Doppler intrusion detector with dual phase processing

In an intrusion detector in which a signal generated by a microwave or ultrasonic generator is radiated into an area under surveillance and reflected from every object in the area;

• a receiver to receive said reflected signal and to present said reflected signal to a pair of mixers, each said mixer mixing said reflected signal with a portion of the transmitter signal, the improvement comprising (a) splitting of said portion of said transmitter signal into two paths, and (b) introducing a relative phase difference between said two paths, the resultant signals being fed into said mixers to produce Doppler signals of equal frequency and amplitude but differing phase, said Doppler signals being presented to phase detecting means having two output channels controlled such that only one such channel may produce an output pulse at any instant, the channel outputs representing incremental and decremental movement of said objects in said area respectively.