Influence of phase position on the performance of chemical sensors based on SAW device oscillators

Sensor systems for the chemical analysis of organic gases based on mass-sensitive sensors, i.e,, bulk acoustic wave and surface acoustic wave (SAW) sensors, are becoming more and more an interesting alternative for highly sophisticated classic instrumentation, Although the use of oscillator-operated SAW resonators as chemical sensors is widely accepted to have the best sensing properties, some important parameters of influence are not sufficient resolved yet. In general, these are effects arising from the electronic circuitry, such as the phase situation on the chemical response of these sensors. We have found that they can be almost dramatic for several types of sensitive coatings and, thus, are not negligible, especially their influence on the sensitivity, We present some exemplary results with SAW resonators, working at 433.92 MHz, coated with different polymeric films such as poly(isobutylene), poly(epichlorohydrin), or poly(dimethylsiloxane) and sampled with toluene as analyte. Depending on the thickness and homogeneity of the polymer film; serious influences of the set phase positions on the quality of the chemical response were observed, such as curve shape and signal-to-noise ratio. Also, a simulation using an equivalent circuit model of the transducers including the polymer coating is used to obtain a deeper understanding of these phenomena.