Patent GB-1509682-A (GB)
Title: Echo signal system for detecting objects
IPC: G01S13/56
Filed: 1975-03-21
Abstract: 1509682 CW intrustion detection alarm PITTWAY CORP 21 March 1975 [27 March 1974] 11997/75 Heading G1G [Also in Division H4] A quadrature detection radar system, preferably CW, for detecting moving objects, i.e. Doppler, comprises TX, RX and first and second quadrature mixing means for mixing signals corresponding to the transmitted and received signals to produce direction sensitive quadrature output signals in accordance with a beat frequency between the TX and RX signals. These quadrature output signals are fed to a phase rotation processor means which may comprise an electro-mechanical phase correlator arrangement or an electronic analogue thereof, and is responsive to the quadrature output signals for producing a bi-polar signal dependent on a quadrature combination thereof, the bi-polar signal is then integrated and thresholded to produce an output signal indicative of a detected object in clutter. The invention is applicable to intruder detection systems, and may use electromagnetic or sonic radiation. The general arrangement of the system Fig. 1 comprises TX oscillator 1 the signal from which is additionally fed to mixer 4 and via quadrature phase shifter 5 to mixer 3 to be mixed with the echo signal received at 2. The mixer outputs are selected by band pass filters 6 and 7 to output the Doppler difference frequency signals from moving targets of interest. These signals are fed to phase rotation processor 10 which produces a bi-polar output signal representing the net or resultant phase rotation forces produced by the concerted action of the quadrature detected mixer output signals. The bi-polar output signal 9 is applied to integrator 10, where clutter tends to cancel out due to its spectial energy distribution whereas the Doppler signal produces a signal component on one or the other side of the transmitted signal frequency component, depending on the sense of the target movement, and integrates cumulatively, and may exceed the threshold in circuit 12 to actuate alarm 13. In a first embodiment Fig. 2 the quadrature filter signals x and y are fed to a phase rotation processor comprising a fixed rotor two phase induction motor. The signals x and y are applied respectively to space quadrature windings 14, 15 and 16, 17 respectively. The resultant b of the flux components b x  and b y  produced by the respective winding currents produces a torque on rotor which results in an output signal in strain gauge 19 which contains the full signal output information. In a further embodiment a mathematical treatment is given to show that the torque m in rotor 18 may be expressed by the equation: is a constant. It is stated that a quantity proportional to the torque may be calculated electronically from this relationship. Each of the terms x@ and y@ contains the full information content of the signal and consequently one only need be calculated, although it is statistically less effective in accumulating Doppler signal energy. In addition, to improve the dynamic range of the system, the signals are calculated using only the first power of the amplitude component of the received signal. In the Fig. 7 embodiment of the phase rotation processor analogue circuit the signals x and y are processed to provide one of the above terms; x@, in which the amplitude component of the differentiated term is removed to produce a normalized derivative Py in which only the polarity information is present. The y signal is differentiated at 41 and applied to zero slicer which produces a polarity dependant signal Py' which is multiplied in four quadrant multiplier 39 with signal x to produce signal 42 which is fed to integrator 10. Fig. 9 shows x and y' signals at (a) and (b), and the Py' signal at c. The products signal, at (d) shows the actual output (continuous line) the rectified Doppler signal component (dotted line) and the clutter and noise components (shaded area), which cancel after integration leaving the true target signal. The system of Fig. 7 is realised in a circuit Fig. 10 (not shown) which may be discrete component or integrated and includes a long tailed pair circuit arranged to act as a four quadrant multiplier in one channel and processes the other channel as polarity information only.