Propeller Head Plaza Technical and scientific discussion of amps, cables and other topics.

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Re: RFI from strange places

Posted by tomservo (M) on January 9, 2007 at 06:42:53

In Reply to: RFI from strange places posted by Lynn on January 8, 2007 at 18:28:09:

Hi

There are Radio’s, like the Ham’s in the article talk about, which are VERY sensitive detectors of RF energy and there is interference (RFI) heard on a stereo or seen on a TV screen.
AS the fellow identified with his radio detector, the washer was radiating (airborne) from its electronics behind a plastic panel.
It is possible modern electronics may be more susceptible to RF interference than previously, but back when the actual levels of stray RF were vastly higher than now, back then, power supply design was easily able to keep up, it was simply a matter of proper design / parts / cost.
Consider that all radios like those mentioned in the article or even your FM tuner, are able to reject power line noise, in there operation, they MUST be able to do that in order to detect the tiny little antenna signal.
If an audio designer were also a Ham or experienced circuit designer, they most likely would already have made the power input side essentially RF proof unless they were cutting cost..

On the other hand, I have seen a number of SS amplifiers that (as near as I could figure) were rectifying radio signals going in at the output terminals.
That detected signal appeared on the speaker output as a very low level mix of several radio stations.
The easy fix on this amplifier was to increase the turns on the small output choke (de-coupling RF) that was inside of the amplifier, coax speaker cable (with the shield being the ground) probably would have fixed it to.

So far as the frequency of having to deal with problems like this, at least for me, noise has been much much more likely to be from ground currents, induced or otherwise. These signals produce a difference on the shield leg of signal cables. The problem goes away when the inputs (ground connection which completes a circuit) are disconnected. It’s a larger subject than can easily be discussed but this is the ”why” behind isolated AC, star ground and such things.
Here one finds with the magnetic induction side, it gets easier to induce the noise signal as the frequency climbs AND it becomes more audible as one climbs out of the fundamental range (60Hz) so it is easy to think it is RF induced.

Basically RFI is a real problem, which has proven solutions and many companies trying to cash in on it by making it seem like more of a problem than normally it is. More often than not, if you hear any noise of faint zizzing, it is from ground loop currents on signal level cables, not RF.
Best,

Tom

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