Electrical Engineering - Re: AM radio: 20 KHz sine-wave modulator signal present on an astronomically-low frequency carrier

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Re: AM radio: 20 KHz sine-wave modulator signal present on an astronomically-low frequency carrier

Karl Uppiano

2007-06-30, 5:25 pm

While ignoring the ridiculous units mismatches, irrelevant hypothetical
values, and unlikely actual interest by the OP, the answer will always be
the same:

The highest modulating frequency for AM must be less than 1/2 the carrier
frequency. Conversely, the lowest carrier frequency must be twice the
highest modulating frequency. Period. I don't care what specific frequencies
and/or energies and/or colors you propose.

If you want to modulate at 20KHz, the carrier must be at least 40KHz. It is
no coincidence that CD audio uses a 44.1KHz sample rate. It is essentially
the same principle. If you exceed the Nyquist criterion, the sidebands
overlap the baseband (i.e., aliasing occurs) and you cannot unambiguously
decode the original modulation.

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