Piezoelectric sensor Q-loss compensation
by Paul, David W.; Beeler, Theodore L.
Describes a Q-loss compensation apparatus for a piezoelectric sensor such as a quartz crystal microbalance or other resonant vibratory device wherein the vibration amplitude of the device is controlled by negative feedback in a manner to obviate the effect of energy loss associated with viscous damping of a large liquid drop on the quartz crystal face serving as an environment for an experiment to measure mass deposited on the crystal face. The apparatus includes an oscillator circuit for the vibratory device in which two generally similar variable gain amplifiers provide the regenerative feedback for maintaining oscillation. The negative feedback amplitude control circuit serves to maintain constant output from the variable gain amplifier following the quartz crystal in the oscillator loop, and it thus maintains at a near constant value the product of the crystal vibration amplitude and the square root of the total gain in the oscillator loop. This results in stable oscillation of the quartz crystal with little influence from changing conditions such as temperature, viscosity of the fluid, evaporation of the fluid, etc., at the same time producing a linear frequency change dependent on the quantity of mass deposited on the crystal face from the liquid environment. Frequency change is measured in a conventional manner with accuracy of about one part per ten million, thereby permitting determination of minute mass amounts on the order of one nanogram.
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