GB609813A - Improvements in and relating to automatic frequency control of electrical oscillating systems and apparatus - Google Patents

Abstract

609,813. Automatic tuning correction. SPERRY GYROSCOPE CO., Inc. April 6, 1945, No. 8623. Convention date, April 7, 1944. [Class 40 (v)] An automatic tuning arrangement for a superheterodyne pulse receiver comprises a separate tuning control mixer, means for ascertaining the sense in which the beat frequency departs from a predetermined value, means for adjustirig the frequency of the local oscillator to correct the tuning and means for initially synchronizing the local oscillator with the received signals, operable if the initial mistuning is so great that the automatic tuning is ineffective. An echo radio system, Fig.. 1, comprises an ultrahigh-frequency transmitter 22 controlled by a pulse generator 21, and a superheterodyne receiver, the local oscillator 26 of which is maintained at a constant frequency difference from the transmitted carrier. Local oscillations are fed to the mixer 25 of the receiver, and, together With part of the transmitter output, attenuated at 31, to a separate mixer 29. The output of mixer 29 is fed via an amplifier 32, controlled by pulse generator 21 through a gatirig circuit 34 to the A.F.C. circuit 33. The suppressor grid of the last I.F. amplifier valve 35, Fig. 2, is connected through resistor 122 to the anode of the gate control valve 121, so arranged that a disabling bias is applied to the amplifier in the intervals between the pulses. A Foster Seeley discriminator 50 produces pulses of amplitude and sense depending on the magnitude and direction of variation of the I.F. from the desired value, amplified by valve 58, are fed to phase-inverter 59, producing pulses of opposite polarity at terminals 60 and 60<1>. These pulses are applied to the grids of detectors 61 and 62, connected in series through potentiometer 65. The application of opposite pulses to the grids of the detectors causes the potential at any point on potentiometer 65 to change from its normal value in a direction depending on the sense of the deviation of the I.F. from its desired value. A variable tapping 75 on potentiometer 65 is connected through switch contacts 77, 79 to the grid of variable resistance valve 81, controlling the output amplitude of oscillator 82. This output is fed through transformer 85 to a heater element 86, which forms part of a variable strut 90, controlling the spacing between flanges 87, 88 of velocity modulation valve 26 and thus the local oscillator frequency. A manual frequency control arrangement may be provided through switch contacts 79, 91, which connect a source of negative potential to the grid of variable resistance valve 81 through a potentiometer 93. In case the I.F. is outside the operating range of frequency discriminator 50, a push-button reset circuit is provided. Push-button 97 energizes relay 96, charging condenser 195 to a high negative potential through contacts 102, 104 and reducing the resistance of valve 81 to a minimum. The output of oscillator 82 and hence the frequency of oscillator 26 rises to a maximum. When the push-button 97 is released, relay contact 101 opens and the negative voltage on condenser 195 is applied through resistances 99, 80 to the grid of control valve 81, cutting off the power supplied to the heater 86, and the frequency of oscillator 26 begin to drop. The condenser 195 gradually discharges through resistances 99, 106, but the output circuit of valves 61, 62 is such that the automatic control voltage predominates when the I.F. passes through its desired value.