The amplitudes of the sideband components are symmetrical (at least for
modulation by a single sine wave), but the phases aren't. The phases of
all the upper sideband components are in phase with the carrier; in the
lower sideband, the odd order components (and only the odd order ones)
are reversed in phase. With multitone modulation, things get a whole lot
more complex. Unlike AM, FM is nonlinear, so there are sideband
components from each tone, plus components from their sum, difference,
and harmonics. The inability to use superposition makes analysis of
frequency modulation with complex waveforms a great deal more difficult
than AM.

Note also that unlike AM, whatever fraction of the carrier that's left
when transmitting FM also contains part of the modulation information.
Of course, at certain modulation indices with pure sine wave modulation,
the carrier goes to zero, meaning that all the modulation information is
in the sidebands. But this happens only under specific modulation
conditions, so you'd certainly have an information-carrying carrier
component present when modulating with a voice, for example.

Roy Lewallen, W7EL

Joel Kolstad wrote:
Avery Fineman wrote:

There isn't any corresponding similarity of
FM and PM to AM for the repetition of sidebands' information when
looking at the spectral content.


Umm... last I looked the spectrum of FM and PM was symmetrical about the
carrier frequency? (Well, the lower sideband is 180 degrees out of phase
with the upper, but that's true of AM as well.) Looking at a single sine
wave input to an FM or phase modulator, this comes about from the Bessel
function expansion of the sidetones and J-n(x)=-Jn(x)?

I know you're far more experienced in this area than I am, however, so I'll
let you explain what I'm misinterpreting here!

---Joel Kolstad