Impedance analysis for lateral field excited acoustic wave sensors

Lateral field excitation (LFE) acoustic wave devices have been recently investigated for liquid sensing applications. In the present study, the impedance characteristic of the LFE devices working in three operational modes was analyzed. The impedance of the pure-LFE coupling device at resonance can be equivalent to that of a Butterworth–Van Dyke circuit (BVD). It is found that the (yxl)-16.51° LiTaO3 LFE device has a much smaller resonance impedance in air due to the large LFE coupling factor and high dielectric constant. For the AT-cut quartz LFE device, there is no significant resonance peak observed in air due to the large resonance impedance. An equivalent circuit was present and used to analyze the impedance of the pseudo-LFE coupling device operating in liquids. It was well explained why the common used AT-cut LFE device is super sensitive to liquid electric property changes compared to a thickness-field-excited (LFE) device. The impedance changes in a single LFE-configuration device, working in a quasi-LFE mode, were also analyzed. And, the experiment results show that the impedance change at the resonance excited by the TFE is much larger than by the LFE in different concentrations of NaCl and 2-propanol water solutions.