Fred McKenzie wrote:
It has been about 35 years since I had a class in school where SSB-FM was
discussed. I recall that if you derive the equations for both AM and FM
SSB, they are identical for practical purposes if the FM signal has low
deviation (low modulation index?).

You're probably thinking of AM vs. narrow band FM. Although the equations
look very similar on paper and the MAGNITUDE spectrum is identical, the
phase spectrum is different in that -- in the phasor domain -- AM always
sits at 0 degrees and just grows and shrinks with modulation (overmodulation
pushes it over to 180 degrees, BTW). NBFM, on the other hand, still has the
carrier at 0 degrees but grows and shrinks along the imaginary axis. Hence
the angle of the phasor is small but time-varying (which implies that the
instantaneous frequency is varying as well -- but of course you already knew
that since we called this whole mess 'frequency modulation'). The angle is
about 15 degrees for a modulation index of 0.3 (what my notes claim as a
good cutoff for NBFM) and about 5 degrees at 0.1.

See the message I posted earlier tonight for a discussion of whether or not
you can recover NBFM with an envelope detector as of course one often does
with AM (the difficulty is due to that phasor's wiggling...). I think not,
but there's plenty I don't have a clue about... yet!

What's the modulation index on two meters anyway?

---Joel Kolstad
....who does know that a wideband FM receiver has no problem whatsoever
receiving NBFM...

Looking at Two Meter FM, the deviation typically peaks at about 5 KHz.
If you listen to your local repeater with an SSB rig such as the IC-706,
it will be obvious that it isn't a clean signal! However, a 3 KHz
deviation FM signal on HF (below 29 MHz) will sound much cleaner when
tuned as SSB, and you may not notice it isn't AM-SSB.