Patent GB-663144-A (GB)
Title: Improvements relating to radar equipment
Inventor: MCQUEEN JACK GORDON
IPC: G01S7/34
Filed: 1949-09-26
Abstract: 663,144. Radiolocation; saw-tooth generating circuits. METROPOLITAN-VICKERS ELECTRICAL CO., Ltd. June 22, 1950 [Sept. 26, 1949], No. 24690/49. Classes 40 (v), 40 (vi) and 40 (vii). Relates to pulse radar systems for marine use in which the gain of the receiver is progressively increased during the pulse repetition period in order to reduce the " clutter " due to short-range reflections from the sea waves. According to the present invention the gain is varied by applying a positive saw-tooth gaincontrol voltage to the grids of the I.F. amplifiers and the instantaneous rate of increase of the gain-control voltage is controlled automatically in accordance with the strength of received signals. Means are also described for controlling automatically the initial voltage level of the saw-tooth and hence the initial value of the gain in accordance with the integrated effect of " clutter " signals received during the first three microseconds of the repetition period. As shown in Fig. 2, the saw-tooth gaincontrol voltage is generated by charging a capacitor C2 through resistors R2 and R3 and diode V2, the capacitor being discharged during transmission of the radar pulse by a negative pulse applied through capacitor C1, potentiometer P and diode V1, the voltage level to which C2 discharges and hence the initial value of the gains being determined by the potentiometer setting. Signals from the video stage of the receiver are applied to a triode V3 connected between the anode of V2 and earth, the triode being biassed so that any video signal above noise level renders it partially conductive and reduces the rate at which C2 is charging in dependence on the amplitude of the video signal. The means for controlling the initial level of the saw-tooth in accordance with the " clutter " signal strength is illustrated in Fig. 3 in which valves V2 and V3 are omitted for simplification. As before, C2, across which the gain-control voltage is developed, is discharged by a negative pulse applied through C1 and V1 but the voltage level to which C2 discharges is now determined by the voltage across a capacitor C3 to which the cathode of V1 is connected via R4. C3 is charged positively through resistor R1 and is discharged through the series-connected valves V5 and V6, positive video signals being applied to the grid of V5 and a positive gating pulse of 3 microseconds duration synchronized by the discharge pulse being applied to the grid of V6 so that any " clutter " signals above noise level occurring within the duration of the gating pulse render V5 conductive in dependence upon the amplitude of the signals and partially discharge C3, thus reducing the voltage which C3 will attain at the occurrence of the next discharge pulse; V4 is a clamping diode which determines the maximum positive value of the gain-control voltage. In order to ensure that large obstacles, e.g. buoys, do not swamp the effect of sea clutter, the signals causirig the discharge of C3 are limited by arranging that V5 passes grid current when the input signal rises slightly above noise level. Fig. 4 (not shown), illustrates a combination of the circuits of Figs. 2 and 3, C2 being replaced by capacitors C2 and C2<SP>1</SP> in series forming a potential divider and the gating pulse generator being shown as a cathode-coupled flip-flop.