Design of a QCM Resonance Measurement System with

Design of a QCM Resonance Measurement System with Automatic Parasitic Impedance Cancellation Electrical Engineering Department Presenter: Greg Mitchell Block Diagram of System Circuit Metal Box Used for Shielding Outside Interference Project Setup & Testing Advisor: Prof. Mustafa Guvench The goal of this project is to design a circuit that will measure resonance characteristics of piezoelectric quartz crystal; in doing so, canceling the effects of parasitic capacitance and conductance. Thickness Shear Mode (TSM) quartz crystals are used as Quartz Crystal Microbalance (QCM) sensors where the resonance frequency of the crystal changes as additional material is deposited on it. However, when the crystal is immersed in a liquid, its resonance characteristics are degraded by viscous loading of the medium and electrical conductance and parasitic capacitance, which render the QCM measurement inaccurate and unreliable. A TSM quartz crystal resonator (which operates at 4 -6 MHz) can be viewed as the following lumped circuit model which shows the effect of loading due to operation in a viscous and lossy dielectric medium with parasitic capacitance and conductance. (Co, Go, R 2) The circuit/system designed, through generation of equal but opposite currents forced to pass through the parasitic branches (ICo and IGo), will cancel out their effect on the output so that the true resonance frequency characteristics can be displayed. The resonance frequency generated by an oscillator circuit will thus be free from those effects. The circuit is designed to generate the cancellation effect in an automated way that adjusts itself to changes in the liquid medium. For this correction scheme, a 4 Mhz narrow band test signal was employed. AD 835 Voltage Controlled Amplifier (VCA) is used for self adjusting feedback. The two synchronous peak detectors shown in the block diagram, one for the detection of the capacitive, the other for the detection of the conductive current at the output , are implemented with analog multipliers. The measurement system will be completed in May 2008. However, test results given on the right have already proven the concept with a complete cancellation of the conductive and capacitive loading demonstrated in an automated manner. The project was supported with grants from: NASA and Maine Space Grant Consortium Input, Output, and VCA Voltage (Auto-Feedback) of the Conductance Loop when feedback is NOT applied Input, Output, and VCA Voltage of the Conductance Loop when Auto Cancellation Feedback voltage is connected