Patent GB-717396-A (GB)
Title: Automatic frequency or phase control systems for electric oscillation generators
IPC: H03B21/02
Filed: 1952-09-30
Abstract: 717,396. Modulating systems; valve generating circuits. RADIO CORPORATION OF AMERICA. Sept. 30, 1952 [Oct. 3, 1951], No. 24470/52. Classes 40 (5) and 40 (6). [Also in Group XXXV] In order to prevent or 'reduce adventitious angular modulation in an amplitude-modulated magnetron or other oscillator, the frequency is partially stabilized by injecting oscillations from a stable source, and means are provided for sampling the oscillator output and using it to vary the phase or frequency of the injected oscillation in such manner that it opposes the adventitious angular modulation. In Fig. 1 an amplitude modulator 6 fed from a power supply 7 varies the cathode voltage of a cavity magnetron 5 which feeds amplitude-modulated oscillations through a lead 10 to a load 8. The injected oscillations are supplied from a crystal source 16 through frequency-multipliers 15, 14 anglemodulated stage 13, and further frequencymultipliers 12, 11, 9, to the lead 10 or to a suitable part of the magnetron 5 With the object of stabilizing its frequency. To render the stabilization more effective, part of the output on lead 10 with its adventitious angle modulation, is diverted by a pick-up 17 to a heterodyne mixer 18 receiving oscillations from a crystal source 21 through a multiplier 20 and amplifier 19. The reduced beat frequency is fed to an angular-modulation detector 22 whose output is passed to an angular modulator 23 associated with the stage 13. In this way an angular modulation is imposed on the output of the multiplier 14 in the opposite sense to that present on the lead 10. The angular modulation is increased in the multiplying stages 12, 11, 9, and counteracts that present on the lead 10. In Fig. 4, the angular-demodulator 22 and modulator 23 are omitted and the oscillations picked up at 17 are fed through a phase-adjuster 30 to a mixer stage 25 interposed in the chain comprising stable source 29 and frequencymultipliers 31, 12, 11, 9. The output of the multiplier 31 is arranged to be at a higher frequency that that of the .magnetron, and the lower sideband is selected from the mixer 25, so that the angle modulation fed from the mixer to the multipliers 12, 11, 9 is in the opposite sense to that present on the line 10. To maintain oscillations on the line 10 during periods of excessively deep modulation, the crystal source 29 is connected through a multiplier 32 as input to the stage 12 beyond the mixer 25. In a further modification shown in Fig. 7, the last stage 37 of the frequencymultiplier chain 37-40 fed by the crystal source 29 operates both as a frequency-doubler and as a mixer to which the pick-up from the line 10 is connected. If F is the working frequency, the crystal source 29 operates at a frequency F(n+1)/n<SP>2</SP> where n is the multiplying factor for the chain. This results in the delivery of injection oscillations to the line 10 at a frequency F(n+1)/n, which beat with the deviating oscillations of frequency F+D in the line 10 to produce an output -F/n+D. This beat is selected and amplified at 41, and passed through a phase adjuster 42 to the input of the chain 40 ... 37, emerging into the line at a frequency FnD, in the correct sense for counteracting the original deviation D. Fig. 8 shows a suitable triode valve circuit for the mixer and doubler stage 37. The grid-cathode circuit is tuned by a resonant cavity to which the input is fed by loop 35. The plate circuit comprises a cavity 43 feeding a doubled frequency to the magnetron 5 by a loop 36. This loop also feeds in the deviating frequency from the line 10, and gives rise to a beat which is selected by the tuned circuit 45, 46, 47 and gives an output at beat frequency to the amplifier 41.