Patent GB-729288-A (GB)
Title: Improvements in or relating to radar relays for transmitting radar images
IPC: H04N7/00
Filed: 1953-11-06
Abstract: 729,288. Radar; multiplex pulse signalling; sawtooth generating circuits. PHILIPS ELECTRICAL INDUSTRIES, Ltd. Nov. 6, 1953 [Nov. 11, 1952], No. 30768/53. Classes 40 (5), 40 (6) and 40 (7). The invention relates to a system for the transmission of radar plan-position indication images to a remote point. The information to be transmitted in each radial scan period comprises (a) the scan trigger pulse, (b) the video signal and (c) the instantaneous value of the alternating voltage corresponding to variation in scan direction. There is also required a reference direction indication signal transmitted only during a small number of periods when the radial scan is aligned with the reference direction. According to the invention, (a) and (c) above are transmitted in the form of pulseduration modulated pulses, the trailing edge of these pulses coinciding with the trigger pulses and leading edges being position modulated by the directional alternating voltage. Fig. 1 shows the transmitter, the signals (a), (b), (c) and the reference signal being fed in at 2, 3, 1 and 4 respectively. The trigger pulses from 2 are sharpened at 5 and fed to sawtooth generator 6 to initiate the generation of a sawtooth waveform in which the quick flyback portions correspond in timing to the pulses. The sawtooth waveform is fed to threshold device 7 in which the D.C. threshold level is varied under the control of the signal from input 1 so that trigger circuit 9 receiving the clipped signal produces a train of width modulated pulses whose trailing edges correspond to the trigger pulse timing. These are shown at e<SP>3</SP> on Fig. 3 (c) where the radial scan period is assumed to be 360 Á sec. and the width modulated pulses to vary in length from 100 to 50 Á sec. A pulse generator 10 derives from the sawtooth waveform provided by unit 6 a gating waveform which opens gate 12 to pass the video signal e<SP>4</SP> for the first 160 Á sec. of each period. Generator 10 also feeds pulse generator 11 which generates a gate pulse opening gate 13 for 10 Á sec. after 200Á sec. of the period has passed. This gate passes a part of the reference signal e<SP>5</SP> if such signal is present. The signals from gates 12 and 13 and circuit 9 are combined and amplified at 14 for transmission to the receiver. At the receiver (Fig. 2) the waveform as shown at Fig. 3 (c) is amplified and applied to lines 17, 18 and 19. The line 17 feeds the signal to integrator 21 whose output is as shown at Fig. 3d and only those parts of the signal above level e<SP>9</SP> are passed by circuit 22. This means in practice that only duration modulated sawtooth pulses corresponding to the pulses e<SP>3</SP> are passed and these are converted to amplitude modulated square wave pulses and fed through amplifier 23 to low-pass filter 24 from which an alternating waveform corresponding to the input at 1 is obtained. Line 17 also feeds differentiating circuit 2.6, the output of which is fed through amplifier 27 to a second differentiating circuit 28. After further amplification at 29, the signal is gated by the width modulated pulses from unit 22 at 30 to give short pulses corresponding to the trigger pulses which, after amplification, are fed out at 33. The trigger pulses at 33 are also fed to control the gate circuits 35 and 36 which respectively select and pass the video signal and the reference direction signal. Fig. 4 shows a circuit suitable for units 5, 6, 7 and 9 of Fig. 1. The trigger pulses are fed to the left-hand section of valve 39 to produce very short output pulses from the normally cut-off right-hand section which is connected as a blocking oscillator. The short pulses are fed to the grid of pentode 42 in the sawtooth generating circuit which comprises capacitor 43 fed through charging resistor 44 and diode 45 and discharged when the pentode conducts on arrival of an input pulse. To linearize the sawtooth the voltage is fed back through cathode follower 47 and capacitor 48 to the remote end of resistor 44. The diode 45 permits the voltage at this point to rise above the diode's anode voltage. The resultant sawtooth waveform is fed to double triode 50 over conductor 49. Valve 50 acts as a trigger circuit and also as a threshold device controlled by the signal fed in by line 8.