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Electronic security system with noise rejection

  • US 4,429,302 A
  • Filed: 10/08/1981
  • Issued: 01/31/1984
  • Est. Priority Date: 10/08/1981
  • Status: Expired due to Fees
First Claim
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1. An electronic security system comprising:

  • transmitter means for producing an electromagnetic field at a frequency repetitively swept through a predetermined range at a predetermined sweep frequency;

    a resonant tag circuit having a resonant frequency within the sweep range;

    receiver means for producing an output signal in response to a tag signal produced by said tag circuit each time the field produced by said transmitter means passes through said resonant frequency;

    circuit means for producing a pulse in response to selected output signals of said receiver means, said selected output signals being an initial output signal and each successive output signal which occurs at an interval from the previous selected output signal which is at least as great as the period of said sweep frequency; and

    means for comparing the frequency of pulses produced by said circuit means with said sweep frequency and producing an alarm signal when said pulse frequency and said sweep frequency are substantially equal.

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