GB641900A - Improvements in and relating to automatic frequency control - Google Patents

Abstract

641,900. Automatic frequency control systems. MINISTER OF SUPPLY. July 28, 1947, No. 20273 [Class 38 (iv)] [Also in Group XL (c)] An automatic tuning system for adjusting the frequency of a first oscillator FX to a given frequency separation IF from that of a second oscillator FL (either of which may be pulsed) comprises means for sweeping the first frequency at high speed through a wide searching band, and means for making fine adjustments when the search has been arrested in the vicinity of the desired frequency. The invention comprises means for carrying the frequency sweep unarrested through one of the two settings that gives the requisite frequency separation IF; and this is effected by a sweeparresting relay C and two auxiliary relays A, B so arranged that if A is energized before C the arrest of the sweep by C is prevented, whereas if B is energized before C the sweep is arrested. The functions of A and B are reversed if the other of the two settings is the desired one. The intermediate frequency IF resulting from mixing the two oscillations is fed to a frequency discriminator, the rectified output of which is shown in Fig. 1 (a) plotted against the sweep frequency FX as abscissa. The outputs for the respective low and high channels are symmetrically placed at a distance representing the IF on opposite sides of the invariable frequency FL. Positive outputs occurring at A and negative outputs at B are applied to the respective grids of a double triode valve V4, the anode circuits of which include the respective relays A/I and B/1. The common cathode lead of the valve V4 is coupled through a diode V2 to the grid of a valve V3, having a relay C/2 in its anode circuit. The cathode voltage of the valve V4 as represented by the curves (b) of Fig. 1, rises to a peak at the central positions C between the areas A and B, so that the relay C/2 is operated when the sweep is in the area C. If the sweep is in the direction of rising frequency, A/1 operates before C/2 at the low setting, and B/1 operates before C/2 at the high setting, whereas the order is reversed during the reverse sweep. The sweep is effected by a motor M1 the field coil of which has its connections reversed by a switch S1 operated at each end of the sweep. The field current is from a source 24V and the circuit is completed by earthing the point T either at contact c1 or at contact d1. To arrest the search motor, both contact cl and contact d1 must be open. Assuming that relays A/1, B/1 and C/2 are not operated, that all the contacts are in the positions shown, that the motor M1 is giving a rising frequency, and that the high channel is the wanted one; then as the low channel is approached A/1 is first energized and its contact a1 removes an earth connection and allows current to pass through a second coil on the relay, through switch-arm S2 and contact c2 to a relay D/1 which throws the contact d1. When relay C/2 operates it will open the earth connection of point T at the contact c1 but this will not stop the search motor on account of the earth connection at dl. When the search approaches the desired high channel, relay B/1 operates first and with the switch S2 in the position shown relay D/1 is not operated, so that when relay C/2 operates, the search motor is arrested by the opening of earth connection c1. The relay E/3 is also released, and its contacts e2, e3 prepare the circuits of forward and reverse fine-tuning stepping motors M2, M3, the magnets of which are controlled directly at contacts a1, b1 by operation of relays A/1, B/1. If the sweep is giving a falling frequency and the high channel is desired, the action is the same, account being taken of the automatic reversal of the position of both the switches S1, S2 which occurs at the limits of the sweep. If on the other hand the low channel is desired, the switch S2 associated with the A/1 and B/1 relays is reversed manually and the switch S1 on the motor field is left unchanged. Reversal of the connections between relays A/1, B/1 and the stepping motors M2, M3 is also required. A single motor with magnetically-controlled reverse gear may replace the motors M2, M3. Specification 641,891 is referred to.