Crystal clock oscillators are typically viewed by digital designers as just another digital component for populating logic boards. In reality, those oscillators represent the antithesis of digital components. While they may use logic circuits to transform sinusoidal oscillations into symmetrical square waves, the crystal oscillator is a quintessential analog device. The crystal and its electrical excitation circuit form a kind of miniaturized, mechanical servo loop. Under closed-loop control, the crystal is driven into resonance. Its own natural frequency sets the vibrating rate.
Particularly remarkable is the way this analog component's specifications are routinely discussed in the rarefied language of parts per million (ppm). The crystal oscillator's drift with temperature, its natural aging characteristics, and the relationship between operating frequency and dc supply voltage are all coefficients with parts-per-million values. Voltage-controlled crystal oscillators, which are widely used in communications circuits, are characterized by their pull range in similar ppm terms.
Among digital circuit designers, the difference between parts per thousand and parts per million is just a longer word length. After all, it's mathematics, not physics. Contrast this digital simplicity with the absolute attention to physical detail involved in holding a crystal oscillator to its parts-per-million stability over −40° to +85°C.
These oscillators demand enormous care in curbing the influence of temperature, aging, oxidation, contamination, outgassing, physical deformity, mechanical stress, chemical degradation—and then some. Yet despite this fabrication complexity, today's advanced production methods yield fully functional oscillators at prices below $10 apiece. Few other man-made "machines" come close to matching an oscillator's price-performance value.
That's why, when developing oscillator specifications, a prospective buyer will save money—not to mention mental wear and tear—by working with the crystal's physical constraints. Attempts to defy the laws of nature are costly and invariably fruitless.