The outputted voltage of a rectifier 3 is detected by a voltage detector circuit 17. When the detected voltage value is low (received radio wave is weak, distance is long), a controller 20 turns off a switch 16 to connect only a capacitor 14 to the output of the rectifier 3, so that the capacitances of the capacitors are decreased and the build up of a power voltage is accelerated. On the other hand, when the detected voltage value is higher than a prescribed level (received radio wave is powerful, distance is short), the controller 20 turns on the switch 16 to connect capacitors 14, 15 in parallel with the output of the rectifier 3, so that the capacitances of the capacitors are increased and a ripple is fully removed. Thus, communications can be made while preventing a malfunction due to the ripple.

A transponder as well as a data communications system equipped with such a transponder are described, with which identification and/or individual addressing of the simultaneously present transponders is guaranteed in a simple manner even in the case of the simultaneous presence of a plurality of transponders in the field range of the reader.

A method of selecting groups of radio frequency RF transponders (tags) for communication between a base station and the tags. The tags are selected into groups according to a physical attribute ofthe signal sent by the tags to the base station, or according to the physical response of the tags to a physical attribute of the signal sent from the base station to the tags. Communication with the tags is thereby simplified, and the time taken to communicate with the first tag is markedly reduced.

A method of selecting groups of radio frequency RF transponders (tags) for communication between a base station and the tags. The tags are selected into groups according to a physical attribute of the signal sent by the tags to the base station, or according to the physical response of the tags to a physical attribute of the signal sent from the base station to the tags. Communication with the tags is thereby simplified, and the time taken to communicate with the first tag is markedly reduced.

The invention provides a reader-transponder system that prevents reading of undesired transponder signals. The reader generates a low-frequency signal that is used to supply power to the transponder. The same low-frequency signal also is used to generate a clock signal for the transponder. The reader varies the frequency of the low-frequency power signal according to a code that is unique to the reader. Because the transponder clock signal is derived from the encoded low-frequency power signal, it also varies in accordance with the reader's code. The coded transponder clock signal is contained in the modulation of the RF signal produced by the transponder. When the reader receives and decodes the RF signal, it also extracts the transponder clock signal from the received RF signal and compares variations in the extracted transponder clock signal to the code that was applied to the low-frequency power signal transmitted by the reader. If the pattern of variations in the transponder clock signal matches the reader's code, then the reader senses that the RF signal it has received is from a transponder being powered by that reader and no other.