A quartz clock is a clock that uses an electronic oscillator that is regulated by a quartz crystal to keep time. This crystal oscillator creates a signal with very precise frequency, so that quartz clocks are at least an order of magnitude more accurate than good mechanical clocks. Generally, 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. Since the 1970s, they have become the most widely used timekeeping technology
Chemically, quartz is a compound called silicon dioxide. When a crystal of quartz is properly cut and mounted, it can be made to vibrate, or oscillate, using an alternating electric current. The frequency at which the crystal oscillates is dependent on its shape and size, and the positions at which electrodes are placed on it. If the crystal is accurately shaped and positioned, it will oscillate at a desired frequency; in clocks and watches, the frequency is usually 32,768 Hz, as a crystal for this frequency is conveniently small, and as this frequency is a power of two and can easily be counted using a 15-bit binary digital counter. Once the circuit supplying power to the crystal counts that this number of oscillations have occurred, it increases the recorded time by one second. This property, of changing shape under an electric current, is known as piezoelectricity. Such crystals were once used in low-end phonograph cartridges: the movement of the stylus (needle) would flex a quartz crystal, which would produce a small voltage, which was amplified and played through speakers.
Many materials can be formed into plates that will resonate. However, since quartz can be directly driven (to flex) by an electric signal, no additional speaker or microphone is required.
Quartz has the further advantage that its size does not change much as temperature fluctuates. Fused quartz is often used for laboratory equipment that must not change shape along with the temperature, because a quartz plate’s resonance frequency, based on its size, will not significantly rise or fall. Similarly, a quartz clock will remain relatively accurate as the temperature changes.
In modern quartz clocks, the quartz crystal resonator is in the shape of a small tuning fork, laser-trimmed or precision lapped to vibrate at 32,768 Hz. This frequency is equal to 215 cycles per second. A power of 2 is chosen so a simple chain of digital divide-by-2 stages can derive the 1 Hz signal needed to drive the watch’s second hand.
In most clocks, the resonator is in a small can or flat package, about 4 mm long. The reason the 32,768 Hz 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 of high frequency crystals, which reduces the life of the watch battery. 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 (in a quartz analog watch), or a liquid crystal display (in an LCD digital watch). Light-emitting diode (LED) displays for watches have become rare due to their comparatively high battery consumption.
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 (87.8°F): 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 its timekeeping characteristics against an atomic clock’s time broadcast, to determine how much time the watch gains or loses per day, and adjustments are made to the circuitry to “regulate” the timekeeping, then the corrected time will easily be accurate within 10 seconds per year. Temperature compensation is usually also included in the electronic circuitry. This is more than adequate to perform celestial navigation.
Some premium clock designs self-rate and self-regulate. 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.
Many inexpensive quartz watches use a technique known as inhibition compensation. The crystal is deliberately made to run somewhat fast, and the digital logic is programmed to skip a small number of crystal cycles at regular intervals such as 10 seconds or one minute. The advantage of this method is that after measuring the frequency of each chip with a precision timer at the factory, storing the number of pulses to suppress in a non-volatile memory register on the chip is less expensive than the older technique of trimming the quartz tuning fork frequency. In more expensive watches, thermal compensation can be implemented by varying the number of cycles to inhibit depending on the output from a temperature sensor.
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 average of a set of time measurements.
Quartz wristwatches are in high demand today as they are more accurate than their mechanical counterparts; they need neither winding nor much maintenance. Light-powered and motion-powered quartz watches represent two innovative types of timepieces. Light-powered quartz watches incorporate a solar cell that transforms the light into electricity. Motion-powered wristwatches have a tiny rotor spinning in response to motion and generating electricity.