Patent GB-625319-A (GB)
Title: Improvements in automatic frequency control in radio receivers
IPC: H03J7/16
Filed: 1946-02-14
Abstract: 625,319. Automatic tuning correction. MARCONI'S WIRELESS TELEGRAPH CO., Ltd. Feb. 14, 1946, No. 8691/48. Convention date, Feb. 14, 1945. Divided out of 625,258. [Class 40 (v)] A superheterodyne receiver for amplitude or phase-modulated signals has separate coarse and fine automatic frequency control operating the local oscillator, the frequency controlling potential being derived from a detector circuit to which is applied intermediate frequency carrier obtained from a highly selective circuit. The Figure shows the invention as applied to the double detection exalted carrier superheterodyne receiver (not shown), described in the Specification 625,258. The second local oscillation (not shown) has a frequency determining circuit 9 across which is, connected a thermionic reactor circuit including valve 123, for fine control of the oscillator frequency. The automatic frequency control voltage which is obtained from an errordetector (not shown), incorporating a crystal tuned to the I.F. carrier frequency, is applied through lead 67 and switch 131 to the grid of the reactor. The coarse. frequency control is effected by means of variable condensers 135, 137 driven through gearing 149, 151 by rotor 140 of a reversible A.C. motor. Winding 142 of the motor is fed directly with A.C. through phasing condenser 146, whereas the other winding 141 is supplied with A.C. through valves 154, 176 under the control of the automatic frequency control voltage. Valve 154 has two control grids 155, 156 to which the A.C. is fed from the secondary 168 of the transformer. Due to the effect of the unbyepassed screen resistor 164 there is a 360 degree phase change between the voltages on the first control grid 155 and the anode and the A.C. voltage on the second control grid may therefore be adjusted by potentiometer 159 so that the A.C. anode currents cancel. The automatic frequency control voltage which is also applied to grid 155 unbalances the circuit and an alternating anode current flows de pending in magnitude and sense upon the magnitude and sense of the automatic frequency control voltage. This current develops a voltage across anode resistor 163 which is amplified by valve 176 and applied to tuned winding 141 to drive the motor and correct the frequency. Manual adjustment of the condenser 135, 137 may be effected by means of knob 152 which when depressed disengages the gears. The resonance indicator 133 has a fluorescent anode 211 and two electrodes 212, 213, which produce shadows varying in width with the potential applied thereto. A.C. is supplied to electrodes 212, 213 in antiphase from transformer winding 168 and also to the anode from controlled motor winding 141. The relative width of the shadows thus depends upon the magnitude of the anode voltage and the phase relative is that of electrodes 212, 213 (i.e. the magnitude and sense of the frequency error). The motor control circuit is disabled when the signals fade by means of a valve 200 connected across motor control valve 154 and biassed by a voltage obtained via lead 134 from the limiter (not shown). When the signal fades, valve 200 effectively short-circuits the control valve.