User`s guide

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All you need to remember is that a 100% Frequency Modulation
amplitude causes the destination frequency to vary between 0 Hz
and twice the specified destination frequency.
In the above illustration, a 100% modulation amplitude causes the
440 Hz destination frequency to vary between 0 Hz and 880 Hz. A
modulation amplitude of 50% would cause the destination frequency
to swing between 220 Hz and 660 Hz (that is, between 0.5 and 1.5
times the specified destination frequency).
Modulation frequencies in the audio range produce complex
sounds, with many more sidebands than are produced by amplitude
modulation. The number and placement of the sidebands depend
on the relative frequencies of the two sounds, their harmonic
content, and the modulation amplitude. Many interesting harmonic
and inharmonic effects are possible, especially if the modulation
frequencies and/or amplitudes are themselves being slowly
modulated. Modulation amplitudes greater than 100% can cause
interesting through-zero FM effects, as the resulting frequency will
become negative part of the time.
If the Frequency Modulation comes from a Pitch Tracker, the
destination’s specified frequency has no effect. It doesn’t matter
what frequency you enter, because the frequency will be determined
by the output of the Pitch Tracker times the Modulation Amplitude.