Issue 91, February 1998
Choosing the Right Crystal For Your Oscillator

Start •Why Quartz Crystals • Timing Budget & Accuracy • Frequency Tolerance • Frequency Stability • Aging • Load Capacitance • Series and Parallel Resonance • Frequency Tolerance and Load Capacitance • AT vs. BT Cut • Mode of Operation • Package Considerations • Crystal Placement • Crystal Clear • References

WHY QUARTZ CRYSTALS

The quartz crystal integrates mechanical and electrical characteristics. If quartz is stressed, an electric field is generated in the direction perpendicular to the applied stress.

Conversely, if an electric field is applied to a quartz crystal, a mechanical stress appears in the direction perpendicular to the applied stress. This effect, known as the piezoelectric effect, is the basis for quartz being used so extensively in crystal manufacturing.

By placing a quartz crystal between two electrodes and applying a changing voltage, the crystal can be made to vibrate. Maximum vibration amplitude occurs when the frequency of the changing voltage matches the crystal resonant frequency. Oscillator circuits using a quartz crystal vibrate at the crystal resonant frequency.

High Q is one of the most desirable features of quartz crystals. It is a measure of how much energy is lost due to vibration. In mechanical terms, Q is:

In electrical terms, Q is the inductive reactance at resonant frequency divided by the equivalent series resistance (ESR).

A crystal with a high Q loses little energy while vibrating. Commercial-grade crystals have Qs ranging between 20,000 and 200,000. High-precision crystals have Qs up to 3 million.

In addition to high Qs, quartz crystals tend to be incredibly stable. The only drift associated with crystals is from temperature fluctuations and aging. Temperature effects are about 100 ppm over the operating range, while aging effects are around ±5 ppm per year.