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Magnetic article surveillance system and method

  • US 4,791,412 A
  • Filed: 01/28/1988
  • Issued: 12/13/1988
  • Est. Priority Date: 01/28/1988
  • Status: Expired due to Fees
First Claim
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1. An article surveillance system, comprising:

  • means for generating and transmitting reference signals at a fundamental frequency in a detection zone;

    a plurality of coded markers, each marker having means for generating harmonic signals of respective and predetermined phase shifts responsive to the reference signals;

    means for receiving the phase shifted harmonic signals generated by coded markers in the detection zone;

    means for determining the relative phase shift between at least two of the harmonic marker signals; and

    ,means for generating a control signal responsive to identification of a valid code by the determining means.

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