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Frequency modulation and phase modulation are physically equivalent.

Suppose you have unmodulated sinusoidal signal, ew t

This signal has the phase of

  • phi(t) = w t,

and frequency of

  • f(t) = 1/(2 pi) * dphi/dt = w/(2 pi) = f0.

Now we apply the frequency modulation with frequency deviation of df and modulation frequency of fm:

  • f(t) = f0 + df Cos(2 pi fm t)

  • phi(t) = Integrate[f(t) dt] = w t + df/fm * Sin(2 pi fm t)

Now think about the phase modulation with modulation depth of m and modulation frequency of fm:

This means that the effect of FM is the same as that of PM. This indicates that the modulation depth of FM is given by

  • m = df / fm.

For given df, FM gives a larger modulation depth with a lower modulation frequency.

FMandPM (last edited 2012-01-03 23:02:40 by localhost)