Patent GB-467717-A (GB)
Title: Improvements in automatic frequency control of high-frequency electric circuits
IPC: H03J7/04
Filed: 1936-10-05
Abstract: 467,717. Wireless receiving circuits. HAZELTINE CORPORATION. Oct. 5, 1936, Nos. 26962 and 26963. Convention dates, Oct. 24, 1935 and Jan. 25, 1936. [Class 40 (v)] In order automatically to compensate for tuning errors due to mistuning or changes in oscillator frequency a superheterodyne receiver has two frequency changing stages ; automatic frequency correction being applied to the second modulator stage. Energy from the second intermediate frequency channel 16 which is highly selective, is applied to an error detector comprising selective circuits 31, 32 tuned above and below the true second I.F. carrier frequency and feeding a pair of diodes 33, 34 the voltages across the load resistances 37, 38 being applied to bias the control grids of triodes 63, 64 respectively through time-delay networks 71, 73, 72, 74 having a constant of at least twice that of the receiver AVC network, e.g. 0À1 to 0À2 sec. The first oscillator-modulator stage 13 of the receiver is tunable and the first IF channel 14 is broadly selective, the second oscillatormodulator comprising a fixed-tuned oscillator 51 the anode circuit of which includes a choke 52 tuned to a frequency higher than the oscillation frequency, the grid circuit comprising a choke 54 such as to tune the oscillator broadly to the oscillation frequency, whereby the capacity of condenser 53 is balanced and the oscillator behaves as a negative resistance, excluding the circuit 50 ; 55 to a feed-back condenser. The grid circuit of 51 also includes circuit 50 the four reactances C1, C2, L1, L2, of which are such that the impedances of the two parallel paths between points 57, 58 are equal at the resonant frequency while the inductance of L2 is a small fraction of the total circuit inductance and its impedance equals that of C2. The reactances L2, C2 are respectively shunted by the anode-cathode space of the valves 63, 64, the impedances of which are varied in dependence on departure from exact tune by the error detector 26, the frequency of circuit 50 being thus varied in compensating sense. With this arrangement, variations in amplitude have no effect on the control system ; any increase or decrease will cause equal rise or fall in impedance of 63 and 64 in the same sense, the frequency of 50 being unaffected thereby. In the control system shown in the Figure, the valves 63, 64 have no initial bias so that their conductance is varied by small variations in frequency of the second I.F. channel 16. In a modification, Fig. 4 (not shown), the valves may have an initial negative bias so that they function somewhat in the manner of Class B or Class C amplifiers. In this case the control only becomes operative when the departure from tune exceeds predetermined limits and the arrangement necessitates change-over of connections from resistance 37 to valve 64 and resistances 38 to valve 63. Visual indication of the amount and sense of initial mistuning or drifting of frequency may be given by means of a centre-zero voltmeter 43 which may be connected across resistances 41, 42 in the error detector 26, or across resistances 75, 76 in the anode circuits of valves 63, 64 (reference 77) or across resistances 79, 80 (reference 81) in the cathode circuits of these valves. Alternatively or in addition, lamps 69, 70 across resistances 75, 76 may give similar indication by variation in brilliance, the lamps being mounted side by side in a unit, separated by a screen, Fig. 3 (not shown). The time constant of the automatic control, normally determined by the filter sections 71, 73 ; 72, 74, may be capable of adjustment by switches 85, 86, which cut in or out additional sections 71a, 73a ; 72a, 74a, and the switches may be controlled by pushing the tuning knob 88 in or out as indicated by line 87 and spring 93. By this means a small time constant obtains during manual tuning, but is increased to the order of 10-30 seconds thereafter to prevent the effects of severe fading on the biases of the frequency control valves 63, 64.