A 0-Phase Circuit for QCM-Based Measurements in Highly Viscous Liquid Environments

摘要

Currently, the series resonant frequency f_(s) and the motional resistance Rm of liquid loaded quartz crystal microbalance (QCM) sensors are extracted either directly, through network analyzer (NWA) impedance measurements, or from QCM-stabilized oscillator circuits. Both methods have serious drawbacks that may affect measurement accuracy, especially if the sensor is operated under highly viscous load conditions and Rm exceeds 1 k(OMEGA). This paper presents a simple passive low-loss impedance transformation LC network which greatly reduces additional electrical loading of the QCM by the measurement system or sensor electronics and maintains a symmetric resonance and a steep 0-phase crossing at f_(s), even if Rm increases by several orders of magnitude as a result of liquid loading. A simple S21 transmission measurement allows direct f_(s) reading at the 0-phase frequency, while Rm is obtained from the circuit loss at f_(s). Circuit operation was verified at 9 MHz by QCM measurements in a liquid with known density and viscosity. The agreement between predicted and experimental data, which was obtained by a temperature-controlled measurement, was within 1percent, even in very high viscosity ranges in which Rm exceeds 10 k(OMEGA).