Doppler radar measuring apparatus

Claim 1

A doppler radar apparatus for surveillance of an area lying within a predetermined operating range, comprising: (a) a transmitter for producing a radio frequency signal at an output thereof; (b) a modulator coupled to the transmitter for momentarily modulating said radio frequency signal during succesive periods of duration T at a predetermined modulation frequency, said modulation frequency corresponding to a wavelength which is at least four times the operating range of the apparatus; (c) a transmit antenna coupled to the transmitter for transmitting the modulated radio frequency signal across the area; (d) a receive antenna for receiving a reflection of the transmitted signal from an object within the area; (e) mixer means coupled to said transmitter output and to said receive antenna for mixing the transmitted and received signals and for producing a difference frequency signal representing the range of said object, said mixer means comprising: (1) a power divider for coupling the transmitter output to first and second outputs thereof; (2) a 90° coupler having an input coupled to the receive antenna and having first and second outputs for providing 90° phase-shifted replicas of the received signal; (3) a first balanced mixer coupled to the first output of each of the power divider and the 90° coupler for mixing the transmitted signal and one of the replicas of the received signal, and for producing at an output thereof a first difference frequency signal; (4) a second balanced mixer coupled to the second output of each of the power divider and the 90° coupler for mixing the transmitted signal and the other one of the replicas of the received signal, and for producing at an output thereof a second difference frequency signal; and (5) a 90° phase shifter coupled to the output of the first balanced mixer for producing at an output of the phase shifter a 90° phase-shifted replica of the first difference signal; and (f) an evaluation device, coupled to the outputs of the 90° phase shifter and the second balance mixer, for sampling the difference frequency signal during each period at a predetermined instant when the difference frequency signal amplitude is a maximum.