Frequency Modulation

In Frequency Modulation (FM) the instantaneous value of the information signal controls the frequency of the carrier wave. This is illustrated in the following diagrams.Notice that as the information signal increases, the frequency of the carrier increases, and as the information signal decreases, the frequency of the carrier decreases. The frequency fi of the information signal controls the rate at which the carrier frequency increases and decreases. As with AM, fi must be less than fc. The amplitude of thecarrier remains constant throughout this process.

Frequency Modulation

When the information voltage reaches its maximum value then the change in frequency of the carrier will have also reached its maximum deviation above the nominal value. Similarlywhen the information reaches a minimum the carrier will be at its lowest frequency below thenominal carrier frequency value. When the information signal is zero, then no deviation of thecarrier will occur. 

The maximum change that can occur to the carrier from its base value fc is called the
frequency deviation, and is given the symbol fc. This sets the dynamic range (i.e. voltage range) of the transmission. The dynamic range is the ratio of the largest and smallest analogue information signals that can be transmitted.