In frequency modulation (FM), the amplitude of the modulated signal is kept constant and equal to the carrier value Vp at rest. The frequency instead varies, in proportion to the instantaneous amplitude of the modulating signal and the maximum deviation frequency of the carrier frequency whith respect to the carrier at rest is called and is equal to 75 KHz being normalized in 1961. The speed with which this change occurs, is determined by the rate of variation of the modulating signal itself