ANTITHEFT PROTECTION METHOD AND DEVICE FOR REALISING SAID METHOD Russian patent published in 2012 - IPC G08B13/26 

FIELD: information technology.

SUBSTANCE: invention relates to antitheft alarms and specifically to alarms signalling theft of commodities from stores, said commodities having special labels. The antitheft method involves determining presence of radio frequency security labels in a control zone, having an antenna with inductance, which is the driving element of a self-excited sweep-frequency generator, wherein the reaction of the antenna to presence of the LC circuit of the label in the control zone is monitored, and alternating voltage in the antenna is generated by applying across the LC circuit, which is formed by the inductance of the antenna, the capacitance of capacitors and a controlled reactive component, e.g., the same capacitors which are in form of varicaps, and loosely coupled with an amplifier output, an amplitude-limited positive feedback voltage obtained from said circuit and applied across the input of said amplifier, and a control voltage, which sets frequency sweeping parameters, is applied across the varicaps and change in amplitude of oscillations of the self-excited generator is monitored. In the antitheft device, the self-excited generator further includes a control voltage generator which controls the varicaps, a current generator and a feedback limiter amplifier which is connected by the output through the current generator to the parallel LC circuit. The device has a unit for monitoring perturbations, consisting of a rectifier of the generated oscillations, which is connected by the input to the LC circuit, a reference voltage source, a differential signal amplifier whose first input is connected to the output of the rectifier, and the second input to the output of the reference voltage source, and the output is connected to high-pass and low-pass filters which are installed at the output of the unit for monitoring perturbations, wherein the unit for processing perturbations and the signalling unit are connected to the control unit.

EFFECT: high sensitivity in the control zone.

14 cl, 11 dwg