Re: How does AM work?

neilsf1975@xxxxxxxxxxxx wrote:
Hello,

This is essentially the same question I posted in the thread on "FM
Photons", and I've just become more puzzled (and I haven't seen any
replies there). Very simply, if I have a source (a "carrier") of
monochromatic photons (at 800kHz, or at visible light - it's not
important) and I modulate this photon source mechanically, by blocking
and unblocking this source, will I get the same spectrum as if I had
keyed the source on and off? (The spectrum predicted by Fourier
analysis of an amplitude modulated signal - a carrier with sidebands)
And in both situations, why do I get a spread of photon energies? At
least one reply to Radium pointed out that photon energy doesn't
change. But if I put a detector between my source and the mechanical
shutter I believe I will see monochromatic photons, yet the same
detector on the other side of the shutter will see a broader spectrum
- a mixture of the carrier and higher and lower energy photons. Is
this quantum physics at work? Or does Fourier not apply in this
situation, and I will not get any sidebands - just a presence or
absence of monochromatic photons.

I replied to my own question in that thread with my own go at
answering it, but I wouldn't mind seeing a reply from somebody more
cluey! I have a little bit of background in electronics engineering,
so I thought I understood AM.

(I suppose only quite modest resources would be required to test this
- an audio frequency oscillator for the carrier, a metal bladed fan
for the modulation, and an audio pre-amp feeding a computer with some
spectrum analysis software. Not sure about the antenna required to
radiate and pickup a useable signal though)

Yes, you will get the same spectrum.

To understand why, you have to remember photons have both particle
and wave characteristics.

If you look at it as a wave, you're are turning the source on and
off and therefore introducing distortion to an otherwise perfect
sine and generating other frequencies.

--
Jim Pennino

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