Deviation

Author: J.B. Hoag

The ratio of the maximum deviation frequency to the maximum audio frequency is called the deviation ratio. Values from 1 to 5 are used in practice. With a value of 5, if the maximum voice frequency is 4,000 cycles, then the deviation frequency will have a maximum value of 5 · 4,000 = 20,000 cycles per second on either side of the carrier. A 40-kHz channel must then be allotted, and all r.f. and i.f. amplifiers must have frequency response curves wide enough to give nearly equal gain throughout this width of band, as contrasted with the usual 10-kHz band for amplitude modulation.

It is not necessary to change the frequency of the oscillator by the total amount of the frequency deviation, say 30 kHz, because frequency multipliers may be used. Suppose, for example, that the oscillator's frequency is changed by the a.f. input by 5 kHz, and that its output is multiplied fourfold. Then the frequency deviation will be increased fourfold to a frequency of 20 kHz.

An advantage of the frequency-modulation method over the amplitude-modulation scheme is the reduction of noise at the receiver. The ratio of strength of the signal to that of the noise is greater with the f.m. scheme than with the a.m. scheme under certain conditions. With a fairly large deviation ratio, say 5 to 1, the signal-to-noise ratio of f.m. is better than with the a.m. method out to a considerable distance from the transmitter, after which the two become comparable. With weak signals, a deviation ratio of 1 to 1 is preferred.