GB637597A - Improvements in or relating to circuit arrangements for producing control signals dependent upon the frequency difference between electrical oscillations - Google Patents

Abstract

637,597. Automatic frequency control. ELECTRIC & MUSICAL INDUSTRIES, Ltd. July 4, 1947, No. 17643. [Class 38 (iv)] [Also in Group XXXVI] To obtain a control signal whose sign is dependent on the sense of the frequency differences between two sets of oscillations, at least one of which is of sawtooth or like asymmetrical form, the oscillations are mixed in a non-linear mixing circuit and the output is differentiated. As shown in Figs. 1-4, one oscillation, Fig. 1, is of sawtooth form and the other of pulse form, Fig. 2. The mixer output takes the form of a series of pulses of varying amplitude, Fig. 3, the envelope of which, shown as a dotted line, is of sawtooth form and of the beat frequency. This envelope is differentiated to produce a series of pulses, Fig. 4, which are of positive or negative sign according to the sense of the frequency difference. The circuit of Fig. 5 employs a mixer valve 2 with two control grids to which the oscillations of Figs. 1 and 2 are respectively applied. The output envelope is derived from the anode across a resistor 8 shunted by a condenser 9. This envelope is amplified in the valve 12 and is differentiated by including an inductance 14 in the anode circuit. The anode is connected to a negative point through resistors 15, 16, 17 and the ends of the resistor 16 are connected through oppositely-poled diodes 18, 19 to a point 20 at which the control voltage is taken off. Both valves 2, 12 have cathode resistors 7, 13, the effect of which is to render the cathode current of the same form as the voltage applied to the inner control grid. The sawtooth voltage, Fig. 1, may be derived from the mains supply and the pulses, Fig. 2, may be the frame pulses of a television transmitter, the control voltage obtained at 20, Fig. 3, being used to adjust the pulse frequency. In a modified circuit, Fig. 6, the mixer 23 is of the kind described in Specification 534,750, the output of which is fed through a differentiating condenser 24 to the grid of a valve 25. Fig. 7 shows the output waveform from the mixer 23, and Fig. 8 the differentiated output applied to the valve 25. A feedback circuit 26, 27 between anode and grid gives the valve 25 a low input impedance. The pulses resulting from the differentiation are amplified in the valve 33, the output circuit of which is similar to that of the valve 12 of Fig. 5. The control voltage is fed to the grid of the valve 41 which has a negative feedback path 42 ... 45 operative only for A.C. components and giving a smoothed output at the terminal 47. When the frequencies of the two inputs to the mixer 23 are the same, the mixer output is a constant or slowly-varying direct voltage dependent on phase difference, and is applied directly to the grid of the valve 41 through a resistor 48 and smoothing condenser 49.