Frequency modulation

Frequency modulation is likely to play an important part in postwar radio developments. The present position of this method of communication is here reviewed; a brief discussion of the features of the three methods of modulation¿amplitude, frequency and phase¿is followed by a survey of the history of frequency modulation, with a consideration of its limitations, advantages and present applications. The first of the two main Sections of the paper is concerned with the production of a frequency-modulated signal; direct and indirect (integrated phase-modulation) methods are examined with particular reference to the variable-reactance valve modulator with automatic mean-frequency correction, and the indirect transposed-sideband modulator with a crystal-controlled master oscillator. Measuring and monitoring equipment is also described. The second main Section deals with frequency-modulated reception, and indicates the essential features of a frequency-modulated receiver, which, except for the amplitude limiter and frequency-to-amplitude converter (examined in detail), is similar to its ultra-short-wave counterpart for amplitude modulation. Also included are three Subsections dealing with tuning indicators, frequency-deviation compression, and distortion and interference. Possible future developments are discussed, and the paper concludes with a bibliography of the most important contributions to the literature of frequency modulation made over the past twenty years.