Circuit arrangement for ultrasonic burglar security systems operating in accordance with the Doppler principle

US4198618A · kind A · utility

Assignee

• Siemens Aktiengesellschaft · DE

Inventor

• Peter Kleinschmidt · Munich, DE

Key dates

• Filing date: Apr 13, 1978
• Grant date: Apr 15, 1980
• Expiry date: Apr 13, 1998

Classification

• Technology area (CPC G): Physics
• CPC primary: G01S15/58
• WIPO field: Measurement
• WIPO sector: Instruments

Abstract

A circuit arrangement for an ultrasonic burglar security system operates in accordance with the Doppler principle and comprises a transmitting transducer which is fed from an oscillator with an ac voltage and continuously transmits ultrasonic radiation during operation, a receiving transducer and a receiving circuit connected thereto and having an analysis circuit and a phase detector which continuously compares the signal received by the receiving transducer with the oscillator ac voltage to form a detection signal which is dependent upon the mutual phase angle. The detection signal is fed to the analysis circuit which responds to the ac component thereof. The circuit arrangement includes a phase-locked loop (PLL) which includes a further detector, a low-pass filter following the phase detector and an oscillator. The oscillator has its frequency controlled at a control input and may be formed as a voltage controlled oscillator (VCO). The cut-off frequency of the low-pass filter is designed to be such that only signals which can pass fundamentally without reduction are those whose frequency is lower than a critical frequency F.sub.c =c/E, where c is the speed of sound of the ultras…