Abstract: An anti-theft system (1) for a motor vehicle detects movement in the motor vehicle, while at the same time avoiding false alarms. A movement detector (2) comprises a transmitter circuit (3) which transmits a microwave signal in bursts of around six nanoseconds duration and a receiver circuit (4) which receives the transmitted signals and reflected signals. The transmitted and reflected signals are mixed, and envelope detected in an envelope detector (22). The signals from the envelope detector (22) are passed through three filters, namely, a low pass filter (25) which passes low frequency signals in the range of 1 Hz to 40 Hz, a high pass filter (26) which passes high frequency signals of greater than 100 Hz and a band pass filter 15 (27) which passes intermediate frequency signals in the range of 40 Hz to 80 Hz. The signals from the filters (25,26,27) are analysed in a microprocessor (28).

Abstract: A method and apparatus for analysing a signal from a movement detector for determining if movement has been detected in an area under surveillance, and an anti-theft system” An anti-theft system (1) for a motor vehicle detects movement in the motor vehicle, while at the same time avoiding false alarms. A movement detector (2) comprises a transmitter circuit (3) which transmits a microwave signal in bursts of around six nanoseconds duration and a receiver circuit (4) which receives the transmitted signals and reflected signals. The transmitted and reflected signals are mixed, and envelope detected in an envelope detector (22). The signals from the envelope detector (22) are passed through three filters, namely, a low pass filter (25) which passes low frequency signals in the range of 1 Hz to 40 Hz, a high pass filter (26) which passes high frequency signals of greater than 100 Hz and a band pass filter 15 (27) which passes intermediate frequency signals in the range of 40 Hz to 80 Hz.