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Range limited coherent frequency doppler surveillance system

  • US 4,565,996 A
  • Filed: 02/06/1984
  • Issued: 01/21/1986
  • Est. Priority Date: 02/06/1984
  • Status: Expired due to Fees
First Claim
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1. A range limited merchandise surveillance system for detecting a presence of a frequency modulating transponder within a surveillance zone comprising:

  • a transmitting system coupled to transmit a first signal at a first frequency, the transmitting system including a transmitter switch selectively enabling and disabling transmission of the first signal in response to a transmitter switching signal;

    a receiving system coupled to receive and detect a second signal at a second frequency different from the first frequency, the receiving system including a receiver switch selectively enabling and disabling detection of the second signal in response to a receiver switching signal; and

    a switching signal generator coupled to periodically generate the transmitter switching signal and the receiver switching signal with generation of the receiver switching signal being independent of actual operation of the transmitting system and with each period of the switching signals having a selected time interval between a disable-to-enable transition in the transmitter switching signal and an enable-to-disable transition in the receiver switching signal to selectively limit the effective range of the surveillance system.

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