GB857539A - Improvements in or relating to electric transfer circuits - Google Patents

Abstract

857,539. Television. ELECTRIC & MUSICAL INDUSTRIES Ltd. April 5, 1957 [April 6, 1956], No. 10520/56. Class 40(3). [Also in Group XL(c)] Relates to a circuit for transferring to a load, in the absence of a D.C. coupling, signals including both A.C. and D.C. components. The invention is stated to be of general application, reference being made to television systems and oscilloscopes, but is described solely in relation to a circuit for applying a brightening pulse to the cathode of the display tube of an oscilloscope. The pulse 20 appears on lead 1 from a low impedance source, such as a cathode follower, and is applied to the cathode 11 by way of an A.C. coupling so that the pulse source is isolated from the large D.C. potential required for operation of the tube. The coupling consists of a first path including capacitor 2 which transmits only the high-frequency components of the pulse, and a second path including a 5 for transmitting the D.C. and low-frequency components of the pulse. In this second path the pulse components are modulated in a stage 3 on a carrier, e.g. of 100 kc/s, provided by oscillator 4 and after passage through the transformer are reproduced by demodulation in a bi-phase rectifier 6 and 7. The sense of the modulation is such that the pulses reduce the carrier amplitude. The pulse components appearing across the rectifier load 9 are then added via resistor 10 to those appearing through capacitor 2 and applied to the tube cathode. In accordance with a feature of the invention, capacitor 2 functions to decouple the carrier components in the output of the rectifier circuit via the low impedance of the pulse source. Capacitors 14, 18 and 19 provide normal decoupling of the respective points in the circuit to earth. Resistor 15 permits the gridcathode bias of the tube to be varied. The part of potentiometer 17 connected between the tube cathode and the slides appears as a shunt load to the signal applied to the cathode, and where the potentiometer is arranged remote from the tube a resistor is preferably included in the cathode lead to isolate the shunt capacitance. Specification 696,362 is referred to.