867,361. Pulse radar; pulse modulation; pulse generating circuits. DECCA RECORD CO. Ltd. Aug. 6, 1957 [July 10, 1956], No. 21396/56. Classes 40(5), 40(6) and 40(7) [Also in Groups XXXV and. XXXVI] Relates to pulse radar systems comprising means for generating a D.C. voltage which is a linear or sinusoidal function of the timing of an input pulse train relative to a reference recurrent sawtooth or sinusoidal wave of the same recurrence frequency by sampling the instantaneous amplitude of each cycle of the reference wave by means of a sampling pulse derived from an input pulse. In a first embodiment, Fig. 1, D.C. voltages proportional to the x and y co-ordinates of a target producing an echo received by the rotating beam of the radar are produced by sampling two sawtooth waves triggered by the transmitted pulse and of amplitude cos # and sin # respectively where # is the instantaneous bearing of the radar beam. The sampling pulse is derived from an echo pulse selected by a gate opened by an output from an amplitude comparator when the instantaneous amplitudes of the sawtooth waves are substantially equal to the previous x and y sampled values and the output x and y co-ordinate voltages may be used to control apparatus or to position an electronic marker on a C.R.T. display. In a second embodiment, Figs. 7 and 8, D.C. voltages proportional to the sine and cosine of the bearing #<SP>1</SP> of an echo pulse received by a narrow beam aerial rotating with an angular speed w are produced by sampling waveforms sin wt and cos w t where wt is equal to the instantaneous bearing # of the radar beam. First embodiment, Figs. 1, 5, 9 and Figs. 2, 3 and 6 (not shown). As shown in Fig. 1 the radar comprises a pulse transmitter 10, a receiver 14 and a narrow-beam aerial 12 rotated in azimuth by a motor 13, the instantaneous bearing of the aerial 12 being denoted by #. D.C. voltages proportional to sin # and cos # control respectively the slopes of two sawtooth sweep voltages produced in generators 40, 41 which are gated on by a pulse from a generator 39 triggered by the transmitter trigger generator 27. An echo video pulse from the receiver 14 is applied through an amplifier 44 and pentone gate 45, Fig. 3 (not shown), to trigger a blocking oscillator "normaliser" 46, Fig. 4 (see below), giving a sampling pulse of constant amplitude which is applied together with the x and y sweep voltages to pulse sampling demodulators. 42, 43 to give x and y co-ordinate voltages proportional to the instantaneous amplitudes of the sweep voltages at the time of occurrence of the sampling pulse. The co-ordinate voltages are applied together with the sweep voltages to a double coincidence comparator 49 such that during each sweep period an output pulse is produced when the instantaneous amplitudes of the x and y sweep voltages are simultaneously equal to the and y co-ordinate voltages and this output pulse is applied to open the video gate 45 so that whenever an echo is received having similar co-ordinates to the previously generated co-ordinate voltages, it is applied to the sampling demodulators 42, 43 and changes the outputs therefrom in accordance with the new co-ordinates. Thus the outputs from the sampling demodulators change in accordance with the movement of the target producing the echo. In order to track a plurality of targets, separate video gates 45, video normalisers 46, coincidence comparators 49 and pairs of demodulators 42, 43 must be provided for each tracking channel but a single pair of sweep generators 40, 41 may be common to all the channels. Pulse sampling demodulator, Fig. 9 and Fig. 2 (not shown). Each demodulator 42, 43, Fig. 1, comprises a high-gain amplifier 80, the sweep waveform to be sampled being applied to an input resistor 84 which is connected to a feedback resistor 86, the junction of these resistors being connected to the input of the amplifier for the duration of the sampling pulse by an electronic switch 82 actuated by said pulse which also actuates another switch 87 to earth the junction of a capacitor 85 and resistor 83 connected between earth and the amplifier output. Thus during the sampling pulse the capacitor 85 is rapidly charged to a value proportional to the sampled input voltage; at the end of the sampling pulse switch 87 opens and switch 82 changes over thereby connecting the junction of capacitor 85 and resistor 83 to the input of the amplifier and so maintains the charge on the capacitor 85 at a constant level. The output thus comprises a stepped waveform, the magnitude of each step being equal to the difference in amplitude of two successive samples. The output may be smoothed as shown in Fig. 9 by combining the stepped wave output in an adding circuit 200 with a correction waveform obtained by differentiating the stepped wave by an RC circuit 205, 204 to give pulses of amplitude equal to the difference between consecutive sampled levels, and integrating said pulses successively