The active-bridge oscillator for use with liquid loaded QCM sensors

This unique bridge-type oscillator design permits liquid-loaded operation of QCM (quartz crystal microbalance) resonators with a simple, low parts count and inexpensive oscillator. This is achieved using "active-bridge" operation, which eliminates the need for the expensive and complex ALC (automatic level control) circuits typically used in high-performance oscillators designed for these applications. The Active-Bridge oscillator absorbs the wide dynamic range of resonator loss by varying the circuit operating Q as a function of the resonator loss. The output amplitude is relatively proportional to resonator loss. This design has demonstrated the ability to servo a loaded QCM resonator to within a few degrees of zero impedance-phase over a wide range of resonator loss, 300 Ω to approximately 3k Ω. This corresponds roughly to a liquid viscosity range of 1 to 120 cP, assuming a density close to one