Patent GB-690442-A (GB)
Title: Improvements in or relating to the automatic frequency control of electrical oscillation generators
IPC: H03L7/04
Filed: 1950-10-13
Abstract: 690,442. Automatic frequency control systems. GENERAL ELECTRIC CO., Ltd., MURRAY, G., and WATKINS, T. B. Oct. 10, 1951 [Oct. 13, 1950], No. 25030/50. Class 38 (iv). [Also in Group XL (c)] An oscillation generator has its very high frequency stabilized by frequency-modulating its output at a lower auxiliary frequency and passing it through a sharply - resonant filter (e.g. a cavity resonator) to a detector. An amplifier for the detected output is provided with automatic gain control and is tuned to the auxiliary frequency and to a further frequency, the amplified output at the auxiliary frequency being compared in phase with the source of the auxiliary frequency to provide a control voltage by which the very high frequency is corrected. Fig. 1 shows the general arrangement, in which the generator 1 is frequency-modulated by an auxiliary source 9 and its output fed through a cavity resonator 12 to a detector 11. The detector output is amplified at 13, gaincontrolled by a rectifier 16, and fed to a phase comparer 14 which is also connected to the auxiliary source 9; and the phase comparer output controls relays 7 for reversibly operating a motor 4 geared at 5 to a tuning piston 3 of the generator 1. The phase - comparer output also controls the voltage supplied by the unit 8 to the generator 1 for giving a fine control of the frequency. The oscillator 1 and supply unit 8, shown in Fig. 2, comprise a velocity-modulated tube with two grids 51, 52, an anode 54, and an earthed electrode 53 formed as a cylinder with one end closed, and with an axial rod extending therefrom into a resonator 2 containing an adjustable piston 3. The output is frequency-modulated by connecting the auxiliary source 9 to the tube cathode 50 through a transformer 45. Supply voltage is taken from the mains 20, full-wave rectifier 25, smoother 26 and regulating valve 27 to the anode and cathode of the generator tube 21. Voltage for the grid 51 is derived by a rectifier 42, smoother 43, stabilizing tube 44 and a potentiometer. Control voltage from the phase-comparer 14, Fig. 1, is fed, in series with a steady voltage derived across the tube 33, to a cathode follower stage 34 which feeds a potential-divider 36 ... 40 including pre-set resistors 38, 40 and a variable resistor 37 linked to the piston control 3. The control voltage thus varies the voltage on the grid 32 of a pentode valve 31 whose anode resistor 29 is in the grid circuit of the valve 27 and thus regulates the anode-cathode voltage of the generator tube 21. Fig. 3 shows the circuits of the amplifier 13, gain-control rectifier 16, phase-comparer 14, and motor-control relays 7, all of which are shown as block diagram units in Fig. 1. The output from the detector 11, Fig. 1, is fed to a three-stage mplifier 61, 62, 63, the anode circuits of which include circuits tuned respectively to the auxiliary frequency and to its second harmonic. Output of both frequencies is taken from the anode of the stage 63 to a diode rectifier 83, from the load resistor 84 of which A.G.C. voltage is fed to the grids of stages 61, 62. In this way, control ia maintained by the second harmonic even when the auxiliary frequency is absent owing to the generator 1 being accurately in tune. The output from stage 63 at the auxiliary frequency is taken from coupled circuit 74 to the diode pair 76 where its phase is compared with that of the auxiliary frequency fed from the source 9 through stages 72, 73. The D.C. output from the stage 14 representing the tuning error is combined with a steady voltage derived across a stabilizing tube 33 and applied through limiter 138 and cathode-followers 111, 34 to the line 113 leading to the power unit 8 of Fig. 2. Connection is also made to control the double triode valves 121, 116 which operate the relays B or C according to the sign of the tuning error, as described in Specification 690,441. The tuning-motor, Fig. 4, comprises a winding 151 fed from the mains 6, and a quadraturephase winding 152 which may be arranged to lead or to lag by adjustment of switches A1, A2, B1, C1, operated by relays A, B, C of Fig. 3. When starting up the generator 1, the switches A1, A2 are in the position shown, enabling the motor to be run in either direction by throwing the manual switch 156. When the correct tuning is approached, the amplifier 13, Fig. 3, becomes active, and its output is rectified at 89 to drive the grid 93 of the double triode 94 positive and to operate the relay A. Contacts A1, A2 are thrown over to disconnect the key switch 156, and to put the contacts B1, C1 into circuit, so that when relay B, or relay C is energized, the motor runs in the direction necessary to correct the tuning error. The relay A also opens a contact A4, Fig. 3, by which the line 113 had previously been earthed; and closes a contact A3 to supply direct current to the winding 152 so as to reduce the motor speed during automatic control. When the oscillator 1 is used as the transmitter in a frequency-modulated pulse code signalling system, the pulses may be used in place of the auxiliary source 9 for A.F.C. purposes. In the case in which each pulse occupies the whole period allotted to it, it may be necessary to superpose on the pulses a further auxiliary frequency of low amplitude.