Quartz Crystal Microbalance Measurement Setup
There are basically three standard approaches of QCM measurement : excitation impulse, network analysis and oscillators. The first approach considers that the quartz crystal after excitation with an ideal impulse will carry out damped oscillations which are determined by crystal properties. Determination of the sensor transfer function, unperturbed by the interface electronics, is a major advantage of impulse excitation as well direct access to resonant frequency and acoustic energy dissipation. But in practice, this approach can be used only in limited range for precise frequency and damping measurement.
Network analysis determines the frequency shift and the shift in bandwidth (proportional to dissipation) by measuring the complex admittance around a resonance frequency and fitting resonance curves to the admittance spectra. Due to passive operation of the quartz, it is possible to minimize parasitic influences respectively to exclude their effects by calibration, which is a remarkable advantage of this method. The essential drawback of network analysis is its high costs and large dimensions of the equipment which is to satisfy the requirements of universal application.
quartz crystal is a part of wideband oscillator circuit whose frequency is controlled by the crystal properties and is measured by a frequency counter. The application of oscillator circuits as sensor interfaces for QCM is the most common method, since quartz as a resonant element can be excited to stable oscillation by simply structured circuits, which deliver a frequency analog output signal for easy processing in digital systems. Some researchers use a reference oscillator to avoid ambient effects. In that case, a counter measures the frequency difference between the output signal of the sensor based oscillator, and the output signal of the reference quartz based oscillator. In order to study fluctuation mechanisms in QCM, we modified active method to measure instantaneous frequency of QCM.