Mode interference study of bulk acoustic wave liquid sensors

The impedance of thickness shear mode (TSM) quartz resonators, which are very sensitive to the liquid electrical property changes, has been investigated. Resonators with a ring shape electrode were exposed to a NaCl solution of varying concentrations and it was found that mode interference between the primary and spurious mode occurred. The results show that as the inner diameter of the ring electrode increases, a spurious mode appears with increasing mode strength and the frequency spacing between the spurious mode and the primary TSM decreases. For a resonator with the same sensing electrode, the strength and center frequency of the spurious mode change as the NaCl concentration varies. Theoretical analysis of the mode interference based on a liquid lossy electrode is presented.