Detector for ultrasonic burglar alarms - uses reflected signal amplifier limited and mixed with transmitter signal and difference frequency then used for alarm tripping

摘要

The detector has an amplifier and a demodulator for the received signal, and retransmits it to an output connected to an alarm tripping device. The detector has also an amplitude limiter (10) for the received signal (7) which cuts off the whole amplitude modulation of the received signal. The signal is much amplified before limiting, so that the limited signal amplitude is sufficient for its retransmission. The limited signal (fr) and a reference signal (fs) derived from the transmitter (1) are applied to a product detector which delivers a frequency difference signal (tD) to the output terminal for transmission to the alarm tripping device.