GB655553A - Improvements in automatic frequency control of thermionic oscillators - Google Patents

Abstract

655,553. Automatic frequency control systems. MINISTER OF SUPPLY. Aug. 14, 1947, No. 22610. [Class 38 (iv)] In an automatic frequency control circuit in which signals from controlled and reference oscillators are applied to a mixer and the difference frequency is fed to a discriminator circuit from which the control voltage is derived, the oscillator is liable to " lock " at a, difference frequency which is a sub-harmonic of that desired, due to the production of harmonic frequencies in the mixer. Accordingly, according to the invention means are provided for disabling the frequency control means when the frequency difference is in the neighbourhood of one or more sub-harmonics of the desired difference frequency. As applied to a control system of the kind described in Specification 641,900, in which the frequencycontrolled oscillator is caused to " search " until the difference frequency falls within a band in which normal discriminator-operated A.F.C. becomes possible, the " searching " means then being disabled, the anode circuit of the valve V1 includes circuits L4, L5 tuned respectively to one half and one third of the desired difference frequency, in addition to the discriminator L1, L2 and circuit L3 tuned to the desired difference frequency. The "search" is normally terminated by operation of a relay in the anode circuit of valve V3 consequent upon a control voltage from the discriminator appearing on its grid; if, however, a voltage is also developed across either circuit L4, L5 it is rectified by a diode V2 and is applied after smoothing to the suppressor grid of the valve V3, thereby disabling it and preventing false interruption of the " search ". The arrangement may be modified by deriving the subharmonic frequencies from a separate mixer and amplifier, Fig. 2 (not shown).