Presence and data labels

Abstract

A system for detecting presence of and for communicating with an electronic coded label. The coded label includes a presence signaling antenna, a label circuit and a data communications antenna. The system includes presence detection means for creating a presence detection electromagnetic field and data communications means for creating an interrogation electromagnetic field. The label circuit is adapted to operate in a first mode when the electromagnetic field to which the label is exposed is at a first power level, said first mode being that of a linear circuit with an impedance dependent upon frequency. The label circuit is adapted to operate in a second mode when the electromagnetic field to which the label is exposed is at a second power level, said second mode being that of a non-linear circuit with behavior dependent upon time.