In general, the frequency deviation is the frequency range between the lowest and highest frequency. In frequency modulation, the frequency deviation is the frequency change caused by the modulation of the carrier frequency. It corresponds to the maximum frequency deviation of the high-frequency carrier frequency caused by the low-frequency modulation signal.
The frequency deviation is proportional to the amplitude of the modulation signal. The frequency of the modulation signal is expressed by how often the frequency deviation is passed through in one second. At a modulation frequency of 1 kHz, the frequency deviation is passed through 1,000 times per second.
With higher level and higher dynamic range, the frequency deviation increases. In VHF broadcasting with its high quality sound, the frequency deviation is about 150 kHz. In contrast, it is only about 10 kHz for voice radio and between 20 kHz and 40 kHz for radio microphones.
The quotient of the frequency deviation and the modulation frequency is called the modulation index.