System including tuned AC magnetic field transmit antenna and untuned AC magnetic field receive antenna

摘要

An inductive magnetic field article surveillance system includes a generator for a first inductive magnetic field having a predetermined frequency. The articles carry a structure which responds to the first magnetic field to derive a second inductive magnetic field having a predetermined frequency; the second field occurs immediately after expiration of the first magnetic field. A receiver for the second magnetic field responds to the predetermined frequency of the second magnetic field to indicate the presence of an article between the generator and receiver, if the frequency subsists for at least a predetermined time interval. The generator for the first magnetic field includes a tuned transmitting coil arrangement, while the receiver includes an untuned receiver coil arrangement. The second coil arrangement has a low Q and a resonant frequency consid- ,erably removed from the predetermined frequency of the second magnetic field so that impulse magnetic noise coupled to the receiver coil is not translated by the receiver coil into components of a signal at the frequency of the second magnetic field. Untuned, narrow band processing circuitry responds to a signal transduced by the receiver coil. The narrow band width of the processing circuitry is achieved by a synchronous demodulator driving an integrator.