Portable SAW Impedance Sensor Using a 1-Port Resonator Approach

All actually used SAW sensors have in common that they analyze the transmission characteristics of propagating SAWs. Here, a portable device using the SAW-based impedance sensor type based on one interdigital transducer (IDT) simultaneously as SAW generator and sensor element (1-port approach) is presented. The input port reflection coefficient S11 is measured at the IDT instead of the commonly used S21 transmission forward gain parameter. As a novelty, the so far required expensive vector network analyzer (VNA) is replaced by a hand-held device to measure the impedance spectrum of the SAW sensor by RF-gain-phase meters. The pivotal aspect of the portable system is the transfer of the sophisticated high frequency approach into a quasi-static one. This enables the use of simple lumped electronics without the need of impedance matching circuits. Proof-of-concept was carried out by measuring conductivities of phosphate-buffered solutions (PBS) and viscosities of glycerin. Sensitivities for conductivity of 64 Hz/S cm−1 and for viscosity of 122 mΩ/mPas were obtained, respectively.