GB1222871A - Improvements in or relating to detection systems - Google Patents

Abstract

1,222,871. Aerials; transistor amplifying circuits. ADVANCED DEVICES LABORATORY Inc. 23 April, 1968, No. 19022/68. Headings H3T and H4D. [Also in Division G4] A detection system for detecting the presence of an object within a volume under surveillance when said object belongs to a selected class, and is moving with a velocity within a selected range, comprising a Doppler system with an effective radiation field covering said volume. Frequency discrimination means eliminates from reflected signals any which correspond to velocities without said selected range, and amplitude clipping means limits the amplitude excursions of the remainder, which operate alarm circuit means. As described, a transmitter 10, Fig. 1, is connected to an aerial 14, which directs radio waves into a space under surveillance. Reflected waves from objects in the space are received by an aerial 20, which is connected to a narrowband detector and mixer 30. A portion of the output of the transmitter 10 is taken to the mixer 30, for heterodyning with the received energy, and, for testing the system, the said portion may be modulated by means of a motion simulator 24, operable by a push-button switch 24<SP>1</SP>. Output from the mixer 30 is taken via a distance range control bias network 32, comprising a manually adjustable potentiometer, to a band-pass amplifier 34, which is adapted to select Doppler signals due to a moving intruder 18 provided that they fall within a frequency range corresponding to a predetermined range of velocities. The bandpass amplifier 34 comprises a high-pass filter amplifier 36 with several stages, an impedance transforming transistor emitter follower amplifier 38, and a low-pass filter 40. In a system in which radio waves of frequency 10 Gc/s are transmitted, the range of the band-pass amplifier 34 may be from 10 c/s to 60 c/s. Output from the said amplifier 34 is taken via an amplitude clipper 42 to a detector 44, which charges an accumulator 46, comprising a capacitor connected across the load resistor of the detector 44 and feeding a transistor emitter follower amplifier. The accumulator 46 operates a limiter and alarm trigger 48, comprising a transistor type Schmitt trigger circuit, provided that the signal due to an intruder persists for at least a predetermined time and exceeds a predetermined amplitude, when an alarm is actuated. An automatic noise compensator 54 is connected between the output of the accumulator 46 and the emitter follower amplifier 38, and it provides adjustment of the quiescent level of the said accumulator in the presence of spurious noise and continuous signals of low amplitudes. A first supervisory alarm actuator 56 compares the level of the output of the accumulator 46 with that of the automatic noise compensator. If the two levels do not differ by more than a predetermined amount, so indicating a fault condition, the alarm trigger 48 is actuated. A second supervisory alarm actuator 58 monitors the level of the output from the mixer 30, and actuates the alarm trigger 48 if it falls below a predetermined value. Circuit details of the system are described with reference to Fig. 2 (not shown). Diodes are used in the mixer 30, the clipper 42, the detector 44, the automatic noise compensator 54, and the supervisory alarm actuators 56, 58. The stages of the high pass amplifier 36 comprise transistor emitter follower amplifiers coupled together by capacitors, the output of each stage being bypassed by a further capacitor. Transistors are also used in the supervisory alarm actuators 56, 58 and in the automatic noise compensator 54, the latter comprising a differential amplifier. The space may be monitored with acoustic waves instead of radio waves, suitable transducers replacing the transmitting and receiving aerials.