quartz crystal

A quartz crystal is a thin wafer of silicon dioxide cut along precise crystallographic planes and mounted between two electrodes. When voltage is applied across the electrodes, the crystal physically vibrates at a specific frequency determined by its thickness and cut orientation. This mechanical vibration generates an electrical signal at that same frequency, making quartz crystals the workhorse frequency reference in almost every electronic device that needs to keep time or maintain spectral purity.

The crystal is typically cut at angles like AT (around 35 degrees 15 minutes from the z-axis) or SC (stress-compensated) to achieve temperature stability over a practical range. Most consumer-grade crystals work between 0 and 70 degrees Celsius with frequency drift of perhaps 20 to 100 parts per million across that span. Industrial and aerospace applications use temperature-compensated oscillators (TCXOs) or ovens to tighten this to single-digit ppm. Frequency ranges span from 32.768 kHz for real-time clock modules up to 100 MHz or higher for RF and digital circuits, though higher frequencies are typically achieved by multiplying a lower fundamental frequency.

The quartz crystal is always mounted inside a sealed package, usually a metal can with two or four leads, and requires a supporting oscillator circuit (amplifier, capacitors, resistors) to sustain oscillation. The crystal itself is passive; it will not oscillate on its own without that circuit driving it. Typical load capacitance values are 15, 20, or 30 picofarads, and if the oscillator circuit does not present the correct load, the crystal will drift off frequency or refuse to start.

Common failure modes include frequency drift from temperature swings, aging (quartz crystals lose frequency stability slowly over years due to charge redistribution in the material), and mechanical damage from shock or vibration. Contamination inside the sealed package, moisture, conductive particles, or outgassing from internal materials, can cause erratic behavior or complete cessation of oscillation. Supply voltage spikes or transients can also fracture the delicate crystal element.

The name "quartz crystal" is literal: it is piezoelectric quartz (crystalline SiO2), exploiting the reverse piezoelectric effect where mechanical strain produces electric charge. The term distinguishes it from other oscillator elements like ceramic resonators (cheaper, less stable, but smaller) or LC tank circuits (less stable, broader tuning range). In schematics, quartz crystals are marked with the symbol Y1, Y2, etc., and specified by frequency, load capacitance, and tolerance (for example, 16 MHz, 20 pF load, ±30 ppm).