Detecting and evaluating the signals of wirelessly interrogational passive SAW resonator sensors

The wireless sensing signal of a passive surface acoustic wave (SAW) resonator sensor is the response of the SAW resonator in a passive circuit to wireless radio frequency interrogation. The response is produced only in the case that the interrogation covers the operational frequency band of the resonator. The wireless response is transient and can only be detectable in a proximity after switching off the interrogation. Due to the fact that, while used as a sensor, the resonant frequency of the resonator is related to and varying with the measurand, the interrogation to a passive SAW resonator sensor has to trace and follow the correspondent variation of the frequency band of the device. The energy evaluation of the response is applied to detect the availability of the sensing response and is used as a feedback argument to roughly localize the operational frequency range of the sensor. A modified frequency estimation is employed to estimate the sensing characteristic frequency in the transient wireless sensing signal with a low signal-to-noise ratio. The estimation is used to further adjust the interrogation frequency to follow the frequency variation of the sensor until the response becomes optimal. The evaluation of signal energy along with the statistical quantity of frequency estimation gives a reference for the confidence of the estimated frequency.