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

• US 4,198,618 A
• Filed: 04/13/1978
• Issued: 04/15/1980
• Est. Priority Date: 05/20/1977
• Status: Expired due to Term

• Current Assignee: Siemens AG
• Original Assignee: Siemens AG
• Inventors: Kleinschmidt, Peter
• Primary Examiner(s): Tubbesing, T. H.
• Application Number:
  US05/895,949
• Time in Patent Office: 733 Days
• Field of Search: 340/3 D, 340/1 R, 340/554, 340/560, 343/5 PD
• US Class Current: 367/94
• CPC Class Codes: G01S 15/58 Velocity or trajectory determination systems; Sense-of-movement determination systems velocity measurement in imaging systems G01S15/8979

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_c =c/E, where c is the speed of sound of the ultrasonic radiation in the area and E is the path of the ultrasonic radiation from the transmitting transducer to a reflective object located in the predetermined maximum range and back to the receiving transducer.

Claims

1. In a circuit arrangement for an ultrasonic burglar alarm system of the type which operates in accordance with the Doppler principle and which comprises a transmitting transducer which is fed an a.c. voltage from an oscillator for continuously transmitting ultrasonic radiation, a receiving transducer for producing a received signal in response to reflected radiation and a receiving circuit including an analysis circuit, a low pass filter and a phase detector which compares the received signal with the a.c. oscillator voltage and forms a detection signal which is dependent on the mutual phase angle and which is fed to the analysis circuit, the improvement therein comprising:

a phase locked loop including a further detector connected to the oscillator and connected to receive the received signal from the receiving transducer and a further low-pass filter connected to said further detector,

and wherein the oscillator is a voltage-controlled oscillator connected to and having its frequency controlled by the output of said further low-pass filter, said further low-pass filter including means defining a cut-off frequency such that the only signals which pass, fundamentally without reduction, are those signals having frequencies less than F_c =c/E, where c is the speed of the ultrasonic radiation and E is the distance from the transmitting transducer to a reflective object located in the predetermined maximum range of the monitored area and on further to the receiving transducer.

2. The improved circuit arrangement of claim 1, further comprising:

an amplifier connected between the receiving transducer and said further phase detector.

3. The improved circuit arrangement of claim 1, further comprising:

an amplifier connected to the receiving transducer, on the one hand, and, on the other hand, to both of the phase detectors.