Importance

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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). Slow movements in the vehicle are determined from the signals from the low pass filter (25), while fast movements in the vehicle are determined from the signal from the band pass filter (27). Interference signals, such as mobile phone signals are determined and identified from the signal from the high pass filter (26) and the low pass filter (25). In the event of interference being caused by a mobile phone, a low frequency energy alarm threshold value (40) of the energy in the signal from the low pass filter (25), at which an alarm condition would be determined as having been detected, is increased to an intermediate threshold value (40''') which is above the 25 energy value of the low frequency component of the mobile phone interference, thereby preventing a false alarm due to the presence of mobile phone interference. However, the intermediate value (40''') of the low frequency energy alarm threshold is such as to permit detection for slow movements in the vehicle during the presence of the mobile phone interference.

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First Claim