Contributions of amplitude measurement in QCM sensors

The characteristics of the amplifier and the feedback loop of a quartz crystal series oscillating circuit are investigated. The fact that the change of the vibration amplitude of the quartz crystal is proportional to the change of its motional resistance is derived. The concept of a characteristic damping coefficient is introduced and the behavior of a quartz crystal vibrating in liquids is analyzed. The experiment shows that the effect of mass loading can be distinguished from that of the liquid damping of a quartz crystal microbalance (QCM) sensor in liquids by simultaneously measuring the amplitude and the frequency change.