Patent GB-682281-A (GB)
Title: Improvements in or relating to automatic control systems especially for electric oscillators
IPC: H03L7/04
Filed: 1950-08-11
Abstract: 682,281. Automatic frequency control systems. GENERAL ELECTRIC CO., Ltd., and HICKIN, E. M. Aug. 10, 1951 1 [Aug. 11, 1950], No. 20047/50. Class 38 (iv). The invention relates to systems arranged to control automatically the frequency of electric oscillators, especially velocity modulation oscillators of the reflection type. As shown in Fig. 1, the main oscillator 1 and the control oscillator 3 feed a mixer 2 and the output of the mixer feeds a filter and I.F. amplifier 4 and main amplifier 5, a voltage pulse being passed when the frequency of oscillator 3 differs from that of oscillator 1 by an amount determined by the filter. The control oscillator frequency is periodically swept over a range by means of a saw-tooth generator 6, and the beat frequency passed by the filter may be either so small that the two pulses generated as the control oscillator frequency approaches and then leaves the main oscillator are almost simultaneous, or alternatively only one of the pulses may be arranged to be effective. The control oscillator 3 also feeds a gas-filled device which may consist of a sealed length of wave-guide filled with, for example, ammonia, and feeding a detector 8 and amplifier 9 so that when during its sweep the control oscillator frequency coincides with the absorption frequency of the gas, a voltage pulse is generated and amplified. The output of the amplifier 5 is applied to one valve of a flip-flop circuit 10 and initiates the generation of a rectangular wave which is subsequently terminated by a pulse derived through a differentiator and pulse former 12 from the fly-back of the saw-tooth generator 6. Simi. larly the output of the amplifier 9 is applied to a flip-flop circuit 11 to generate an oppositely directed rectangular wave which is also terminated by a pulse derived from the fly-back of the saw-tooth generator 6. The two outputs are combined in a circuit 13, to produce rectangular pulses, whose duration and sign depend on the relative durations of the input pulses, and are amplified and integrated in circuit 14 to produce a control voltage which is applied to the main oscillator in such sense as to make the main oscillator frequency more nearly equal to a frequency differing from the absorption frequency of the ammonia gas by the required amount. Fig. 2 shows the circuit details, the main oscillator 1 being a velocity modulation oscillator of the reflection type, part of whose output is taken by a waveguide 15, branched from the main output waveguide 21, to a crystal mixer 2. A similar control oscillator 3, whose frequency is swept by a saw-tooth oscillator 6, also feeds the mixer 2 through a wave-guide 17 and is connected by a wave-guide 18 to the sealed, gas-filled waveguide 7 which feeds a detector 8 and amplifier 9. The output from the mixer 2 is taken through a filter and I.F. amplifier 4, a crystal detector 20, and amplifier 5, to a differentiating circuit C1, R1. In the flip-flop circuit 10 in the normal state the valve V2 is conducting and the valve V1 cut off, but a positive pulse formed across the resistor R1 causes the valve V1 to conduct and V2 to be cut off to initiate a positive-going rectangular pulse at the anode of V2 which is fed to an amplitude limiter 30. Similarly, the output from the amplifier 9 feeds a differentiating circuit C3, R6, and the positive pulse formed across resistor R6 triggers the flip-flop circuit 11 to initiate a negative-going rectangular pulse at the anode of V3 which is fed to an amplitude limiter 31. The valves V13, V14, amplify a pulse produced across differentiating circuit C22, R30, by the fly-back of the saw-tooth oscillator 6, to produce negative pulses at the anodes of V13 and V14 which are applied to the control grids of valves V3 and V1 respectively, to terminate the opposite going rectangular pulses from the rectangular wave generators 10, 11. The outputs from the limiters 30, 31 are combined at 13 in a resistor R14, and amplified at 40. Two output pulses in the same phase are taken from the amplifier 40, one being inverted at 41, to produce both positive and negative going pulses whose sense will be interchanged if the sense of the error in the main oscillator frequency changes, these pulses being applied to a balanced rectifier circuit 42, and a control voltage is obtained across resistors R20, R21, in series, which varies in magnitude and sense with the variations in error. The control voltage is applied through a delay network to prevent instability, to the reflector electrode 48 of the main oscillator 1, coupling being effected by means of the resistors R24, R25, connected in series with the biasing supply to the reflector electrode. A centring control for the control oscillator 3, for ensuring that the oscillator sweeps out an equal frequency either side of the absorption frequency of the gas, comprises diodes V11, V12, rectangular waves being fed from the anode of valve V4 to the cathode of V1 and the anode of V12. If the control oscillator is sweeping out equal frequencies the positive going portion of the wave will commence at the mid-point of the saw-tooth waveform and the negative going portion will start during the fly-back, producing equally timed positive and negative rectangular waves, and no voltage difference will appear across the resistor R26. If the frequency of the control oscillator is too high the positive going portion of the rectangular waveform occurs at an earlier part of the sawtooth waveform with the result that V12 conducts for a longer period than V11 and a potential is produced across R26 which varies the biasing voltage on the reflector electrode 49 of the oscillator 3 to decrease the frequency. Similarly, when the frequency of oscillator 3 is low, the biasing on the reflector electrode is adjusted to increase the frequency. In a modification, Fig. 3 (not shown), the limiters 30, 31, the pulse combining means, and the amplifier 40, are combined in a circuit consisting of two tetrode valves having a common anode resistor. The control grids of the valves are negatively biased to produce amplitude limiting and the oppositely directed rectangular waves are respectively fed to the two control grids.