Patent GB-711513-A (GB)
Title: Improvements in or relating to radar system
IPC: G01S19/26
Filed: 1952-01-09
Abstract: 711,513. Radio-location. GILFILLAN BROS., Inc. Jan. 9, 1952 [Jan. 29, 1951], No. 684/52. Drawings to Specification. Class 40 (7). [Also in Group XXXV] In a pulse radar system wherein the Doppler effect is used to provide indications of moving targets only, and of the type wherein the received echo signals corresponding to each pulse transmission are mixed with a reference signal coherent in phase with a signal derived from the corresponding exploring pulse, delayed for an interval corresponding to the pulse, period and subsequently combined subtractively with the directly received echo signals corresponding to a subsequent pulse transmission, gain control means, which may be manually or automatically controlled, are provided to maintain the. mean signal levels in the delayed and undelayed channels equal, and frequency control means' are provided to vary the pulse recurrence frequency of the exploring pulses in dependence upon variations in the delay introduced in the delayed channel to ensure that delayed and undelayed echo signals corresponding to the same target occur simultaneously and' thus effectively cancel for stationary targets. The exploring radio-frequency pulses are converted to intermediate frequency by a stable local oscillator and subsequently control the phase of a secondoscillator providing the coherent reference signal. The echo signals after conversion to intermediate frequency are heterodyned with the reference signal to modulate those corresponding to moving target with a beat-frequeney signal due to the Doppel effect. The resultant signal is detected and then modulated upon a 15 Mc/s carrier which is applied to a delayed channel and an undelayed channel in parallel. The' delayed channel comprises a mercury delay line which introduces a delay of one millisecond, equal to two pulse periods, and a highgain amplifier to compensate for the attenuation introduced by the delay line. The undelayed channel comprises a resistance attenuator and high-gain amplifier corresponding to that in the other channel. The outputs of the two amplifiers are applied to a subtraction circuit the output from which is. amplified and then passed' to a signal-selecting circuit to which unmodified echo signals, are also applied, control pulses applied to the selecting, circuit determining the range limits between which moving-target indication is provided and the limits between, which normal echo indication is provided. The output from the signal-selecting circuit is then passed to a display unit or control circuit. The radar system also comprises an automatic temporal control system wherein the frequency of the master oscillator controlling the pulse-recurrence frequency is. controlled in accordance with the delay introduced by the mercury delay line and a related automaticgain control' system whereby output pulses from the two channels corresponding to stationary targets have equal amplitude. The output from the master oscillator, which is of the resistancecapacity type, is converted' to sharp pulses by a blocking oscillator circuit, these pulses being used to. develop pairs of closely spaced gating pulses. A further tracking pulse signal, delayed so that it lies midway between the gating pulses, is developed, and modulated on to the 15 Mc/s carrier for transmission through the delayed channel comprising, the. mercury delay line and subsequent amplifier. The amplifier, output is demodulated and applied to a gating circuit controlled by the gating pulses. The gating circuit delivers an output, corresponding to the relative timing of the input "tracking" pulse, which is applied to the master oscillator, to control its frequency so that under conditions of balance the "tracking" pulse lies midway between the two gating pulses. A further 15 microsecond " sampling " pulse is derived from the trailing edge of the later of the gating; pulses and is also passed through the delayed and undefayed channels to derive an automatic-gain-control voltage for controlling the gain in the undelayed channel. The output from the subtraction circuit is applied to a circuit which is gated by a pulse signal corresponding to the " sampling puke. The output of the circuit includes an integrating circuit which accordingly develops a gain-control signal in accordance with the polarity and magnitude of the sampling pulse at. the output of the subtraction circuit.. In order to ensure that' the temporal control and gain control circuits are not affected by the reception of noise or of echoes from distant targets,- the receiver is blanked off for the later portion of each pulse cycle, e.g. at a time occurring, later than 350 microseconds after the. transmission of each exploring; pulse.. Alternative manual gain-control means are also provided, switching from. automatic to) manual control being effected by a change-over switch controlling a. relay.