GB1025709A - Resistance-capacitance type oscillators and selective amplifiers - Google Patents

Abstract

1,025,709. Amplifiers etc.; oscillators; frequency changers, MUIRHEAD & CO. Ltd. Jan. 23, 1964 [Feb. 8, 1963], No. 5132/63. Headings H3F and H3W. [Also in Division G1] An electronic oscillator or frequency selective amplifier in which the frequency of operation is determined by a resistance capacitance network in a positive feedback loop of an amplifier has a photo-sensitive element in the frequency determining network and means for varying the light falling on the element and therefore the frequency of oscillation, or operation, of the circuit. Fig. 1 shows one embodiment of a device for varying the intensity of illumination of a photo-sensitive element in which the mask 4 and an opaque disc 1 with transparent section 3 are interposed between a source of light and the photo-sensitive device, the rotation of the disc 1 controlling the light and therefore the operating frequency of the circuit. The mask 4 may be adjustable vertically to provide a zerosetting means. If a greater range of operation is required two discs geared to move in unison and with their transparent sections 3 crossing nearly perpendicularly may be provided, Fig. 3 (not shown), to give a variation of light intensity which is approximately the square of the variation provided by one disc alone. Alternatively the disc or discs may be replaced by film strips, Fig. 7 (not shown), one of which may have the frequency scale 38 photographically produced on its edge. The discs may also be replaced by a pair of polaroid plates one of which is turned relative to the other to modulate the light source. A low frequency oscillator circuit embodying the invention is shown in Fig. 10 in which photo-sensitive resistors 14 and 14<SP>1</SP> are subjected to the same illumination from a source 8 via a light modulating device not shown. The two photo-sensitive resistors may be replaced by a pair of fixed resistors having one photo-sensitive resistor connected between tapping points on the fixed resistors, in addition the push-pull output arrangement may be replaced by a transformer if the extreme low frequency range is not required (Fig. 5, not shown). The output signal of the oscillator may be modulated by providing a pendulum having an arcuate filter, opaque at its two extremities and linearly graded in optical density to transparency at its centre, arranged with the filter interposed between the light source and the mask in the frequency determining arrangement of the oscillator. When modulation is required the pendulum is maintained in oscillatory motion, the frequency of the swing being adjusted by adjustment of the pendulum bob weight. The light output of the source may be checked by having a further photo-sensitive resistor mounted to be illuminated by the light source and connected in a wheatstone bridge circuit which is balanced when the illumination is correct, the bridge output may be connected to automatically maintain the light output correct.