A General-Purpose Online Measurement System for Resonant BAW Sensors

A novel general-purpose online measurement system for the resonant piezoelectric bulk-acoustic-wave sensors is presented. The system supports the various sensor types for the different physical measurands, such as the temperature, humidity, the viscosity, density, and speed of sound of liquids, and the film thickness. The measurement principle is based on the determination of the locus of the admittance curves of the respective sensors for one or several resonances (fundamental and/or overtones). The measurement results are calculated from the measured resonance frequencies or quality factors by applying the appropriate sensor function or calibration data for each sensor. The system covers sensors with resonance frequencies ranging from 30 kHz to 30 MHz. Special electronics have been developed to transform the sensor-admittance values (real and imaginary parts) to voltage signals that are acquired by the computer system. The disturbing effects caused by the capacitance of the sensor connecting cable are minimized by a special electronic circuitry. System performance was tested successfully in combination with two different types of temperature sensors (30-MHz quartz crystal and 6-MHz GaPO₄ crystal) and two different types of viscosity sensors (56-kHz torsional mode and 2.8-MHz thickness-shear-mode quartz crystals)