Abstract: A surveillance system based upon generation and detection of phase shifted harmonic signals from markers on articles or persons, responsive to transmission of a reference signal at a fundamental frequency in a detection zone, may be adversely affected by the presence in the detection zone of large metal objects, such as shopping carts. Such objects can alter the phase of the harmonic signals from predetermined values. An alternate signal processing technique for such a surveillance system recognizes that immunity to such disturbances can be achieved by measuring the phase between two or more of the harmonic signals returned from the marker, because the effect of such large metal objects has been found to have the same effect on all of the harmonic signals. Correspondence to a predetermined phase difference is a reliable indicator of marker detection.

Abstract: Markers carried in an interrogation zone by articles to be monitored are subjected to an electromagnetic field varying in time at a fundamental frequency and respond by generating signals at harmonic frequencies of the fundamental frequency, characteristic of soft magnetic materials. The system comprises: a transmitter for the electromagnetic field signal; a receiver and processor for signals generated by the article markers; and, a clock for phase locking the transmitter, the receiver and processor with one another. Signal information corresponding to at least one even order harmonic of the fundamental frequency in the received signals is tested for, and the duration thereof is measured. Signal information corresponding to at least one odd order harmonic is tested for, and if present, the phase angle thereof is measured.

Abstract: A surveillance system is based upon generation and detection of phase shifted harmonic signals responsive to transmission of a reference signal at a fundamental frequency in a detection zone. Phase shifted harmonic signals may be generated by markers comprising a core of soft magnetic material and an electrically conductive material at least partly surrounding the core. Phase shifted harmonics are not accidentally generated by biassed ferrous alloys, the cause of most system failures.