The difference between frequency modulation and phase modulation is that with frequency modulation the angular frequency of the signal is modified while with the phase modulation, the phase angle of the signal is modified.

AM - Amplitude Modulation FM - Frequency Modulation

Modulation Index is the ratio of the maximum deviation frequency to the frequency of modulation. In other words it is the ratio of the spread in frequency spectrum to the frequency that was used to modulate the carrier.

For FM,

modulation index is given by the formula

mf= df/f

where,

mf=modulation index for FM

df=difference in carrier frequency

f=frequency of the signal

FM means frequency modulation AM means amplitude modulation