Effects of the Set Phase Position on the Response of a SAW Resonator Oscillator Based Chemical Sensor

Theoretical calculation results are described to illustrate the experimental results of the dependencies of surface acoustic wave resonator sensor´s sensitivity on the set phase position within an oscillator circuit. The experiment highlights that the dependencies are insignificant for thin coatings but quite significant for thick coatings. The calculations were performed using an equivalent circuit model for polymer loading that incorporates a sensitivity factor or phase dependent factor. The datasheet parameters of the device used in the experiment are utilized to simulate the electrical behavior of the sensor. Based on a ratio R of the coating film strain magnitudes induced by cross-film to in-plane gradients, the thickness regime of operations are defined such that if R <;<; 1, the film is acoustically thin and if R ≥ 1, the film is thick. It is shown that, with film thickness as a parameter, while the sensitivity factor curves are unaffected for R in the range from 0.46 to 0.91, the curves exhibit a prominent spike shape with varying magnitude around resonance peak for R in range from 1.37 to 3.65. With shear modulus as parameter, varying R from 0.65 to 8.23 also results in a spike shape sensitivity factor curves with its peak magnitude varies from 1.026 to 1.92. These results show that in the thick regimes, depending on the magnitude of the sensitivity factor at a point of circuit oscillation, the sensitivity of the sensor may not only increase or decrease but also change its direction. The calculation results thus show consistency with the experimental result.