Quartz clock
From Wikipedia, the free encyclopedia
A quartz clock is a clock that uses an electronic oscillator which is made up by a quartz crystal to keep precise time. This crystal oscillator creates a signal with very precise frequency. Usually some form of digital logic counts the cycles of this signal and provides a numeric time display, usually in units of hours, minutes, and seconds.
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[edit] Explanation
[edit] Quartz
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. This property is known as piezoelectricity.
Many materials can be formed into plates that will resonate. However, since quartz can be directly driven by an electric signal, no additional speaker or microphone is required.
Quartz has the further advantage that it does not change size much as temperature changes. 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.
[edit] Mechanism
In modern quartz clocks, the resonator is tuning fork shaped, laser-trimmed or precision lapped to vibrate at 32,768 Hz. This frequency is exactly 215 Hz. A very simple electronic circuit can be built with a series of 15 divide-by-2 stages cascaded to get the base time of 1 second. In most clocks, the resonator is in a small can or flat package, about 4mm long. The reason 32,768Hz resonator has become so common is due to a compromise between the large physical size of low frequency crystals for watches and the large current drain high frequency resonators demand from watch batteries. During the 1970s the introduction of Metal Oxide Semiconductor (MOS) integrated circuits allowed a 12 month battery life from a single coin cell when driving either a mechanical stepper motor, indexing the second hand (Quartz Analogue), or a liquid crystal display (LCD Digital). Light-emitting diode (LED) displays for watches have become rare due to their very high battery consumption.
[edit] Accuracy
The relative stability of the resonator and its driving circuit is much better than its absolute accuracy. Standard quality resonators of this type are warranted to have a long-term accuracy of about 6 parts per million at 31 °C, that is, a typical quartz wristwatch will gain or lose less than a half second per day at body temperature.
If a quartz wristwatch is "rated" by measuring it against an atomic clock's time broadcast, and worn on one's body to keep its temperature constant, the corrected time can easily be as accurate as 10 seconds per year, more than good enough to perform celestial navigation.
Some premium clock designs self-rate. That is, rather than just counting vibrations, their computer program takes the simple count, and scales it using a ratio calculated between an epoch set at the factory, and the most recent time the clock was set. These clocks usually have special instructions for changing the battery (the counter must not be permitted to stop), and become more accurate as they grow older.
It is possible for a computerized clock to measure its temperature, and adjust for that as well. Both analog and digital temperature compensation have been used in high-end quartz watches.
[edit] Fine-tuning adjustment
One practical issue for the mass-production of quartz oscillators was how to adjust their frequency without requiring a human operator to fine-tune them. Several automated methods were developed; in the most common one, the tuning fork as made has a small amount of metal on its ends, and runs slightly slower than rated frequency. An automatic machine then measures its frequency while using a laser to vaporize the metal on its ends, slowly raising its frequency, until it reaches the specified frequency; then it is sealed into a small can and is ready for use.
[edit] Chronometers
Quartz chronometers designed as time standards often include a crystal oven, to keep the crystal at a constant temperature. Some self-rate and include "crystal farms," so that the clock can take the centroid of a set of time measurements.
[edit] History
Warren Marrison, working for Bell Laboratories, developed the first quartz clock in Canada. Seiko produced the world's first quartz wristwatch, the Astron, in 1969. The inherent accuracy and low cost of production has resulted in the proliferation of quartz clocks and watches since that time. Quartz timepiece production has emerged from Asia, notably Hong Kong and Japan. Many traditional European clockmakers have continued to produce the less accurate but popular geared timepieces.
