A 0-phase circuit for QCM-based measurements in highly viscous liquid environments

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 kΩ. 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 1%, even in very high viscosity ranges in which Rm exceeds 10 kΩ.