A New Broadcast-Transmitter Circuit Design for Frequency Modulation

The problem of generating wide-band frequency-modulated waves is first reviewed in order to ascertain specifically the desired performance capabilities for a commerical transmitter circuit. The factors which influence or limit these performance capabilities in the two methods available for the generation of frequency-modulated waves, compensated phase modulation, and direct frequency modulation, are then explored. It is found that each method possesses desirable fundamental characteristics not present in the other, but with the circuits now generally employed with either method the modulation characteristics and carrier frequency stability are interrelated so that one has a limiting effect upon the other. A new circuit is described in which these two important characteristics are independent of each other. Owing to this independence and to other circuit refinements the modulation capabilities are unrestricted with low distortion over an exceedingly wide range. A balanced electric oscillator operating at one eighth the radiated frequency is modulated by balanced reactance-control tubes and negative feedback is used to minimize amplitude modulation and harmonic distortion. A system of frequency division is employed together with a crystal-controlled oscillator and synchronous motor in such a manner as to control mechanically the mean frequency of the modulated wave with the same stability as that of the crystal-controlled oscillator. The carrier, or mean, frequency stability is that of a single crystal-controlled oscillator and is independent of any other circuit variations. A carrier frequency stability of 0.0025 per cent is possible without the use of temperature-controlled crystals or apparatus.