in integrators 206, 207 to give a sawtooth waveform, the slope of each sawtooth being proportional to the amplitude of the pulse from which it is derived. Each integrator comprises a circuit similar to the demodulator but in this case the input is applied to the resistor 83 and the resistor 84 is earthed. The integrators 206, 207 are reset to earth just before sampling by applying the sampling pulse directly to the integrator switch control and via a delay to the demodulator switch control. Alternatively the stepped waveform may be smoothed by providing a second similar demodulator and applying potentials representative of the required rate of change to the end of the resistor 83 remote from the amplifier 80. Double coincidence comparator, Fig. 5. The comparator 49, Fig. 1, comprises 2 pairs of triodes 110, 111 and 118, 119, Fig. 5, each arranged as a long-tail pair with a valve cathode impedance, the two pairs of voltages to be compared being applied to the grids of the triodes. The anodes of the triodes are connected through diodes 120 . . . 123 to the input of a cathode-coupled pair of triodes 126, 127 such that all the triodes 110, 111, 118, 119 are cut off only if the amplitudes of each pair of comparison inputs are substantially equal within predetermined tolerance limits in which case triode 126 is conductive and triode 127 is cut off thereby producing a highly positive control voltage at its anode output; if this condition is not fulfilled at least one of the triodes 110, 111, 118, 119 will be conductive thereby rendering triode 127 conductive and reducing the output control voltage. The magnitude of the equality tolerance determines the duration of the gate pulse applied to the video gate 45, Fig. 1, and is controlled by an adjustable resistor 117 between the cathodes of each pair of comparator triodes, the resistor 117 being adjusted manually or automatically in steps until a response is passed by the gate 45 when the tolerance is reduced to its minimum value corresponding to the possible departure of the target from its linearly predicted position in the internal between consecutive samplings. All the echoes received by the receiver 14 may be displayed on a P.P.I. C.R.T. 17 having fixed x and y deflector coils energized respectively with sweep waveforms from generators 25, 26 whose slopes are controlled by the cos # and sin # control voltages which are produced by applying the output from an a.c. generator 19 to the rotor of a magslip 18 ganged to the arial 12, the resolved cos and sin outputs being applied together with sampling pulses derived from the generator 19 to pulse sampling demodulors 22, 23 similar to the demodulators 42, 43 described above. In order to initiate tracking a particular response on the screen of C.R.T. 17 an electronic mark is set on the response by means of manually controlled x and y voltages which are also applied to those inputs of the double coincidence comparator 49 which are normally connected to the outputs from the demodulators 42, 43, thereby causing the video gate 45 to accept the wanted echo corresponding to the marked response. As shown the x and y co-ordinate voltages from the demodulators 42, 43 are also applied to control the position of a spot or raster on a second C.R.T. 50, the raster being intensity modulated in a known manner by means of a monoscope or flying-spot scanner to indicate the course, speed or identity of the target. Other data may also be displayed on the C.R.T. 50, the respective shift and scan control voltages being applied sequentially to the sweep generators 55, 56 through electronic switches 57, 58 and 64, 65 controlled by a selector unit 63. Instead of a single spot, a circle of radius proportional to the width of the gating pulses from the double coincidence comparator 49 may be produced by applying phase quadrature A.C. voltages from a generator 68 to the shift control inputs of the sweep generators 55, 56. In one position of the selector switches 57, 58 and 64, 65 the C.R.T. 50 may give a P.P.I. display of targets detected by another radar controlled by a trigger circuit 76. The application of brilliance control signals to the C.R.T. 50 is controlled by a switch 74 and gate 75. The x and y co-ordinate voltage outputs from the demodulators 42, 43 may also be applied to a navigation computer to control a craft along a predetermined track. The sweep generators 25, 26 and 55, 56 for the C.R.T's. 17 and 50 may be as in Fig. 6 (not shown) and in Specifications 679,722 and 818,477. Second embodiment, Fig. 7 and Fig. 8 (not shown). Fig. 7 is shown as a modification of Fig. 1, the sinusoidally varying D.C. control voltages cos # and sin # from the demodulators 22, 23 being applied to pulse sampling demodulators 162, 163 similar to the demodulators 42, 43 in Fig. 1. An echo pulse is applied to the demodulators 162, 163 via a gate 164 and normalising circuit 165 as in Fig. 1 but in this case the duration of the sampling