Function tester for ultrasonic intruder alarms - produces simulated Doppler frequency shift and RC network to render phase-locked loop unstable

摘要

The operational check system for ultrasonic intruder alarms consists of a variable-frequency oscillator which normally drives an ultrasonic transmitter at a given, fixed frequency. This frequency is also fed to a frequency comparator on the output of the Doppler receiver. The function tester, when operated, causes the oscillator to deliver a frequency to the comparator which varies from the normal reference frequency used to drive the transmitter. The comparator reacts to an apparant frequency difference in the same way as it would if a Doppler frequency shift had occurred and triggers the alarm. The main alarm cuicuit, however, is not activated but a secondary indicating is energised to show that the equipment is operating correctly.