[sdiy] AM and FM conventions

[Scott Stites]

Hi Michael,

I work in the RF industry (test equipment), and have always assumed that FM
and AM in synthesizers were analogous to the FM and AM in RF.  Someone on
this list please correct me if I'm wrong....

In FM, the frequency of the carrier is modified by the amplitude of the
modulator at the rate of change of the modulator.

In AM, the amplitude of the carrier is modified by the amplitude of the
modulator at the rate of change of the modulator.

While on the subject..

When using AM, sidebands will appear offset from the carrier at the
frequency of the modulator.  In other words, a 1 kHz tone modulating a
carrier will cause a sideband to appear offset 1 kHz above the carrier and 1
kHz below the carrier.  If you filter out the carrier and one of the
sidebands, transmit the remaining sideband, then re-inject the carrier, you
will recover the audio information (this is done in SSB radios to decrease
the amount of power required to send a signal a certain distance by not
having to transmit the carrier and  both sidebands).  If you shift the
re-injected carrier slightly, the audio will shift as well, resulting in
that 'Donald Duck' kind of audio.  I gather this is the way some frequency
shifters work?

Take care,
Scott