Patent GB-690339-A (GB)
Title: Improvements in plan position indicator scanning circuits
Inventor: JONES ANTHONY MERVYN HAROLD
IPC: G01S7/14
Filed: 1950-02-14
Abstract: 690,339. Radiolocation; saw-tooth generating circuits. COSSOR, Ltd., A. C. Jan. 22, 1951 [Feb. 14, 1950], No. 3799/60. Classes 40 (vi) and 40 (vii). A circuit for feeding a deflecting coil of a plan position indicator comprises two valves having the coil connected between their cathodes. The cathode of one valve is connected through a resistive device to a negative supply source and an alternating voltage is applied to vary the current through the two valves in opposite directions to drive a current of saw-tooth or truncated saw-tooth waveform through the deflecting coil. The magnitude and sense of the Direct Current in the coil is varied by variation of the effective resistance of the two valves in opposite directions. The Figure shows a circuit for producing a linear saw-tooth current in a deflecting coil L1 in which a saw-tooth voltage is applied from a wave generator G via a potential divider P to the control grid of a normally conductive valve V1. The potential at point T2 is maintained constant by applying the voltage across resistor R7 to the grid of amplifier valve V5 so that any slight variation in voltage of point T2 is amplified and applied in reverse sense to point T3 thus varying the effective impedance of valve V4 to maintain the voltage at T2 constant. If it is assumed that a constant Direct Current passes through inductor L2 and that the potential at point T2 is constant then the current flow into resistor R7 may be derived from either valve V4 or in part from valve V2. If the potential divider P1 is adjusted so that maximum current flows through valve V1 so that valve V2 is cut-off then the current which is normally passed by valve V2 is diverted through valve V4 and valve V3. Since valve V3 is connected as a constant current pentode the D.C. current flow through the deflecting coil L1 will have a maximum value in the direction of the arrow A1. By adjusting the divider PI to reduce the current flow in valve VI so that valve V2 is conductive the whole of the current for valve V3 now passes through valve V2 and thus the D.C. current flow through the deflecting coil L1 will have a maximum value in the direction of arrow A2 which will be equal to the maximum value of that in the direction A1. Thus the potential divider PI may be used to vary the D.C. current flow through the deflecting coil L1 and thus to offset the oscilloscope beam from the centre to, or a little beyond, the edge of the screen, the saw-tooth current flow in the deflecting coil L1 being produced by the application of a suitable waveform-from the wave generator G. Since the current flow in the deflecting coil L1 is limited to either of two maximum values (in directions A1 or A2) corresponding to the two conditions in which valve V1 or valve V2 is cutoff, whatever voltage is applied through resistor R1 these maximum current values cannot be exceeded, the scale of the display is increased but the maximum deflection is not exceeded. Thus the beam of the oscilloscope may be offset by a large amount by adjustment of divider PI without the need to generate large deflecting currents to secure deflection of the beam on the screen. Normally two such circuits would be used in a P.P.I. display, one associated with each pair of deflecting coils, the input voltages to the two circuits being modulated in accordance with the sine and cosine respectively of the angle made by the radar antenna with some datum direction. In order to further limit the current flow in the direction A2 in deflecting coil L1 to the maximum value, valve V6 is connected in a subsidiary negative feed-back loop between the junction-point of coil L1 and resistor R5 and the control grid of valve V2; the constant current pentode V3 may be replaced by a resistive or a resistive inductive network.