| 摘要 |
A monitoring system, particularly suitable for monitoring conditions (e.g. pressure or temperature) of a vehicle tire comprises a passive sensor ( 102, 104 ), an antenna ( 106 ) associated with said passive sensor, and switching means ( 108 ) for controlling the connection between the passive sensor and the antenna such that, in a first state of said switching means, said antenna is prevented from transmitting a signal generated by the passive sensor, and, in a second state of said switching means, said antenna is permitted to transmit a signal generated by the passive sensor. The switching means is operated remotely by means of a control signal. The control signal may be derived from the sum or difference of RF signals (F<SUB>1 </SUB>and F<SUB>2</SUB>) applied to a filter ( 112 ) In alternative embodiments the control signal is the modulating frequency of an AM or FM modulated radio signal or a radio signal carrying a digital code.
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