GB1474928A - Intruder detection device - Google Patents

Abstract

1474928 Intruder alarms SHORROCK DEVELOPMENTS Ltd 8 Jan 1976 [15 Nov 1974] 49433/74 Heading G4N A receiver for an intruder detection system of the type in which a modulated radio frequency signal is applied to the receiver comprises an aerial RA, a radio frequency detector RFD, signal processing means for providing a D.C. signal the level of which is proportional to the amplitude of the received signal, means for sampling said D.C. signal level at predetermined time intervals, and means for detecting a change in said D.C. signal level, as compared with the level detected in the immediately preceding sample, greater than a predetermined amount, and, in the event of such a change, initiating an alarm. As shown in Fig. 1 the aerial receives an audio frequency modulated microwave signal and the audio frequency modulation is extracted and applied to circuitry comprising a pre-amplifier, a logarithmic amplifier and filter, a rectifier and a low pass filter from which the D.C. signal is obtained. The D.C. signal is applied to an R.C. circuit having a switch arranged between the resistor R and the capacitor C, the switch being turned on for brief intervals of time only by a pulse generator. The D.C. signal is applied via resistor R to one input of a comparator, the other input being coupled to the junction between the switch and the capacitor C. In the static state both inputs to the comparator will be equal. Should any change in D.C. signal level occur, this will immediately be seen by the comparator which may or may not give an output according to the extent of the signal change. If the amplitude change is sufficient to cause a comparator output, this output deenergizes an alarm relay. However, if the amplitude change is insufficient to cause a comparator output, e.g. due to ambient condition changes, on closure of the switch the capacitor C will endeavour to recharge to the altered input signal level. This change reduces the differential input to the comparator and after sufficient pulses from the pulse generator, the input levels to both comparator inputs will equate. As shown in Fig. 2, the D.C. signal is applied to one input of a first comparator and to an R.C. circuit XC1, the junction between X and C1 being connected to one input of a second comparator, the other inputs of both comparators being connected to the output of a voltage follower which follows the charge on a capacitor C2. If the D.C. level change is sufficient, comparator 1 produces an output to de-energize an alarm relay. If the change is small, then the charge of capacitor Cl will alter. When the pulse generator operates to close the switch, the capacitor C2 will attain a state of charge corresponding to that of C1 which in turn, via the voltage follower, will reduce the voltage differential or comparator 1 and change the charge on C1. This sequence repeats until the imbalance is effectively zero.