[sdiy] Yet another try at explaining beat tones, etc.

[Needham, Alan]

>I am still a bit fuzzy on what Makes a system non-linear so to clarify
>lets use a Voltage Control Amplifier.

Beat frequencies do NOT involve amplitude modulation, just mixing.
I feel terminology is responsible for some confusion here.

In a simple AM radio transmitting a steady audio sine (amplitude
modulation of RF carrier), there are two sidebands generated, one above
the carrier and one below, they are both spaced from the carrier by the
frequency of the audio signal (assume RF is 100kHz and audio is 1kHz -
sidebands are at 101kHz and 99kHz, both are transmitted, along with the
100kHz carrier).
These sidebands DO exist --- to transmit with a "single sideband" (SSB)
radio all I need to do is filter out the carrier and either unwanted
sideband, transmit, say, 99kHz, and an SSB receiver will have enough
information, by simply 'mixing' in a replacement of the missing 100kHz
carrier, to correctly reproduce the original 1kHz audio as if it were
simple amplitude modulation, even though one sideband is still missing.

With mixing, there is no mechanism to produce anything other than the
sum of the inputs, unless there is some 'waveshaper' non-linearity, at
which point it ceases to be simple summing and, potentially, starts
giving "product" terms?

OK, I'm gone - apologies for wasting bandwidth!

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