In message , isw
writes
In article ,
Rich Grise wrote:

On Tue, 03 Jul 2007 22:42:20 -0700, isw wrote:

After you get done talking about modulation and sidebands, somebody
might want to take a stab at explaining why, if you tune a receiver to
the second harmonic (or any other harmonic) of a modulated carrier (AM
or FM; makes no difference), the audio comes out sounding exactly as it
does if you tune to the fundamental? That is, while the second harmonic
of the carrier is twice the frequency of the fundamental, the sidebands
of the second harmonic are *not* located at twice the frequencies of the
sidebands of the fundamental, but rather precisely as far from the
second harmonic of the carrier as they are from the fundamental.

Have you ever actually observed this effect?

Sure. (In a previous life, I designed AM and FM transmitters for RCA).
Just get a short-wave radio, locate yourself fairly close to a standard
AM transmitter, and tune to the harmonics. you'll find, in every case,
that the audio sounds just the same as if you were listening to the
fundamental.

Works for FM, too, but the situation is somewhat more complex.

Isaac


Yes, I think I'm missing something obvious here. Let me have another
think (aloud)....

If you FM modulate a 1MHz carrier with a 1kHz tone, you get a spectrum
consisting of a 1MHz carrier in the middle, plus a family of sidebands
harmonically spaced at 1kHz, 2kHz, 3kHz etc (to infinity).

[One obvious difference between the FM spectrum and that of an AM signal
is that the AM spectrum only has sidebands at 1kHz, and the amplitude of
the carrier does not vary with modulation depth. With the FM signal, the
amplitudes of the carrier and each pair of sideband do vary with the
amount of modulation.]

So, if you FM modulate a 1MHz carrier with a 1kHz tone, you get a 1Mhz
carrier and the family of 1kHz 'harmonic' sidebands. Demodulated it, and
you hear a 1kHz tone.

Now double the signal to 2MHz. You might expect the sidebands to appear
at 2, 4, 6kHz etc. However, if you demodulated the signal, you still
hear the original 1kHz tone (which should now be double the amplitude of
the original 1MHz signal). You definitely don't hear 2kHz. This at least
proves that the original 1kHz FM modulation is preserved during the
doubling process.

So, would it be simplistically correct to consider that, during the
doubling process, the original family of 1kHz sidebands also mix with
the new 2MHz carrier, and create a family of 1kHz sidebands centred on
2MHz?

Or, alternatively, does the original family of 1kHz sidebands (on the
1MHz signal) mix with the original 1MHz carrier to produce a family of
baseband 1kHz 'harmonic' signals, and these then mix with the new 2MHz
carrier to create the family of 1kHz sidebands centred on 2MHz?

Or are both equally valid (invalid)?

A possible flaw in my simplistic 'explanations' is that I would have
thought that, while the doubling process occurs as a result of 2nd-order
intermodulation, surely the two-step process in both 'explanations' is
really 4th-order intermodulation?

However, my explanations work equally well (?) for FM and AM.

Am I wrong, or am I wrong?

Ian.
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