Modulation Percentage

Author: J.B. Hoag

In the absence of modulation, the carrier wave has a constant amplitude, E_c, of Fig. 16 C. Modulation increases and decreases the voltage above and below the constant value by a maximum amount designated in the figure as E_a. The ratio E_a/E_c is called the modulation coefficient (m). When expressed in percentage, by multiplying by 100, it is called the percentage of modulation, M. We may write,

Fig. 16 M. Under-, complete-, and over-modulation

Figure 16 M shows cases of under-modulation, correct or 100 per cent modulation, and over-modulation. It is to be noted that the peak r.f. voltage for 100 per cent modulation is exactly twice that of the unmodulated voltage. In order to produce 100 per cent modulation, the a.f. voltage's peak value must be equal to that of the r.f. The strength of the carrier wave will determine how far the radio wave can be transmitted, whereas the modulation percentage will determine the audible output at the receiver. Modulation should never exceed 100 per cent or the a.f. will be distorted and contain harmonics not present in the original sounds which entered the microphone.