🚀 go-pugleaf

On May 27, 9:02 am, GreenXenon <glucege...@gmail.com> wrote:
> Hi:
...
> Increasing the modulator’s frequency will increase the rate in which
> the carrier frequency changes. So why is it impossible to transmit a
> higher-frequency modulator signal on a lower-frequency FM carrier
> signal?

It's not impossible but it gets really weird.   You get "negative
frequency" sidebands which wrap around zero and often become noise,
depending how they line up with the non-wrapped sidebands.

>
> I can understand how increasing the modulator’s amplitude will
> increase the bandwidth the carrier uses.
>
> I don’t understand how increasing the modulator’s frequency will
> increase the bandwidth taken up by the carrier. Higher modulator
> frequency simply means the carrier’s frequency will vary faster; it
> will not take up more frequencies unless the mod’s amplitude
> increases.

Somewhere out there, there's a Fourier Transform applet which will
show you what happens to the spectrum.   You get sidebands spaced at
multiples of the modulating frequency.

Executive summary:  FM (and its conjoined twin, phase modulation)
operate by advancing and retarding the progress of the carrier around
its "phase circle" if you will.  The changes in phase-progress
generate frequencies you might not at first expect.  It comes out to C
+- nM for a lot of integer "n".  The amplitude of each one  can be
predicted by something called Bessel Functions.