In frequency modulation the variation in frequency of the carrier wave happens due to the instantaneous value of the modulating voltage. In this case the amplitude is made constant.
The equation of the wave before modulation is shown below:
x, A, ω, Φ represents the angular frequency, maximum current or voltage, phase angle and instantaneous value of the voltage which the modulation is made proportional to the change in frequency. The change in frequency is equal to the modulating frequency.
In part (a) of the diagram a low frequency modulating signal is shown.
In part (b) a high frequency carrier signal is shown.
Part(c) shows an overstated view of a Fm signal.
Important note: Same amplitude signals will change the value of carrier frequency by same amount.
