Patent GB-757699-A (GB)
Title: Improvements in or relating to pulse amplitude sensitive trigger circuits
Inventor: KANDIAH KATHIRKAMATHAMBY
IPC: H03K21/02
Filed: 1953-02-24
Abstract: 757,699. Pulse amplitude discriminating circuits. NATIONAL RESEARCH DEVELOP. MENT CORPORATION. May 24, 1954 [Feb. 24, 1953], No. 5135/53. Class 40 (6). A pulse-amplitude sensitive circuit comprises a discharge-tube which is operated by pulses exceeding a predetermined level and a further tube regeneratively coupled to it to form a trigger circuit the first tube being in a stable state of continuous conduction in the stand-by or quiescent condition. The Figure (Provisional), shows a pulse amplitude discriminating circuit to which positive-going pulses are applied at input 20 and which furnishes a positive pulse at output 21 in respect of each input pulse exceeding a predetermined amplitude. Tubes 1 and 3 are reciprocally-coupled via common cathode resistor 2 and resistor-capacitor network 4, 16, 5 but the stability of the circuit with both tubes normally conducting is maintained by the negative feedback between the anode and con. trol grid circuits of tube 1 via diode 6 and by the use of diode 6, the impedance of which is a function of the current flow, as a load for tube 1. When a positive pulse is applied at input 20 to tube 1 no appreciable change of anode voltage takes place until almost all of the current in diode 6 is switched into tube 1 when the circuit rapidly triggers to the state in which tube 1 conducts heavily and tube 3 is cut-off. As the anode current of tube 1 and the current flow through diode 6 are stabilized the input pulse amplitude to cause triggering of the circuit is exactly determined. The sensitivity can be increased by reducing the current in diode 6 by increasing the values of resistors 7 and 8 in the same ratio. After triggering, the circuit reverts at a rate determined by network 4, 16, diode 25 limits the grid swing of tube 3 and a fast positive-going pulse is produced at 21. Rectifier 15 and resistor 17 prevent the cathode of diode 6 from falling slowly when the circuit is in the triggered condition the current flow through resistors 14, 17 being equal to that through tube 3 in the steady state thus rendering the sensitivity substantially independent of input pulse recurrence frequency. A variable discrimination level is obtained by adjusting the bias on diode 18 by potentiometer 19. In a modification shown in the Figure (Complete) the feedback over tube 1 is obtained via diode 6 and cathode-follower stage 22. The circuit of diode 6 is completed through tube 24b and the current through diode 6, the negative feedback to the grid of tube 1 and the sensitivity of the discriminator are all adjustable by varying resistor 23. When the circuit is triggered over triode 24b is cut-off at its grid thus'isolating the cathode of diode 6 and the sensitivity of the circuit is independent of input pulse recurrence frequency without the provision of a compensating circuit. Resistor 16a is provided for charging capacitor 4 through diode 26 independently of grid resistor 16 thus preventing the reduction in sensitivity, when the input pulses have a slow rate of rise, due to. differentiation of the pulse at the anode of tube 1 by network 4, 25. The circuits may be adapted to operate with negative input pulses by reversing the connections to diode 6 and adjusting the load of tube 1 so that diode 6 passes the required stabilizing current.