Computation of FM Distortion in Linear Networks for Bandlimited Periodic Signals

In spite of the efforts of a large number of investigators who have studied the problem over three decades there is no way to compute the distortion caused by filters and other networks for arbitrary angle modulated signals of large index or large baseband band widths. However, by use of the Fourier method 1-4 introduced by Roder in 1937, it is possible to compute the exact responses of networks to a frequency-modulated signal for bandlimited periodic modulation signals. In addition to deterministic signals of this class, noise modulation can also be simulated and the resulting network distortion computed by a Monte Carlo procedure. In an excellent paper, Medhurst and Roberts 4 have described the procedure and given some results for low index FM, pre-emphasized in accordance with CCIR standards. 1043 1044 T H E BELL SYSTEM TECHNICAL JOURNAL, JULY-AUGUST 19(58 Their computer program was written in Extended Mercury Autocode. The same method, coded in FORTRAN II, and extended to include the effects of amplitude as well as phase distortion is being used to study large index FM systems. The results presented are for single sine wave modulation and random noise modulation.