Patent GB-780736-A (GB)
Title: Improvements in and relating to electronic apparatus
IPC: H03B29/00
Filed: 1954-05-03
Abstract: 780,736. Automatic frequency control system. ISOTOPE DEVELOPMENTS, Ltd., and EPPSTEIN, J. S. B. Aug. 3, 1955 [May 3, 1954], No. 12780/54. Class 38 (4). [Also in Group XL (c)] Apparatus for generating electrical pulses randomly distributed in time comprises a radioactive source, a detector producing pulses in response to incident radio-activity, an amplifier and discriminator for providing an output pulse in response to each detector pulse above a minimum amplitude and means responsive to the output pulses to control the sensitivity of the detector to control the mean repetition frequency of the output pulses. A source of radioactive material such as a foil coated with strontium 90 emits Beta particles which fall on a fluorescent screen which may be a plastic phosphor on which the foil is mounted. The light emitted by the screen energizes the cathode of a photo-multiplier M (Fig. 3) the operating voltages of which are derived from an E.H.T. generator F. Negative-going pulses developed across resistor R1 are applied to amplifiers V1, V2, V3 and V4; V3 is D.C. coupled to V4, which acts as a cathode follower. The output pulses from this amplifier are fed to a voltage discriminator and pulse former comprising pentodes V6, V7 connected as a cathodecoupled flip-flop. A double triode V8 has both sections connected in common as a cathode follower and ensures rapid recovery of the " flip-flop " by reducing the time taken to recharge timing capacitor C1. The positive-going pulses appearing at the cathode of V4 are applied to the control grid of V6 to trigger the flip-flop. This grid is negatively biased by means of a potential divider PD2 while V5a acts as a clamping diode and prevents the grid of V6 going more negative than the bias voltage thus only pulses of more than 20 volts will trigger the multivibrator. Sections V9a, V9b of a double diode act as clamping diodes at the anode of V6 and thus define the pulse duration. The cathode of V9a is maintained at 80 volts by a gas-filled regulator tube V12, while the anode of V9b is connected to the cathode of a double triode V11 the grids of which are connected to a reference voltage derived from potentiometer RV1, thus setting the lower limit of the anode voltage of V6. The positive-going output pulses at the anode of V7 are applied to the grid of V10. Rectifier X1 also connected to this grid acts as a D.C. restorer and applies a bias voltage which holds V10 cut-off. The amplitude of negative-going pulses appearing at V10 is limited by grid current in the valve. These pulses are taken from socket SK1. These pulses are also fed through condenser C3 to the circuit which controls the mean pulse frequency. This circuit comprises a bi-stable multivibrator V4 to both anodes of which the anode of V 10 is coupled via condenser C3 and double diode V13. The multivibrator is then triggered by the output pulses and the two halves of the multivibrator are alternately cut-off and limited by grid current. Rectifiers X2, X3 clamp the grid voltages of V4 thus ensuring that the recovery time is less than the pulse duration. The grids of V14 are coupled to the grids of a similar double triode V15 the two halves of which are cathode coupled. When one grid of V14 is taking current the corresponding grid of V15 is held at earth potential while the other is cut-off. Because of the cathode-follower action the cathode voltage of V is set by the highest grid voltage so that the current through the conducting half of V15 and thus the anode voltages excursions of V15b are well defined. The two halves of a double diode V16a, V16b with condenser C4 and resistor R2 act as a diode pump which produces a grid voltage at pentode V17 dependent on the pulse rate frequency. At a particular value of grid voltage V17 conducts, its anode voltage falling rapidly. The pentode output is coupled via cathode followers V18 to the screen of E.H.T. oscillator V14 to control its output voltage and thus the mean pulse rate at the photo-multiplier tube.