| 摘要 |
A system for detecting presence of and for communicating with an electronic coded label (2). Coded label (2) includes a presence signalling antenna (9, 22), a label circuit (10-21) and a data communications antenna (9, 25). The system includes presence detection means (3, 4) for creating a presence detection electromagnetic field and data communications means (5, 6) 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 (2) 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 (2) is exposed is at a second power level, said second mode being that of a non-linear circuit with behaviour dependent upon time.
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