Patent GB-1114496-A (GB)
Title: Automatic gain control system
Inventor: CHAPMAN THOMAS HENRY
IPC: B07C5/342
Filed: 1966-02-08
Abstract: 1,114,496. Automatic gain control. GUNSON'S SORTEX Ltd. 12 Jan., 1967 [8 Feb., 1966], No. 5525/66. Headings H3Q and H3T. [Also in Division G1] The invention relates to a system for controlling automatically the gain of a circuit adapted to receive a pulsed waveform, the pulses of which may vary in amplitude and may have signals superimposed thereon. It is described as applied to automatic sorting equipment using photoelectric viewing heads: objects 10, Fig. 1, to be sorted on the basis of their light reflection are allowed to fall past a highly reflecting white surface 13, illuminated by lamp 12. The reflected light is scanned by means of a rotating disc 16 with apertures 15 in its periphery, being transmitted to photo-multiplier 11 by means of light conductor 14, e.g. a bundle of optical fibres. The photomultiplier output comprises a train of rectangular pulses (Fig. 2, not shown), each extending from the black level A<SP>1</SP>G, Fig. 3, to a peak white level CF corresponding to the reflecting white surface, upon which is superimposed a pulse DE of lesser amplitude produced by the corresponding object being sorted. The pulses A<SP>1</SP>CFG vary in height due to (1) variations in the areas of the scanning apertures, and (2) A.C. mains energization of the lamp 12. To compensate for such variations, the invention provides means for charging a capacitor with the pulses and superimposed signals and switch means, actuated automatically during each excursion of the pulsed waveform in a predetermined direction to discharge at least partially the capacitor prior to the expected arrival of the next superimposed signal, together with means for comparing the voltage across the capacitor with a reference voltage to produce a gain control voltage dependent on the comparison. Means are provided utilizing the gain control voltage to control the gain of the circuit, whereby on receipt of each pulse the gain is increased to and maintained at a high value which is substantially constant during the presence of the signal and whereby amplitude variations of the pulsed waveform on which the signal is superimposed are substantially eliminated. The output of photomultiplier 11, Fig. 4, is fed via cascaded emitter-followers Q1, Q3, Q4 to charge capacitor C1; at a point B<SP>1</SP>, Fig. 3, in each pulse a pulse passed by emitter follower Q2, coupled to emitter follower Q1, causes a succeeding biased-back transistor to become conductive. A pulse is passed from the collector of this transistor to the base of transistor Q15, which with transistor Q16 forms a Schmitt bi-stable trigger circuit, whereby it reverses its state. A pulse from the collector of transistor Q15 is then passed to capacitor C2 which differentiates it, producing a sharp negative-going pulse which is applied to the base of transistor Q17: this is normally non-conductive but is momentarily rendered conductive and so partially discharges capacitor C1 which it shunts. The gain is thereby momentarily increased by the gain control voltage, so that the incoming pulses rise rapidly to the peak reference level C, Fig. 3, and are thereafter stabilized at that level by the gain control voltage. The gain control voltage is derived from capacitor C1 via emitter-follower Q5 and adjustable potentiometer RV2 which applies the voltage to the base of one transistor Q6 in an emitter-coupled pair: the base of the other transistor QY derives a reference voltage from Zener diode MR1. A voltage dependent upon the difference between those applied to the bases of transistors Q6, Q7 is derived from the collector of transistor Q6 and is applied over cascaded emitterfollowers Q8, Q9 to vary the output of an oscillator Q10. A feedback path comprising resistor R3 and capacitor C3 between the emitter of transistor Q9 and base of transistor Q6 stabilizes the arrangement. The output of the controlled oscillator Q10 is applied to a rectifier circuit to produce at terminals 1, 2, 3 smoothed voltages which are applied to the corresponding dynodes 1, 2, 3 of photomultiplier 11, thereby controlling its gain.