A crystal oscillator is an electronic device that uses the mechanical resonance of a physical crystal of piezoelectric material to create an electrical signal with a very precise frequency. It is an especially accurate form of an electronic oscillator. The crystal is usually made of quartz, but can also be made of rubidium or ceramic. This frequency is used to keep track of time (as in quartz wristwatches), to provide a stable clock for digital circuits, and to stabilize frequencies for radio transmitters. Crystal oscillators are the most common source of time and frequency signals. The crystal is sometimes called a "timing crystal". They can be embedded in integrated circuits.

Crystals for timing purposes

A 4.000 MHz crystal oscillator.

A crystal is a solid in which the constituent atoms, molecules, or ions are packed in a regularly ordered, repeating pattern extending in all three spatial dimensions. This means that a crystal is very predictable, because of its shape.

Almost any physical object could be used in a similar way to the crystal, with appropriate transducers, since all objects have natural resonant frequencies of vibration. These frequencies depend on size, shape, material, molecular structure, etc. They can be approximately calculated. However, you probably don't want to, because calculating this value for your kitchen table can become extremely complex. That's why crystals are used. Their repeating pattern forms an ideal base for calculating their natural frequency.

Many materials can be formed into plates that will resonate. However, since quartz can be directly driven by an electric signal (it is piezoelectric), no additional electrical-to-mechanical transducer is required.

Chemically, quartz is a compound called silicon dioxide. When a crystal of quartz is properly cut and mounted, it can be made to bend in an electric field. When the field is removed, the quartz will generate an electric field as it returns to its previous shape. The crystal strains (expands or contracts) when an electrical voltage is applied. When the voltage is reversed, the strain is reversed. This property is known as piezoelectricity.

Quartz has the further advantage that it does not change size much as temperature changes. It could be made of either natural or synthetic quartz, but all modern devices use synthetic quartz. Fused quartz is often used for laboratory equipment that must not change shape as the temperature changes. This means that a quartz plate's size will not change much with temperature. Therefore, the resonant frequency of the plate, which depends on the plate's size, will not change much, either. This means that a quartz clock will be relatively accurate as the temperature changes.

More than two billion (2 × 10⁹) quartz oscillators are manufactured annually. Most are small devices built for wristwatches, clocks, and electronic circuits. However, quartz oscillators are also found inside test and measurement equipment, such as counters, signal generators, and oscilloscopes.

Crystals and frequency

Schematic symbol and equivalent circuit for a quartz crystal in an oscillator

If you put your hands on the table, and start shaking it, you'll notice that when you're doing it slowly, it might go easier than when you're shaking very fast. This all depends on the kind of table you have. The same thing happens to crystals, only electricity is used to initiate the crystal's movement.

A regular timing crystal contains two electrically conductive plates, with a crystal sandwiched between them. A quartz crystal inside the oscillator is the resonator. The circuitry around the crystal then applies an AC signal to it, and the crystal will start oscillating at this frequency. However, because (let's assume in this situation that this is so) the natural frequency of the crystal is different than the applied frequency, it won't get to a large amplitude. This is because when the crystal is pushed forward (for example) and the crystal's natural frequency makes it go back at the same moment, the forces are working in opposite directions and we're not getting anywhere.

When the crystal oscillator circuit notices this, it will adjust its frequency. When the frequency gets closer to the natural frequency of the crystal, the amplitude of the crystals movement will suddenly get a lot bigger. For the crystal oscillator circuit this means that there is suddenly almost no resistance at all, and that the current frequency is very close to the natural frequency of the crystal. This is called resonance.

This is usually used by microcontrollers, which can be optimized for system on chip-like situations. Because of the use of crystals, the generated clock signal can be extremely accurate, making it useful for timing systems at high speed.

The crystal oscillator circuit sustains oscillation by taking a voltage signal from the resonator, amplifying it, and feeding it back to the resonator. The rate of expansion and contraction is the resonance frequency, and is determined by the cut and size of the crystal.

The output frequency of a quartz oscillator is either the fundamental resonance or a multiple of the resonance, called an overtone frequency.

A typical Q for a quartz oscillator ranges from 10⁴ to 10⁶. The maximum Q for a high stability quartz oscillator can be estimated as Q = 1.6 × 10⁷/f, where f is the resonance frequency in MHz.

Environmental changes of temperature, humidity, pressure, and vibration can change the resonance frequency of a quartz crystal, but there are several designs that reduce these environmental effects. These include the TCXO, MCXO, and OCXO (defined below). These designs (particularly the OCXO) often produce devices with excellent short-term stability. The limitations in short-term stability are due mainly to noise from electronic components in the oscillator circuits. Long term stability is limited by aging.

Due to aging and environmental factors such as temperature and vibration, it is hard to keep even the best quartz oscillators within 10^-10 of their nominal frequency without constant adjustment. For this reason, atomic oscillators are used for applications that require better long-term stability and accuracy.

Notation

An abbreviation used for a crystal is "X" (or "XTAL"), and for a crystal oscillator is "XO". These abbreviations are used in electronic schematics and in radio specifications.

Types of crystal oscillators include voltage-controlled crystal oscillators (VCXO), temperature-compensated crystal oscillators (TCXO), oven-controlled crystal oscillators (OCXO), temperature-compensated-voltage controlled crystal oscillators (TCVCXO), oven-controlled voltage-controlled crystal oscillators (OCVCXO), microcomputer-compensated crystal oscillators (MCXO), and rubidium crystal oscillators (RbXO).