Patent GB-757464-A (GB)
Title: Improvements in or relating to frequency stabilized oscillatory systems
IPC: H03L7/04
Filed: 1954-09-28
Abstract: 757,464. Automatic frequency control systems. WESTERN ELECTRIC CO., Inc. Sept. 28, 1954 [Oct. 1, 1953 ; Oct. 1, 1953], No. 27977/54. Class 38 (4). [Also in Group XL (c)] The frequency of an oscillator is stabilized by including in the feedback path of the amplifier, e.g. a travelling-wave tube 21, a serially-connected high Q cavity or other resonator 26 tuned to the desired frequency, measuring the change in phase shift of the oscillation in traversing the resonator 26 resulting from frequency drift from the desired value, and using this detected error to correct the frequency. As shown, the waveguide input and output of the resonator 26 may be connected by directional couplers 27, 28 to the collinear arms of a magic- T phase comparator 30, after one input has been phase-shifted by #/2. In the shunt arm 4 the inputs are added, and in the series arm 3 subtracted. Crystals 3, 4, respectively rectify the sum and difference and have outputs connected in series opposition, so measuring in sign and magnitude any departure of the input phases from quadrature relation in correspondence with the sense and magnitude of the drift in frequency. This error voltage may then be amplified by a D.C. amplifier 35 and applied to a motor 36 controlling through a potentiometer 38 the voltage of the gun 40 of the travelling-wave tube 21 relative to the helix 22 and so varying the effective electrical length due to the amplifier and the frequency generated. Preferably the gain condition for oscillation is not satisfied for frequencies outside the pass-band of the resonator 26. In a modification the directional coupling outputs are applied to a frequency mixer 56 (Fig. 4), first one output being transposed in frequency by a 60 c/s. source 55 driving a synchronous motor 54 rotating the phase of the output in a rotary mechanical phase changer 53 and hence altering its frequency by 60 c/s. The 60 c/s. beat from the mixer 56, after amplifying at 57, is then compared with the 60 c/s. source 55 in a phase detector 58, any frequency drift of the oscillator producing departure from normal quadrature phase of the inputs to the detector 58 and hence an output at 46 in sense and magnitude corresponding the to frequency drift. This may be applied to a variable phase shifter in the feedback path to stabilize the frequency. The variable phase shifter may comprise five waveguide sections : (1) a tapered section producing a circular waveguide ; (2) a 90 degrees phase-shift section converting from linear to circular polarization; (3) a rotatable 180 degrees phase -shift section (4) a 90 degrees phase-shift section for reconverting to linear polarization ; (5) a tapered section restoring to a rectangular waveguide. Phase shift is then varied by rotating by a reversible motor the 180 degrees phase-shift section. Or, the phase shifter may comprise vanes of ferromagnetic material in a rectangular waveguide variably magnetized by coils con. trolled by the control voltage, to produce a corresponding phase shift. The amplifier 21 may be overdriven to limit the output, or may be followed by a separate limiter consisting of a hybrid junction having four arms. Energy from the first arm subdivides between two others terminating in crystals whose impedance varies with increasing amplitude of input to them so that a correspondingly reduced proportion of the incident wave is reflected and becomes available in the output fourth arm. The cavity resonator may be hermetically sealed and enclosed in a temperature controlled oven. Specification 671,989, [Group XL (b)], is referred to.