Apparatus and method for interrogating a passive sensor

摘要: Apparatus for determining the resonant frequency of a passive sensor, example saw device, comprises: means (5, 7, 3) transmitting signals plurality frequencies to said sensor (2); (3, 7) receiving from simultaneously with signals; (7) comparing received transmitted identify characteristic change in relationship between and which occurs when is being driven at its frequency. Typically, as passes through phase and/or amplitude signal generated by will change. By identifying this point resonance can be detected. The moment comparison indicates that sensor.