GB995204A - Improvements in or relating to cw radar apparatus - Google Patents

Abstract

995,204. C.W. Doppler radar; parametric amplifiers. ASSOCIATED ELECTRICAL INDUSTRIES Ltd. Feb. 7, 1964 [Feb. 22, 1963], No. 7260/63. Headings H3B and H4D. A low noise C.W. Doppler apparatus comprises a degenerate parametric amplifier 7, Fig. 1, arranged to operate at a pump frequency 2f1+f3, approximately twice the operating frequency f1 of the radar apparatus, means by which the reflected radar signal at a frequency f1+f2, where f2 is the Doppler frequency, is applied to the input of the parametric amplifier, first mixing means 15, by which the idler output of the parametric amplifier is combined with a signal on line 17 at a frequency f1, derived from the radar transmitter 1, to produce an intermediate signal at a frequency f3Œf2 equal to the sum or difference of an intermediate frequency f3 and the Doppler frequency f1, and second mixing means 23, by which a signal at a frequency f3, derived from the pump frequency 2f1+f3, is mixed with the intermediate signal to eliminate the intermediate frequency and leave as output, the Doppler frequency f1. The two mixers 15 and 23 are balanced crystal mixers. The signal at a frequency f3 is derived from the pump signal, by means of a 3db coupler 31 feeding part of the pump signal to pump a second degenerate parametric amplifier 27 having part of the transmitted signal picked off by 3db coupler 29 applied to its input and producing the idler frequency f1+f3 at its output. The idler frequency is then mixed in a balanced crystal mixer 33 with part of the transmitted signal picked off by two 3db couples 19 and 37, to produce the signal at frequency f3. In a second embodiment, Fig. 2 (not shown) the signal at frequency f3 is produced by mixing part of the pump frequency 2f1+f3 with a signal at a frequency of 2f1 derived from the transmitted signal via a varactor frequency doubler. The parametric amplifier may comprise, Fig. 4, an input/output waveguide 101, having a wide band resonant cavity 115 formed at its far end by an iris diaphragm 111, a pump waveguide 103, and a voltage variable capacitive diode 105 connected between the two waveguides and positioned in an aperture 107 joining the two waveguides. The signal frequency f1+f2 is fed into the resonant cavity 115 which has its resonant frequency varied by means of the variation of the capacitance of diode 105 under the action of the pump signal. As well as the amplified signal frequency, a signal at the idler frequency, f1+f3 -f2, and a stray part of the pump signal, are produced and radiated from the waveguide 101. A ferrite circulator, Fig. 3, is provided to separate out the various frequencies being fed to and taken from, the input/output waveguide of the parametric amplifier. The circulator comprises a ferrite core 59 within a box 51 and subject to a magnetic field normal to the paper. Four waveguides lead into the box, and the magnetic field is such that energy transfer can only take place between the waveguides in an anticlockwise direction. The signal frequency is applied to waveguide 53 and is transferred to waveguide 54 and thence to waveguide 101 of the parametric amplifier 63. The amplified signal frequency f1 + f2, passes along waveguide 54, is transferred to waveguide 55 from where it is rejected by filter 67 and finally absorbed in a matched load 69. The stray pump frequency 2f1 + f3 is rejected by filter 65. The required idler frequency f1 + f3 - f2 is however passed by both filters 65 and 67 to the output waveguide 13.