Patent GB-1387579-A (GB)
Title: Doppler radar systems
IPC: G01S13/58
Filed: 1961-11-27
Abstract: 1387579 Radar DEFENCE SECRETARY OF STATE FOR 20 Nov 1962 [17 Nov 1961] 42401/61 Heading H4D In a Doppler radar wherein a phase sensitive detector detects a Doppler signal using a reference signal at the same frequency as the Doppler signal, the Doppler and the reference signals being obtained from two coherent detectors which detects signals from a moving target, using a signal derived from the transmitter as a reference signal; phase shift means are provided for introducing a 90 degrees phase shift between the inputs to the two coherent detectors, and a further phase shift means introduces a further 90 degrees phase shift between the Doppler signal and the reference signal applied to the phase sensitive detector so that signals in the output of the phase sensitive detector due to unwanted double sideband incoming signals caused by noise modulation of reflected transmission power oppose each other. In the C.W. system shown in Fig. 1, received signals pass to detectors 6 and 7 directly and through quadrature phase-shift circuit 11 respectively. A reference frequency is supplied to detectors 6 and 7 from the transmitter, and the Doppler outputs of the detectors are amplified in units 8 and 9. The outputs of amplifiers 8 and 9 pass through quadrature phase-shift circuit 12 and directly, respectively, to a phase-sensitive detector 10. In another arrangement, Fig. 2, signals are fed to units 6, 7 and 11 at intermediate frequency and the Doppler frequency outputs of amplifiers 8 and 9 pass to speedgate mixers 16 and 17 where they are mixed with the output of speedgate oscillator 15. The outputs of mixers 16 and 17 pass through narrow band amplifiers 18 and 19 to the phase shift circuit 12 and detector 10. Thus Doppler signals at the discrete frequency corresponding to a given target velocity may be selected by adjustment of the frequency of the speedgate oscillator. The phase-shift circuits 11 and 12 are shown as situated with one in each channel, but one of them may be provided with switching means enabling it to be switched into either channel according to the sense of radial movement of a wanted target. An A.G.C. circuit 20 may be included, operating an amplifier 18 in accordance with the output of detector 10. For use in a tracking system channels 4 and 5 may be extended to two mixers supplied with a reference oscillation from a common oscillator operating at speedgate amplifier frequency, Fig. 3 (not shown). The outputs of the mixers are fed via low-pass filters to a second phase sensitive detector, the channel 4 including a differentiator. The output of the detector is used to control the speedgate oscillator 15 to track the target in velocity. When tracking in direction, Fig. 4 (not shown), using an aerial system comprising a four-horn cluster arranged to produce a sum signal and two difference signals, the sum signal is applied to a channel similar to channel 4 of Fig. 2 and the two difference signals are applied to two further channels similar to channel 5 of Fig. 2; the outputs of the two phase-sensitive detectors then represent the azimuth and elevation misalignments. A limiter may be provided in the reference signal feed to the two phasesensitive detectors from the quadrature phaseshift circuit, and may be regenerative to ensure constant amplitude for the reference signal irrespective of breaks in the input signals.