Patent GB-675033-A (GB)
Title: Improvements in or relating to electrical oscillation generators
IPC: H03L7/04
Filed: 1950-01-11
Abstract: 675,033. Automatic frequency control systems. GENERAL ELECTRIC CO., Ltd., and WATKINS, T. B. Jan. 11, 1951 [Jan. 11, 1950], No. 779/50. Class 38 (iv). The frequency of an oscillation generator is maintained equal to a natural frequency of a cavity resonator by frequency modulating the generator output at an auxiliary frequency and feeding it to excite standing waves of a first mode in the resonator, these waves being picked up and used to bring the generator frequency into agreement with the resonator frequency in this mode; and the resonator is itself stabilized by feeding thereto oscillations modulated in frequency or phase and derived from a source of standard frequency to excite therein standing waves of a second mode, these waves being picked up and used to control the resonator frequency in both modes by adjusting the position of a movable part thereof. Referring to Fig. 1, the generator 1 to be controlled is frequency-modulated by an auxiliary source 3 of low frequency, and its output is fed to a loop 4 inserted through the curved wall of a cylindrical cavity 2 to excite therein TM111 waves which are picked up by a loop 5 inserted through the flat end wall of the cavity and are detected at 6. If the cavity is off-tune, the detector yields an output at the auxiliary frequency, the phase sense of which is dependent on the sense of the mis-tune. A phase comparer 8, which may be of the kind described in Specification 670,765, is fed from the amplifier 7 and the source 3, and yields A correcting voltage for the generator 1, which may be of the velocity-modulated type. In order to stabilize the frequency of the cavity despite changes of temperature &c., a second A.F.C. loop is provided, comprising a crystal oscillator 16, phase or frequency modulated at 17 by an auxiliary source 19, and feeding a harmonic generator 18, which through a bandpass filter 21 excites the cavity 2 in the TE011 mode by a loop 22 inserted through the curved wall of the cavity at the side opposite to the loop 4. Pick-up for waves of this mode is taken by a loop23, 90 degrees removed from the input loop 22 and is supplied to a detector 24, amplifier 25 and phase-sensitive device 26 where its phase is compared with that of the source 19. The output from 26 controls a servo-motor 9 which is geared to a piston or plunger movable within the cylinder 2 so that the frequency of the cavity is maintained equal to the selected harmonic of the crystal source 16. The cavity may be roughly tuned with the help of a calibration scale to a selected harmonic of the crystal source, before the automatic control is brought into operation. The arrangement of the coupling loops 4, 5, 22, 23 is such that there is little stray coupling between the two control circuits; the positions of the loops being shown in Fig. 2 which is a section along the line 22, 5, 4 of Fig. 1. The piston 11 has a quarter-wave flange 14 to provide substantially zero impedance between the piston and the cylinder wall. The generator 1 may operate at 4500 mc/s. and the filter 21 may pass the range 4400-4800 mc/s. The crystal source 16 may operate at 20 mc/s. The auxiliary source 3 may frequency-modulate a separate oscillator the output of which is mixed with the unmodulated output of the stabilized source 1 to provide the frequencymodulated input to the resonator 2.