In article ,
Cecil Moore wrote:

FM usually occupies about twice the bandwidth of AM.

Thanks for adding "usually" to your original statement.
I was just pointing out that if the FM peak deviation
is equal to the maximum modulation frequency, then the
FM signal occupies the same bandwidth as AM.

I believe that you're mistaken on this point, Cecil.

As I understand it, the spectrum of a frequency-modulated carrier
will have energy at the carrier frequency (except at certain very
specific modulation indices), and energy at offsets from the carrier
which are equal to the modulating frequency and all of its harmonics.

It's entirely possible (and in fact common) for an FM signal to have
energy at frequencies which are further away from the nominal carrier
frequency, than the maximum instantaneous deviation of the carrier
would suggest. For example, a carrier which is modulated with a 1000
Hz tone, but only to the level of having a maximum peak deviation of
500 Hz from nominal, will still have sideband energy out 1000 Hz and
2000 Hz away from the nominal carrier frequency.

This is *really* counter-intuitive, and I haven't been able to fully
wrap my brain around the question of just how the math works... but
the math says that it's true, and my own spectrum measurements show
that it's true.

An FCC-approved NBFM phone signal, which has a modulation index of no
more than 1.0 at the highest modulating frequency, *will* have energy
out further than the peak deviation would suggest. There will be some
amount of sideband energy located out at twice the highest modulating
frequency, and a bit at three times.

The levels of these further-away-from-nominal-carrier sidebands will
be relatively low - they don't start to become appreciable until you
get to a higher modulation index.

An AM signal being modulated by the same intelligence signal would not
have any energy out at the multiples, *if* it was generated and
transmitted in an entirely-distortion-free manner.

My guess is that in practice, NBMF (per FCC regs), and ham-grade AM,
probably have very similar bandwidths. All it would take for the AM
signal to be spread out as far as the FM signal, would be a bit of
nonlinear distortion in the audio chain, mixer, or amplifier chain...
this will create a second-harmonic sideband.

I did an introductory presentation to a local hamclub last year (with
some math, admittedly simplified and approximated and covered up with
moderate amounts of hand-waving) which explains some of these
concepts. I included some plots of actual measured spectra, generated
by an HP signal generator and captured using an old Systron Donner
spectrum analyzer.

http://www.radagast.org/~dplatt/hamr...modulation.pdf

--
Dave Platt AE6EO
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