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Shoplifting detection system of the transmission type

  • US 5,051,727 A
  • Filed: 03/16/1990
  • Issued: 09/24/1991
  • Est. Priority Date: 03/17/1989
  • Status: Expired due to Term
First Claim
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1. A shoplifting detection system of the transmission type, suitable in particular for the use of high frequency interrogating signals, in which system an electronic label provided with a resonance circuit can effect an electromagnetic coupling between at least two antenna coils, at least one of which is a transmitting antenna coil which, in operation, is fed with an A.C interrogating signal from a transmitter circuit, and at least one other of which is a receiving antenna coil which supplies a received signal to a receiver circuit, characterized in that said receiver circuit comprises a phase-sensitive synchronous detector to which the received signal is supplied, and to which a reference signal is supplied of such a phase that a component in the received signal, caused by an electronic label, provides a maximum output signal of said synchronous detector, and a signal phase-shifted through 90°

  • relatively to said component provides a minimum output signal of said synchronous detector.

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