ELECTRONIC SURVEILLANCE SYSTEM

摘要

An electronic surveillance system in which a passive label is attached to goods to be placed under surveillance and the label is interrogated by electromagnetic signals from a transmitter, the label acting to transmit a reply signal to a receiver which gives a characteristic response in the presence of a label, the transmitter being arranged to transmit signals, preferably two, at widely different frequencies and the label acts to mix these frequencies to produce a reply signal which is distinct from the original transmitted signal. The receiver is provided with a signal processing system which analyses and compares characteristics of the transmitted signal and the reply signal and produces different responses in the presence or absence of a label.