Modeling of double SAW resonator remote sensor

The usage of SAW resonators as remote sensors is well known. Double resonator sensors for reducing disturbing influences from the radio channel as well as the separation of the sensor element from the SAW device into the matching network are already presented in M. Binhack et al. (2000). Here the change of the difference frequency is the sensor signal. Corrected matching was used to integrate the sensor function in the matching network as stated in S. Klett et al. (2002), e.g. as a capacitive sensor. For passive wireless sensors it is very important to optimize the sensitivity and the resolution while keeping the energy conditions sufficient to be able to interrogate at longer distances. The system design for e.g. strain gauge sensors without sensitivity to the temperature demands to special properties of the used resonators. This paper shows a systematic calculation of the resonator behavior by using the PEC (parameters of equivalent circuit). The influence of the matching conditions and the connected SAW devices to different frequency detuning are calculated and compared to specified pairs of resonators. To extend the difference frequency shift by the sensor variation, resonators with obvious unequal Q factor have to be used. Furthermore a certain derivation of the single resonator impedance results in different frequency dependencies from the detuning capacitor. This can be influenced during the design and fabrication process of the SAW devices. Typical values like the Q factor, the serial and parallel resonance frequency or the impedance of the SAW devices were taken to build the model of the double SAW resonator sensor. Finally the model is used to draw conclusions to the design parameters like reflection coefficient or aperture of the resonators to optimize the sensor behavior.