An electronic oscillator with automatic gain control: EQCM applications

An electronic oscillator for use with thickness-shear-mode acoustic wave sensors is described. The oscillator monitors not only the resonance frequency of the sensor, but also provides an automatic gain control signal that monitors the energy dissipated by the sensor. Electrochemical quartz-crystal microbalance (EQCM) experiments have been performed in which one of the sensor's electrodes serves as the working electrode in an electrochemical cell. Electrochemical deposition of Tl+ on the sensor's surface results in a frequency shift that is attributed to mass loading, while a negligible amount of energy loss from the sensor is observed. Results from the cyclic voltammetry of the K3Fe(CN)(6)/K4Fe(CN)(6) couple, which does not alter the mass on the sensor, indicate that the energy loss from the sensor can be directly monitored by the automatic gain control voltage and is related to the microviscosity of the solution adjacent to the sensor. The addition of automatic gain control to an oscillator circuit provides energy-loss information that is usually only given by impedance analyzers. The circuit is applicable to situations where remote sensing or cost are important factors.