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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, R2)
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. AD835 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