Progress in modeling of sensor function for matched SAW resonators

SAW resonators are particularly suitable for passive wireless sensing because of the high Q factor. The returned sensor signal consists of a frequency shift and a change in the amplitude. To ensure a good performance, such passive systems have to be carefully designed for energy efficiency and for the demands of the interrogation regime. For well designed sensors, the parameters of equivalent circuitry (PEC) of the resonator have to be estimated, in order to simulate the resonator in combination with the matching network. Hereby, the matching network does not only match the resonator - it transforms the whole resonance behavior and influences it. So a sensor is constructed by the implementation of a sensing element (e.g. variable capacitor) into the matching network. Based on measured data of different resonator types, the PEC have been precisely calculated by an improved method. The PEC model is extended by a matching network to simulate the interaction between the two parts, which leads to a modified resonance behavior. Consequently, different matching circuits have been calculated and measured for the applicability of pulling the resonance frequency inside a reasonable region of mismatching. For practical purposes, an approximation of the sensing capacity and the pulling sensitivity is possible. This paper discusses the simulations, which have been verified by experiments.