Evaluating the Feasibility of a Highly Time-Resolved, Multi-Frequency Quartz Crystal Microbalance

Sammanfattning

Quartz Crystal Microbalance (QCM) is a highly sensitive mass-sensing technique that exploits the piezoelectric effect of a thin quartz crystal: when an alternating current drives the crystal at its resonance frequency, any mass added to its surface induces a measurable shift in that frequency. Because this frequency change is directly proportional to mass loading at the nanogram scale, QCM has become a cornerstone technology for monitoring thin-film deposition, probing interfacial phenomena, and real-time biosensing applications. However, achieving both high mass-resolution and high time resolution in a compact, low-cost platform remains challenging. This thesis implements three measurement techniques on an FPGA-based platform: impedance analysis, ring-down, and multi-frequency lock-in amplification (MLA). A streaming architecture enables continuous, simultaneous capture and generation of arbitrary excitation and response waveforms, limited by the available memory on the FPGA board. Impedance analysis reliably tracked responses at both the fundamental frequency and the third overtone, with a ≈14 Hz root-mean-square (RMS) precision at one-second resolution. The ring-down and MLA methods generated and recorded the intended waveforms but did not yet yield meaningful data; the most likely cause appears to be limitations and design errors in the analogue circuitry connected to the quartz crystal, though other factors cannot be ruled out. Overall, these findings do not disprove the potential for multi-frequency QCM on inexpensive FPGA hardware but indicate that further investigation, particularly into analogue circuitry and FPGA design refinements, is needed before a multi-frequency system with high time resolution can be achieved.