GB666464A - Improvements in or relating to electrical oscillation generators - Google Patents

Abstract

666,464. Valve generating circuits; frequency modulation. GENERAL ELECTRIC CO., Ltd., and CORMACK, A. Oct. 25, 1950 [Nov. 22, 1949], No. 29957/49. Classes 40 (v) and 40 (vi). A phase shift oscillator of the type comprising a first valve arranged as a resistance capacity coupled amplifier connected in a loop with at least three cathode follower stages has its frequency varied or modulated by varying or modulating the anode load impedance of the first valve. As shown, a resistance capacity coupled amplifier 1 is followed by four cathode followers 2, 3, 4, 5, the output from the final cathode being fed back to the grid of amplifier 1. The loop will oscillate at the frequency at which the phase change produced by each cathode follower as a result of the interelectrode and stray capacitances is 45 degrees. The output is derived from the cathode of stage 2 and is fed to an output stage 23. A further cathode follower 26 is connected across the anode load 10 and owing to the grid cathode capacitance of the valve its output impedance is inductive and may be varied by varying the grid bias. In the circuit described a variation of frequency between 40 and 100 mc/s. may be obtained without changing the output amplitude by simultaneously varying the grid bias of the cathode followers 2-5 and between 100 and 150 mc/s. by varying the grid bias on the cathode follower 26, a continuous variation throughout the two ranges being obtained by means of a potentiometer 54. With the slider at its lowest point all the cathode followers are heavily biased but as it is raised an increasing positive voltage is applied through resistor 43 to the control grids of stages 2-5. This continues until the positive bias applied from a potentiometer 46, 47 to the cathode of a diode 38 is overcome when the diode conducts preventing further rise. At this point however the negative bias applied from a terminal 52 to a further diode 40 is also overcome and this diode conducts and the bias on stage 26 is thereafter reduced below the cut-off value applied from terminal 52. The voltage obtained from the potentiometer 54 may be replaced by a modulation voltage and chokes may be included in the filament leads for the cathode followers 3-5 to reduce the cathode-earth capacitance and thus increase the maximum oscillation frequency.