Patent GB-935069-A (GB)
Title: Limited circuit for a time modulator
IPC: G01S13/70
Filed: 1959-09-14
Abstract: 935,069. Pulse radar; pulse circuits. HOLLANDSE SIGNAALAPPARATEN N.V. Sept. 14, 1959 (Sept. 15, 1958], No. 31230/59, Classes 40 (6) and 40 (7). In a radar system employing a non-overlapping double gate target selector controlled in known manner by a pulse derived via an adjustable delay means from the synchronizing pulses which initiate each transmitter pulse, the reception of echo signals from the area in the immediate vicinity of the apparatus (ground echoes) is prevented by limiting the minimum delay which the adjustable delay means can introduce. As shown in Fig. 1, the target echo (video) signals are supplied at 141 to a pair of gates 139, 140 rendered conductive sequentially by pulses from gate generators 116, 117. Signals phased by the gate are supplied to integrators 142, 143 and thence to a differential amplifier 144 the output of which controls a cathode-follower 146. From the latter a signal is derived via an adjustable potentiometer 150 and employed to control a Miller integrator comprising valves 151, 153 and capacitor 156 the output of which is supplied over conductors 157, 110 to bias a diode 108 such that the latter can only conduct when the amplitude of a saw-tooth wave from a generator 103, 105, 107, controlled by the synchronizing pulses of the system at terminal 119, exceeds such bias level. The pulse produced by such conduction is then supplied via stage 111 as a reset pulse for a trigger circuit 124, 126 (set to a first state with 124 conducting by each synchronizing pulse) which on resetting supplies a pulse, via amplifier 114, to the display tube via terminal 115 and to the gate generator 116, the latter supplying a pulse to gate generator 117 on the termination of its own input pulse. In this manner when once the desired target echo has been selected by control 150 the gates 139, 140 continue automatically to track the target. When the range becomes very short the target echoes are obscured by ground echoes and the invention therefore provides means to prevent the gates from passing such echoes. This is achieved by setting a minimum value to the delay which the circuit can introduce between the production of the pulses controlling the gates and the occurrence of the corresponding synchronizing pulses. As shown, a capacitor 137 (bridged by a high value resistor 136) is charged via 135 when the latter conducts under the control of the output of cathode follower 131 during the interval between the occurrence of a synchronizing pulse and the resetting of trigger 124, 126. The potential across 137 is thus proportional to target range and by a suitable arrangement of circuit parameters conduction of a diode 138 holds the input potential to the Miller integrator 151, 153, 156 (which increases positively with decreasing range) at a predetermined value so that further decrease in the potential on conductors 157, 110, which determines the reset time of trigger 124, 126, is prevented. For limiting the maximum range of the apparatus a diode 155 is employed. In a modification of the elements shown within the dotted line area of Fig. 1, the cathode-follower 131 is dispensed with and the grid of valve 135 supplied from the resistor 129 (Fig. 2, not shown). When the range at which limitation recurs is very short correct operation requires that capacitor 137 must charge to the required potential in a very short time. This requires a steep fronted voltage step which, it is stated, may lead to instability and to avoid this a fixed delay is introduced between the synchronizing pulses and the saw-tooth wave generator (103, 105, 107, of Fig. 1), as described with reference to Fig. 3 (not shown). In a modification of this arrangement the fixed delay is introduced between the synchronizing pulse source and the radar transmitter (Fig. 4, not shown). In a further modification the charging of the capacitor 137 controlling the minimum range (Fig. 1), is controlled by synchronizing pulses applied via an adjustable delay device (Fig. 5, not shown).