Patent GB-546677-A (GB)
Title: Improvements in or relating to systems for maintaining a constant mean frequency in transmissions by frequency modulated carrier wave
IPC: H03C3/09
Filed: 1941-03-18
Abstract: 546,677. Automatic control systems ; frequency modulation. ELECTRICAL RESEARCH PRODUCTS, Inc. March 18, 1941, No. 3689. Convention date, March 30, 1940. [Class 38 (iv)] [Also in Group XL] Oscillation frequency.-The mean frequency of a frequency modulated carrier wave oscillator is corrected for drift by deriving from the modulated wave by frequency division a subharmonic oscillation having diminished frequency modulation and a mean frequency such that the side bands of higher order than the first representing the diminished modulation are suppressed and controlling the carrier oscillator by beat currents between this harmonic and oscillations of a fixed frequency. The carrier oscillator comprises two vacuum tubes 23, 24 in push-pull, a tuned circuit comprising inductance 25 and fixed and variable condensers 27, 29 and feed-back condensers 30, 31. Amplitude modulation is prevented by a choke coil 36 in series with a bias resistance 32. Frequency modulation by a signal source 40 is produced by control tubes 37, 38 in push-pull across the tuning inductance 25 a loose coupled coil 85 and phase-shifting network 47 producing at the input terminals of the control tubes a radio-frequency voltage exactly in quadrature with the voltage across the inductance 25. The coil 85 also furnishes current to the output leads 48, 49 from which a cascade connection of frequency dividers 52, 58 ... 59, 60 feeds two balanced modulators through transformers 61, 62. A master frequency oscillator 67 provides potentials to the grids of the modulator valves 63 ... 66 through two phase-splitting networks 69, 70 so that two beat currents in quadrature arise from any change in the mean frequency and the modulated carrier wave. A two phase motor 78 actuates the condensers 29 in such case to alter the tuning of the oscillator 23, 24 and correct the change. In a frequency modulated transmitter operating at about 40 megacycles the oscillator frequency has a normal value of 5120 cycles. The maximum frequency swing due to modulation is taken as 100 kilocycles above and below the radiated carrier frequency or a deviation of 12.5 kilocycles in the oscillator frequency. To reach a modulation of the subharmonic having an amplitude less than the lowest significant fre-. quency of speech signals the frequency is divided to the 1024th part by 10 dividers in tandem each reducing by one half. The phase libration representing the modulation amounts then to only 12 degrees and the resulting rapid oscillation of the motor 78 with zero beat frequency is suppressed by the inertia and friction of the mechanical parts which may include a worm drive with or without eddy current damping, a torsionally flexible shaft or spring coupling. The frequency divided shown is of the type described in U.S.A. Specification 2,159,595 and comprises a vacuum tube 57, a bridge network 50 of copper oxide rectifiers, a tuned output circuit and a feed back to the terminals of the rectifiers through a pair of bridging inductances.