Theft detection system.

摘要

A theft detection system comprising an interrogation unit and a detection unit for generating an electromagnetic interrogation field with a predetermined frequency and detecting the presence of electronic labels belonging to the system in the interrogation field. The electronic labels generate a reply signal in the interrogation field by means of a resonant circuit which comprises at least a coil and a capacitor and which also functions as receiving circuit for the interrogation field. The labels comprise means which provide a frequency-switched signal with the aid of which the signal generated by the interrogation field in the resonance circuit is modulated via a first switching element in such a manner that at least two high-frequency signal components arise which are located within the resonance curve of the resonance circuit, in such a manner that these signal components are strongly present in the secondary magnetic field formed by the coil. <IMAGE>