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Anti-theft system for a motor vehicle

  • US 5,905,444 A
  • Filed: 11/12/1996
  • Issued: 05/18/1999
  • Est. Priority Date: 11/09/1995
  • Status: Expired due to Term
First Claim
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1. An anti-theft system for a motor vehicle, comprising:

  • a transceiver to be disposed in a motor vehicle, said transceiver having an antenna oscillating circuit with components including an antenna, said antenna oscillating circuit having a variable transceiver resonant frequency determined by said components for selection to a first resonant frequency and a second resonant frequency;

    an oscillator imposing an oscillation on said antenna oscillating circuit for transmitting energy signals;

    a portable transponder for transmitting code information to said transceiver, said portable transponder including a transponder oscillating circuit having a variable transponder resonant frequency for selection to a high resonant frequency and a low resonant frequency as a function of the code information, said portable transponder including an energy reservoir for activating said transponder oscillating circuit, said energy reservoir charged by the energy signals;

    an evaluation unit receiving the oscillation generated by said transponder and received by said transceiver, said evaluation unit detecting the received oscillation and outputting the code information;

    a comparator demodulating and comparing the code information from said evaluation unit with desired code information and issuing a release signal for a security unit if the code information and the desired code information correspond; and

    the transceiver resonant frequency of said antenna oscillating circuit selected to the first resonant frequency for transmitting the energy signals and to the second resonant frequency for receiving the code information from said transponder, the second resonant frequency having a value between the high and low resonant frequencies of said transponder oscillating circuit.

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