Mike Knudsen wrote:

I have a cheap AM/FM clock radio that sounds like light-dimmer power line noise
even on FM. I've tried tacking caps all over the place, to no effect. The DC
supply is clean on a scope. You have to wonder how such a sound gets into FM
-- it sounds exactly like the sort of noise that sends you reching for the ANL
switch on a real radio. --Mike K.


A lot of that kind of noise comes from solid state rectifiers in the
powersupply. The HV rectifiers don't switch off quickly enough, and
essentially short out the supply for an instant around the zero crossing
point of the input sinewave. This always sounds like a very raucus
60Hz (half wave rect.), or 120Hz (full wave rect.) buzz, and it comes in
thru some RF or IF stage in the receiver.

This can be solved one of two ways:

1) add a series resistor to each each diode to limit how much current
can flow, 100 ohms, or some such. (Note, one for each diode!)

2) switch to fast recovery diodes.


As to why cheapy FM radios respond to this kind of AM noise, it is all
in the use of a ratio detector vs a limiter/discriminator. Ratio
detectors are pretty good at eliminating AM, but limiter/discriminator
stages are very good at eliminating AM. Ratio detectors are basically
simpler, and as such much cheaper.

-Chuck Harris, WA3UQV