damage either this or other equipment. The problem would be that it doesn’t work as satisfactorily while they are off (or possibly something else nearby doesn’t work as satisfactorily).

Slight nitpick… while it is true its reactance increases with frequency, an inductor can also be configured to “choke off” low frequencies, such as a first order HP filter using a series resistor and shunt inductor…

But in this case it isn’t choking low frequencies through the wire it’s on, it’s choking high frequencies, and indirectly preventing the low frequencies from going down some other wire.

The “choke” name comes from the world of RF, where they consider that it chokes or stops the signal. My point was that the origin of the term “choke” also explains why it’s added to wiring, answering the OP.

You could use it like your example, and from a systems point of view this could be said to choke off the low frequencies. You could also unwind the wire and garrote someone, choking the life out of them. But these don’t relate to the origin of the term “choke” or explain why chokes are “added to wiring”.

If you leave them off, it’s quite possible that your neighbors suddenly find their TV or radio or cordless phones suddenly stop working well, and after they pay for various repair attempts and eventually pay for a service tech to come out, and the tech identifies your improperly set up satellite radio as the cause of the problem in the neighborhood …

Well, I can see where that might indeed be “dangerous”. I can even see the possibility of “damage”, but rather than to the equipment it’s more likely to be damage like a black eye or a bloody nose to you personally.

Put them on. Sony included them for a purpose, use them. If you don’t like the way it looks, put a nice lace tablecloth over the whole thing, or something like that.

FWIW, we did an experiment with these in Physics at University, and they do actually work quite well under the proper applications. This is a case where a very deceptively simple-appearing device actually can deliver a bit of benefit.

>>If you leave them off, it’s quite possible that your neighbors suddenly find their TV or radio or cordless phones suddenly stop working well…

Oh, yeah, I guess this is a possibility, and t-bonham is right.

Generally people that sell electonic products go through some sort of process to ensure that they won’t interfere with other people’s electronic products. I hope whatever that process is, it doesn’t let a product through by relying on those little snap-on ferrites. But I don’t know.

We’re in agreement, Napier. I just didn’t want folks to think that the statement “an inductor chokes high frequency” always means that a filter w/ choke attenuates high frequencies. The opposite could be true.

>>I just didn’t want folks to think that the statement “an inductor chokes high frequency” always means that a filter w/ choke attenuates high frequencies. The opposite could be true.
Just so.

Since you can make filters stop either high or low frequencies, and you can build them with either capacitors or inductors (or both), how do they decide whether to use capacitors or inductors?

One way is the cost of storing energy. I studied this point closely for the case of power supply filters and found that inductors cost 270 X what capacitors cost per Joule stored.

Another way is the mutual inductance you can get wrapping several wires through the same core - that is, you can wrap your video cable through a Ferrite and choke the common-mode high frequencies, and yet still transmit the differential-mode video signal to your monitor, for example. I don’t see a corresponding multichannel ability for capacitors.