GB596803A - Improvements in time base circuit arrangements - Google Patents

Abstract

596,803. Saw-tooth waveform generators ; radio navigation. WILLIAMS, F. C. March 30, 1945, No. 8134. [Class 40 (v)] In a time-base circuit of the type described in Specification 593,491 for use in an airborne P.P.I. display conforming to true ground ranges, correction for height within predetermined limits is achieved automatically without varying circuit parameters by deriving the exponential components of the time-base voltage from a network comprising a plurality of branch circuits which are charged or discharged for a time dependent on the height h and have time constants in geometric progression. Fig. 1 shows a five-branch network, associated with a Miller time base circuit. A. gating wave-form of width proportional to h is applied at terminal A and terminals D, Bare clamped at earth potential and at normal anode potential respectively for the duration of the gate. When the steady potential at terminal A is removed, terminals D, B are unclamped and the network discharges symmetrically to terminals A, D. The exponentially decreasing currents at D are added to the constant current delivered through resistor R1 to give a resultant potential at B approximating to the required hyperbolic form. The approximation may be made closer by unclamping terminal B shortly before the termination of the gating wave-form at terminal A in order to increase the initial rate of change of voltage at B. Fig. 3 shows a modified network N which is charged in the quiescent condition and has its terminal voltages limited by diodes D1, D3 an D4 respectively. Gate valve V2, the Miller time-base valve V3 and the first valve of biassed multi-vibrator V4, V5, are biassed beyond cut-off and the anode voltage of V3 is limited to 100 volts by diode D5. The positive gating voltage renders valve V2 conducting and network N discharges through resistor R6 and valve V2 for the duration of the gate, and thereafter the anode voltage rises suddenly to 100 volts, producing a positive pulse through differentiating condenser C6 which causes the biassed multi-vibrator to change over and cutoff valve V5. A positive wave front is communicated back via capacitor C7 to cut-off diode D4, and unclamp the grid of valve V3, while a similar pulse reacts on the grid from differentiating capacitor C10 to ensure rapid initiation of the time-base sweep at the anode of valve V3. Capacitor C1 determines the scale of the timebase which is terminated when the biassed multi-vibrator reverts to its original condition. Valve V3 is then clamped in cut-off condition and capacitor C1 recharges'rapidly through the anode inductance. Potentiometer VR1 determines the amplitude of the exponential components and resistors R1, R6 are matched to make the charge transferred from the network N equal to the charge which would be fed through resistor R1 during the period of the gate, if the grid of valve V3 was not clamped.