ELECTROMAGNETIC SURVEILLANCE SYSTEM WITH IMPROVED SIGNAL PROCESSING

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. An alarm condition is signaled whenever the at least one even order harmonic endures for a predetermined time period, and the at least one odd harmonic, if present, is in proper phase with the transmitted signal. The alarm is inhibited whenever the phase angle of the at least one odd harmonic is improper, or broad band noise with random phase angles is detected and measured. Sensitivity to valid alarm conditions is substantially increased and occurrence of false alarms is substantially eliminated.