Editing Humidity Detector Based on Quartz Crystal Oscillator (section)

=== Core Architecture: The Colpitts Crystal Oscillator ===
The central engineering challenge of this project lies in designing a robust electronic interface capable of driving the quartz crystal and precisely tracking its frequency changes. Rather than relying on commercial QCM equipment, this project focuses on the implementation of a custom-built Colpitts crystal oscillator <ref> A. Alassi, M. Benammar, and D. Brett, ``Quartz Crystal Microbalance Electronic Interfacing Systems: A Review,'' \textit{Sensors}, vol. 17, no. 12, 2799, 2017.</ref>. 

The Colpitts topology is highly regarded for its exceptional frequency stability and low phase noise, making it an ideal choice for sensor applications where minute frequency deviations must be detected. Our circuit utilizes a bipolar junction transistor (specifically the 2N2222 BJT) as the active gain element to sustain oscillation. A 6 MHz quartz crystal is integrated into the feedback loop. In this configuration, the crystal acts as a highly selective inductive element that dictates the oscillation frequency. 

A critical aspect of the circuit design is the careful selection of feedback capacitors and biasing resistors to ensure reliable startup and to suppress any high-frequency parasitic oscillations that the 2N2222 might otherwise introduce due to its wide bandwidth. By ensuring the circuit oscillates cleanly at the crystal's fundamental 6 MHz mode, the system can reliably translate the mass variations from the water glass coating into a stable, measurable output signal.