It follows from the theoretical and experimental investigations that the error of measurement by the above method for a zero intermediate frequency and a sinusoidal modulation is equal to $$\frac{\pi }{{2(\Delta f/F)}}.$$ 100%, i.e., it drops with a rising modulation index. The deviation of the intermediate frequency from zero has a small effect on the measurement results [the additional error due to this factor is of the order of (f if/Δf)2]. Since the error of the frequency counter can normally be neglected, the only source of error for a large modulation index, in fact, consists of the modulating function deviation from a sinusodial shape. For a small nonlinear distortion coefficient this error is of the same order as the coefficient.