Front-end Electronics for the Characterization of Quartz Tuning Forks in QEPAS Sensors

In the Quartz-Enhanced Photoacoustic Spectroscopy (QEPAS) technique, a laser source modulated with a sinusoidal signal interacts with the target gas, generating acoustic waves. These waves are detected by a resonant piezoelectric sensor, the quartz tuning fork (QTF), whose response amplitude is proportional to the gas concentration, which is the desired measurement. The key advantage of this technique lies in leveraging the resonance properties of the QTF, which distinguishes it from conventional photoacoustic methods. To fully exploit this benefit, the QTF must be excited at its resonance frequency, requiring a reliable characterization process to accurately identify this frequency. Typically, this is accomplished using electrical characterization of the QTF, i.e. by exciting the sensor with an external sinusoidal signal and finding the frequency corresponding to the peak of the QTF response. However, it has been observed that this process does not guarantee that the peak frequency obtained in this way corresponds to the optimal frequency for the QEPAS technique during the normal operation. In particular, the difference depends on the front-end used to read-out the QTF. This work aims to highlight the existence of this discrepancy, emphasizing the importance of selecting an appropriate preamplifier to achieve an optimized system. Furthermore, it seeks to identify the optimal circuit configuration that minimizes this mismatch, thereby ensuring full exploitation of the QTF's resonance properties for high-sensitivity gas detection.