Patent GB-690480-A (GB)
Title: Improvements in radar and like apparatus
IPC: G01S13/70
Filed: 1949-01-11
Abstract: 690,480. Radiolocation; valve circuits. MURPHY RADIO, Ltd., and McMULLIN, T. A. Nov. 7, 1949 [Jan. 11, 1949], No. 748/49. Classes 40 (vi) and 40 (vii). Relates to a secondary pulse radar in which a single strobe pulse is caused to drift in range until it intersects enough of an echo pulse to cancel the drift, the strobe then remaining locked to the echo. In such a system, if the echo signal fades the strobe resumes its normal search drift rate so that when the echo reappears the strobe may not be in a position to lock on and to obviate this means are provided according to the present invention which are operative during the fade period for (a), Fig. 3, causing the strobe to drift in the same direction at a reduced rate; or (b) Fig. 4 (not shown), shifting the strobe abruptly in the opposite direction and then drifting it as in (a); or (c), Fig. 5 and Fig. 6 (not shown), maintaining the strobe stationary at its pre-fade position. It is also stated that the strobe pulse may be caused to drift during the fade period at the rate at which it was drifting immediately prior to the fade period and that preferably the strobe pulse should shift abruptly in the opposite direction and then remain stationary for 10 seconds.  Some embodiments of the invention are described as applied to a system, Figs. 1 and 2, in which a recurrent positive saw-tooth waveform a synchronized by the transmitter pulses A is generated by valve V1 and a slowly increasing positive voltage b is produced at the anode of valve V3 when switch S1 is open due to the changing of capacitor 7 through resistors 6, 8 and 9, the negative side of the capacitor being connected to the control grid of V3 causing the grid potential to decrease slowly. The waveforms a and b are applied respectively to the grid and cathode of valve V2 which becomes conductive when the two waveforms are of equal amplitude and is cut off at the flyback of waveform a to produce negative pulses c whose leading edge is progressively retarded and which is differentiated at 11, 12, the negative pulse d of the resultant waveform d, e being selected and inverted by valve V5 and applied as a positive pulse f to the suppressor grid of a coincidence pentode V4, the echo signals B being applied to the control grid of this valve. When the strobe and echo signals partially overlap a negative pulse is produced at the anode of V5 which, due to the clamping action of diode V6, causes a positive potential to be applied to the grid of V3 which increases the conductivity of the valve. The circuit constants are such that when the echo and strobe pulses overlap completely the voltage b will decrease tending to reverse the strobe drift so that the strobe will assume an equilibrium position locked to the echo, the voltage b being then proportional to the range of the object producing the echo and this range is indicated by the voltmeter I. The circuit constants are also such that a number of cycles are needed to effect any substantial change in the grid potential of V3 so that interfering signals of different recurrence frequency do not affect the reading of the voltmeter I. In Fig. 3, the negative pulse from the anode of the coincidence valve V4 is inverted by the pentode V7 to give a positive pulse which is fed directly to the grid of V3, the anode of V7  being coupled to its suppressor by a capacitor 21 so that when the echo signal fades the posi. tive pulse at the anode of V7 decays slowly and the strobe pulse will drift outwards at a reduced rate during the fade period. Such a system is unsatisfactory if the echo pulse decreases in range during the fade period and in order to meet this case the screen grid of V7 is also capacity coupled to the suppressor, Fig. 4 (not shown), so that when the echo fades V7 is momentarily cut off and the strobe is shifted abruptly in the opposite direction to its normal drift after which it begins to drift outwards at a reduced rate. Fig 5 shows a circuit for arresting the strobe drift during a fade period. When the strobe and echo pulses are coincident a negative pulse from the coincidence valve (not shown) is applied through diode V6 and smoothing circuit 34, 35 to the grid of a pentode V9, the resultant positive anode voltage being applied directly to the grid of V3 (not shown) via a potentiometer tapping 63 in a similar manner to the Fig. 3 embodiment. The output from the smoothing circuit is also directly coupled to the grid of a pentode V10 which has a relay YA and rectifier 38 connected in series between the anode and screen loads 37 and 36 the circuit constants being such that the valve is cut-off or conductive when the strobe and echo pulses are coincident or non-coincident respectively, but in either case the potential of the anode and screen will be equal and the relay will not be energized. When the echo and strobe pulses are coincident and the echo then fades, the screen voltage will decay immediately but the anode voltage decay will be delayed due to the capacity coupling 71 between the anode and suppressor and the relay will operate to break the connection between the potentiometer tapping 63 and the grid of V3 as illustrated in Fig. 6 (not shown), so that the voltage at the anode of V3 remains constant. In Fig. 6 (not shown), the strobe pulse is produced by differentiating the square-wave cathode output of an anode-triggered phantastron the duration of the square-wave and hence the timing of the strobe being progressively varied by applying the anode voltage of V3 through a diode to the anode of the phantastron. The output from the coincidence valve is applied to V3 via a drift modifying circuit V9 and V10 similar to that of Fig. 5, but manually operated switches are provided which short-circuit resistor 72 (V10) and the control-grid input of V9 when it is desired to unlock the strobe from an echo. Capacitor 7 (V3) is also discharged automatically when the anode voltage of V3 reaches a predetermined upper limit by a current-operated relay in the anode circuit which connects the control grid to H.T.+ when the anode current reaches the corresponding lower limit.