Patent GB-569044-A (GB)
Title: Improvements in control circuits, particularly for automatic frequency control and remote tuning
IPC: H03C3/14
Filed: 1942-05-14
Abstract: 569,044. Automatic control systems. MARCONI'S WIRELESS TELEGRAPH CO., Ltd. May 14, 1942, No. 6586. Convention date, May 14, 1941. [Class 38 (iv)] [Also in Groups XXXVII and XL] Frequency.-A circuit arrangement comprises one or more thermionic valves having grids tn which an alternating current is applied, and means for applying to at least one further grid a control voltage, such that the alternating output is of one phase or the reverse or zero according as the control voltage is positive, negative or zero. Fig. 1 shows the invention applied to the control of a tuning element TE by driving a two-phase motor 3 in one direction or the other according as a control voltage applied at C is positive or negative. Alternating current from a mains supply is fed to terminals A, B and also to wind- 5 of the motor 3 through a condenser 6 which ensures the necessary quadrature relationship between the currents in motor windings 5 and 32. One end of a resistance potentiometer 15 feeds to a control grid G1 of a valve 1 having an unbyepassed screen resistor 11. A second control grid G3 is fed from a variable tapping on 15 through a condenser 14. The output from 1 is amplified in valve 2 which feeds the motor winding 32 tuned by a condenser 34. In the absence of a control voltage at C, the tapping on 15 is adjusted so that the voltages amplified on grids G, and G3 are equal and opposite in resistor 18. Then according as the control voltage applied at C is positive or negative, the phase of the output is predominantly that of grid G3 or of G1 thus causing the motor 3 to run in one direction or the other. This may be used to vary the tuning of a frequency modulator or the oscillator of a superheterodyne receiver in accordance with deviations from an assigned frequency or for remote tuning. Fig. 3 shows a modified valve arrangement for phase reversing. One secondary winding of a mains transformer 120 feeds one winding 5 of a two-phase motor 108. Another secondary winding feeds in push-pull control grids G3 G<SP>1</SP>3 of valves 50, 60, condensers 124, 126 and resistances 128, 130 ensuring a quadrature relationship between the currents in motor windings 5, 32. The anodes of valves 50, 60 are coupled by condenser 122 and the resultant output is fed through an amplifier 112 to the second motor winding 32. Grid G1 of valve 50 is earthed while grid G<SP>1</SP>1 receives at C a control voltage which, according as it is positive or negative, causes the output from 60 or 50 to predominate, so to drive motor 108 in one direction or the other. In the' application shown, motor 108 drives a condenser 113 forming part of the reactance of an oscillator 80 arranged to be push-pull frequency modulated in known manner by reactance valves 37 and 38. A control voltage dependent on deviations of the mean frequency of oscillator 80 from the assigned frequency is derived by feeding part of the output from 80 together with the output from a reference oscillator 90 to a limiter valve 88 in which the frequency modulation component of the resultant has a sharply peaked wave form which corresponds to Fig. 4c or Fig. 4D according as the mean frequency of oscillator 80 is on one side or the other of the frequency of the reference oscillator 90. The modulation component is detected by feeding the output from 88 through a blocking condenser 97 to one grid 99 and through reactive circuits 91, 93 to a second grid 95 of a multi-grid detector 96. A voltage amplifier 98 passes the output to biassed differential diodes 102, 104 which derive the desired positive or negative control voltage for application at C. In a modification, Fig. 5 (not shown) the detection of the frequency modulation component is effected by a phase modulation receiver with an audio-correction network in its output of known form, for example that described in U.S.A. Specification 2,063,588.