Patent GB-596314-A (GB)
Title: Improvements in voltage wave generators
IPC: G01S13/42
Filed: 1945-02-23
Abstract: 596,314. Radiolocation; pulse modulation ; valve circuits. WESTERN ELECTRIC CO., Inc. Feb. 23, 1945, No. 4650. Convention date, Feb. 24, 1944. [Class 40 (v)] [Also in Group XXXV] Means for derving ai substantially continuous waveform corresponding to the modulation envelope of an amplitude modulated pulse train comprises means for applying the pulse train to a storage element, e.g. a condenser, the energy content of which is adjusted to a reference level before the application of each pulse and is then varied by an amount proportional to the pulse amplitude and remains substantially constant until the operation of the next reference level adjustment. The invention is described as applied to a pulse radar system employing a conical aerial-scan in which the pulses received from a target displaced from the scan axis are amplitude modulated at the scan frequency, the phase of this modulation being used to derive two error voltages proportional to the azimuthal and elevational misalignment of the scan axis with respect to the target line of sight. The error voltages are then applied to control azimuth and elevation motors which automatically orientate the aerial to reduce the misalignment to zero. The system may be combined with a known form of automatic ranging to provide complete automatic tracking of the target. As shown in Fig. 2, the amplitude modulated train of short pulses 201, Fig. 6, is applied to the grids of the parallel connected valves V8 and 9, and to the grid of V 10 is applied a train of constantamplitude positive " clearing " pulses of greater width having the same pulse recurrence frequency as the modulated pulses, each " clearing " pulse preceding a modulated pulse by a short time interval. Each clearing pulse renders V10 conductive causing the condenser 108 to be charged up to a constant reference potential 222, Fig. 7, and the succeeding modulated pulse renders V8 and 9 conductive causing the condenser 108 to discharge by an amount proportional to the amplitude of the modulated pulse. In the interval between a modulated pulse and the succeeding clearing pulse, valves V8, 9 and 10 are non-conductive and the charge on the condenser 108 remains constant, 224, Fig. 7, giving a stepped Waveform 224 with a " spike " 225 on the front of each step. The '' spikes " are removed by the filter circuit 110, 111 giving a smoothed waveform of the same shape as the modulation envelope 221 but of reverse phase, plus an additional small phase displacement equal to half the width of a step 224. The smoothed waveform is then fed to an output through a cathode follower V11. In a modification, Fig. 5 (not shown), the heater circuits of V10 and 11 are connected to a tapping across the cathode load of V11 in order to minimize the heatercathode potential difference and consequent leakage. Alternatively, the condenser 108 could be discharged by the clearing pulses and then charged by an amount depending on the amplitude of the modulated pulses. The reference voltage to which the condenser is charged need not be constant as shown in Fig. 7, but may vary in accordance with some modulation with which it is desired to combine the pulse amplitude modulation. Radar system, Figs. 1 and 8. In the radar system shown in Fig. 1, the transmitter 10 and receiver 19 are connected through the TR box 11 to the aerial system 12, the directive lobe of which is scanned conically by rotating a waveguide about the axis of a parabolic reflector 14, the complete assembly being orientated in azimuth and elevation by the motors 15 and 16. Echoes from a particular target 201, Fig. 8A, are selected by applying a trigger pulse 200, Fig. 8, from the transmitter 10 to the range unit 21 to generate a variable phase pulse 202, Fig. 8B, which is inverted and then applied to shock excite a tuned circuit connected to the grid of an unbiassed valve in the pulse generator 22, Fig. 4 (not shown), producing a positive clearing pulse 206, Fig. 8E, at the anode of the valve, the duration of which is determined by the tuning of the shock-excited circuit; the trailing edge of this pulse initiates a second pulse 212, Fig. 8I, of similar duration by a process of differentiation and shock-excitation, Figs. 8F, G and H, and this pulse is applied as a gating pulse to the signal selecting circuit 20. In this circuit, Fig. 3 (not shown), the positive gating pulse is applied through a cathode follower to the cathode of a mixing valve and the negative video signals from the receiver are applied to the grid giving an output, Fig. 8K, in which the selected echo 201 is superimposed on a pedestal 217, the selected echo being separated from the pedestal by limiting the output at a cut-off level 215. The selected echo signal is fed together with the output from the range unit 21 to a known form of tracking unit 23 which automatically maintains the selected echo signal in the centre of the gate pulse. The selected echo signals which are modulated at the scan frequency are also fed to a detector 24 of the type described with reference to Fig. 2 and the output modulation frequency 226 is applied through the amplifier 25 to the azimuth and elevation phase detectors 30 and 31 together with phase-quadrature reference scanning-frequency voltages 227, 228 from the generators 18, 18A which are driven in synchronism with the rotating wave-guide. In each phase detector which may be similar to that disclosed in U.S.A. Specification 1,539,903, the reference voltage is applied in push-pull and the modulation voltage 226 is applied in parallel to the grids of a pair of valves to produce a D.C. error voltage of positive or negative polarity depending on whether the modulation voltage is more nearly in phase or phase-opposition with the reference voltage, the amplitude of the error voltage depending upon the degree of phase coincidence. The error voltages are applied to bridge type modulators 34 and 35, together with 90 degrees phase-shifted 60 c/s. voltages from the source 38 to give 60 c/s. output voltages which are 90 degrees phase advanced or retarded with respect to the input 60 c /s. voltages depending on the polarity of the error voltages, the amplitude of the output voltages depending on the amplitude of the error voltages. These output voltages which are thus in phase or phase opposition with the voltage source 38 are applied together with a voltage direct from the source 38 to reversible two-phase motors 15 and 16 which rotate the aerial in azimuth and elevation until the alignment error is zero. In order to minimize distortion of the scan frequency modulation by extraneous signal fluctuations, a portion of the output from the detector 24 may be applied through a longtime constant circuit to control the gain of the receiver 19..