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