Patent GB-670739-A (GB)
Title: Improvements in or relating to apparatus for generating electrical oscillations
IPC: H03L7/087
Filed: 1949-07-21
Abstract: 670,739. Automatic frequency control systems. GENERAL ELECTRIC CO., Ltd., BRYDEN, J. E., EASTER, B., and WATKINS, T. B. Oct. 16, 1950 [July 21, 1949], No. 19254/49. Class 38 (iv). An unstable oscillation is kept in step with a reference oscillation F by beating the latter with quadrature phases of an auxiliary low-frequency f (such as the main supply), and passing the side-frequency outputs (FŒf) after a relative phase shift of 90 degrees to two combining circuits in which they are respectively added and subtracted so as to separate the sum frequency (F+f) and the difference frequency (F - f). These are then separately modulated with the unstable oscillation to produce difference frequencies. When the unstable oscillation deviates from its desired frequency F to a value (F+d), these difference frequencies, respectively (f+d) and (f - d), are supplied to synchronous motors which drive two shafts of a differential gear, the third shaft being geared to a tuning-member of an oscillation generator, which supplies the unstable oscillation either directly or by a heterodyning stage. In a modified system, one motor is operated continuously at mains frequency f, and the other frequency alone varies with the deviation. When the system is used for the reception of single-side-band radio signals, as shown in the diagram, the controlled generator may be a local heterodyning oscillator 1, the unstable oscillation may be the weak incoming carrier 6 reduced to an intermediate frequency, and the reference oscillation may be the local source 9 employed for de-modulating the received sideband. The source 9 is connected through a filter 3 and amplifier 4 to a balanced modulator 5 where it is mixed with the incoming side-band and a weak carrier, which may have been reduced in frequency by a first heterodyne stage. The carrier is separated by a filter 7 to give an output of frequency (F+d), where F is the desired value and d is an undesired deviation. A stable local source 9 oscillating at the desired frequency F is connected through an amplifier 10 to two balanced modulators 11, 12, where it is mixed with quadrature phases of a source 13 of low-frequency f (which may be the mains supply), a network 14 giving 90 degree phaseshift being provided. The two outputs, each including frequencies (F+f) and (F - f), are given a further relative phase shift of 90 degrees; and are then added at 16 to give a frequency (F+f), and are subtracted at 17 to give the frequency (F - f). Those frequencies are separately applied to balanced modulators 18, 19 where they are mixed with the incoming carrier of frequency (F+d) from filter 7 and amplifier 8. Low-pass filters 20, 21 respectively select the difference frequencies (f + d) and (f - d) from the mixer outputs and feed synchronous motors 24, 25 through limiters 22, 23. These motors are normally running at a speed corresponding to f so that the shaft 27 of the differential gear 26 is stationary. The occurrence of a deviation d speeds up one motor and slows down the other with the result that the shaft 27 rotates and through reducing gear 28 adjusts the condenser 2, or other tuning impedance, in the sense to correct the frequency of the source 1. The signal is detected by a de-modulator fed with incoming side-band from the modulator 5, and with locally-produced carrier from the source 9. The received carrier is also fed from amplifier 8 through a further amplifier 30, rectifier 31, and a low-pass filter to yield A.V.C. voltage. Should the carrier fade, the low-frequency source 32 is rendered operative to supply the motors 24, 25 through the limiters 22, 23 with current of frequency f so that the position of the shaft 27 is maintained during the fade. The source 32 also operates a relay 33 which during fading periods disconnects a clutch in the shaft 27, and applies a brake to the spindle of the tuning condenser. Specification 666,088 is referred to.