GB1017236A - Improvements in or relating to doppler radar apparatus - Google Patents

Abstract

1,017,236. Doppler radar. GENERAL ELECTRIC CO. Ltd. Aug. 28, 1962 [Aug. 30, 1961], No. 31279/61. Heading H4D. In a Doppler radar system wherein sinusoidally frequency modulated continuous wavesignals are transmitted, a beat IF signal is produced by mixing the received echo signal with a signal obtained by offsetting the transmitted signal, and the upper and lower first order sidebands are separated out from the IF signal, whereby the amplitude of the sidebands varies as a first order Beseal function of the target range to give minimum system sensitivity to leakage signals and interference signals from zero range targets, the side bands are combined to produce a compound signal from which the Doppler signal can be obtained and one sideband is phase-shifted with respect to the other prior to the combination, whereby the range at which the sensitivity of the system is a maximum can be controlled. In the system shown, oscillator 4 produces a signal centred at X mc/s. and frequency modulated at 100 kc/s. by the output of oscillator 5. The modulated signals fed to aerial 3 for transmission, via circulator 6. The echo signal is mixed in mixer 11 with a portion of the transmission signal offset by 40 mc/s. by means of oscillator 12 (40 mc/s.) and mixer 13. The resultant beat signal centred on 40 mc/s. is further reduced in frequency in mixer 15 by means of mixer 17, oscillator 12 and oscillator 16 (2 mc/s.), such that it is centred about 2 mc/s. Band-pass filters 23 and 24 respectively separate out the sidebands spaced from the centre frequency of 2 mc/s. by plus and minus the modulation frequency, i.e. those centred at 2 mc/s. + 100 kc/s. and 2 mc/s. - 100 kc/s. Each sideband comprises two frequencies separated from the centre frequency by plus and minus the Doppler frequency, and having amplitudes varying as indicated above. Each sideband is amplified and converted to a common centre frequency of 550 kc/s. in mixers 27 and 28 with the aid of mixers 30, 31 and 33, frequency double 32 (200 kc/s.) and oscillator 29 (450 kc/s.). The signal from mixer 28 is applied directly to amplifier 35, while that from mixer 27 is applied to the amplifier via phase shifter 34. The output of the amplifier, comprising the combined outputs of the mixers, is fed to receiver stages 36, where the Doppler frequency is extracted and the speed of the target determined. Means for cancelling out transmitter energy leaking through circulator 6 to output 10, similar to that shown in Specification 916,486, are described.