GB364603A - Improvements in and relating to systems of frequency modulation - Google Patents

Abstract

364,603. Valve modulating circuits. BRITISH THOMSON - HOUSTON CO., Ltd., Crown House, Aldwych, London.- (Assignees of Dome, R. B.; 13, 9th Street, Schenectady, New York, U.S.A.) July 3, 1930, No. 20204. Convention date, July 22, 1929. [Class 40 (v).] In a system of frequency modulation a relatively low constant carrier frequency is varied in accordance with signal amplitude and at a rate dependent on signal frequency, the frequency-modulated carrier frequency being multiplied until the sidebands are raised above audibility; the amplitude modulation component is simultaneously suppressed. The oscillator 5, oscillating at the frequency of the tuned circuit 6, derives its anode voltage from the battery 7 through the choke 8. The signal from the microphone 11 is passed on to a modulator 9, the corresponding anode current variations through the choke 8 producing variation in high tension supply to the oscillator 5. As the oscillator frequency varies with anode voltage a mixture of frequency and amplitude modulated energy is present in the output circuit of valve 5. This energy is passed to a series of saturated frequency multipliers 17, 18, 19 having tuned anode circuits 20, 21, 22, each tuned to a harmonic of the grid circuit frequency. By this means the amplitude modulation is removed due to the saturation of the amplifier, while the sideband frequencies are simultaneously raised above audibility.