937,542. Automatic frequency-control systems. COLLINS RADIO CO. July 20, 1961 [Jan. 3, 1961], No. 26374/61. Class 40 (5). An oscillator is frequency stabilized by mixing its output with a pulse train forming a spectrum of harmonic frequencies nf 1 equi-spaced at the pulse recurrence frequency f 1 to derive through a narrow pass-band filter, tuned to a multiple of, e.g. twice the pulse recurrence frequency f 1 , an output at around this frequency amplitude modulated by the oscillator frequency drift, if any, from an exact multiple of f 1 . The filter output is then rectified and after superimposing on a mean negative bias voltage, applied as A.F.C. voltage to a variable reactance, e.g. capacitor, across the oscillator tuned circuit. Further, if the A.F.C. output exceeds a predetermined positive voltage a circuit 19 renders normally blocked transistors 86, 91 conductive (the latter for a short time only due to the time constant of R.C. circuit 95, 96) and short-circuits the base/collector path of NPN transistor 83 and hence the A.F.C. voltage so that a sawtooth form of the latter is produced and sweeps the oscillator frequency until it locks in with a harmonic nf 1 , whereupon the sawtooth circuit 19 is inoperative. The harmonics nf 1 may be derived by squaring the output of a stable oscillator of frequency f 1 , then differentiating it across the output inductor 38 of a transistor pulse spectrum generator 12, rectifier 39 removing positive peaks. The negativegoing pulses are applied to the collector of a mixer 11, with the oscillator frequency to be stabilized applied to the emitter, a mixed output (fully modulated) being taken from the collector and applied to the filter 15 tuned to 2f 1 . Specification 937,541 is referred to.
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