Pulling of SAW resonators for wireless sensor application

Abstract

SAW resonators are particularly suitable for passive wireless sensing because of the high Q factor (storage of rf energy) and the possibility to change their resonance be- haviour influenced by an external measurand (frequency pulling). The sensor is build not only by a physical af- fecting of the acoustic wave but also by an electrical interaction outside the resonator (e.g. capacitive or inductive). The sensor signal is determined by the ex- ternal circuit elements and manifests in a fre- quency and amplitude shift. The pulling frequency and the optimum pulling span, also the transmitted power are estimated by the external reactive circuit ele- ments, as well as by the SAW-Resonator characteristic. To obtain the maximum power transmis- sion, a matching between the sensor element and the antenna for wireless interrogation is necessary; simultaneously, one element of the matching network is the sensing element that pulls the frequency. For well-designed sensors, the parameters of equivalent circuitry of the resonator have to be estimated with a high accuracy, in order to simulate the resonator pulling in combination with different matching networks. Thus, from all the L-, Pi- and T-sections, the eligible con- figuration were selected and tested in order to ensure the maximum power transmission in a wide pulling range. This paper presents simulation and ex- perimental results for designing proper sensor systems.