"coustanis" ) writes:
....and what does it sound like?
I used to think it was a type of radio but since reading this group
I see it is a sound.

Did you look in a dictionary for the basic definitions? Even if the
dictionary doesn't give a technical explanation, it should help you
when adding the context of radio around it.

A heterodyne is when two frequencies are beat together, causing a third
frequency to be generated.

So when two stations are too close together, their carriers will beat
together to cause a beat note in your receiver. If one is at 1MHz, and
the other at 1.001MHz, then you get a 1KHz beat note. This is one reason
AM broadcast stations are 10KHz apart. It's high enough that it will
be out of hearing range for most people, and high enough that if it's
a bother one can filter it out without really bothering the actual
contents of the modulation.

The less structured a service is, the more likely the heterodynes. Listen
to CB when the band is open, and it's full of beat notes, because the
stations aren't all on the same frequency. (Amateur radio bands could
also be the case, but when single sideband replaced AM, the carriers
which are what beat together disappeared.)

A beat note sounds like an audio note, because it's the difference of
two higher frequencies. You'd only hear the audio beats, because of
the frequency response of the receiver, and your hearing range. They
won't sound exotice because they aren't. They may vary in amplitude,
if the stations that are causing the heterodyne are varying in amplitude.

If you've got a receiver with a BFO, turn it on while listening to an AM
station. You will hear the heterodyne, or beat note, of the BFO and
the station's carrier. Vary the BFO tuning, and the frequency of
the beat will change.

Tune to a CW (ie a code station) and turn on the BFO, and what originally
sounded like thumping or the on and off of noise, is now a tone. The
BFO heterodyned the keyed carrier down to an audio frequency, where
you can hear it.

A superheterodyne receiver, which must be what you are thinking of,
just extends this idea. It heterodynes the incoming signal to an
intermediate frequency, where it can better be amplified and filtered,
before going on to further stages of heterodyning, or the ultimate
detection to audio.

Michael