FM modulation (frequency modulation) refers to the superposition (modulation) of a lower frequency analog data (information) signal onto a higher frequency sinewave carrier signal.
With AM modulation (amplitude modulation), the information signal varies the amplitude of the carrier signal i.e. the size of the carrier signal varies but the frequency remains constant. With FM modulation, the amplitude remains constant, but the frequency varies.
Figure 1. AM and FM Waveforms
The higher the frequency of the carrier signal, the shorter the size of the antenna (aerial) required to detect and receive the signal. FM radio stations can be received using an antenna typically less than 1 meter in length, a convenient size for a typical FM receiver.
Generation of an FM signal requires a voltage-controlled oscillator (VCO) circuit. This can be designed and then built using discrete components, but they are also available as integrated circuits (ICs).
Once the modulated signal has been detected and received, the analog data signal can be recovered using a Phase-Locked Loop (PLL) circuit.
While the carrier signal is an analog signal, the information signal can be digital. In this scenario, which is referred to as Frequency Shift Keying, only two frequencies are used – one frequency to represent ‘0’ and the other (higher or lower) frequency to represent ‘1’.