Quartz clocks and quartz watches are timepieces that use an electronic oscillator regulated by a quartz crystal to keep time. The crystal oscillator, controlled by the resonant mechanical vibrations of the quartz crystal, creates a signal with very precise frequency, so that quartz clocks and watches are at least an order of magnitude more accurate than mechanical clocks. Generally, some form of digital logic counts the cycles of this signal and provides a numerical time display, usually in units of hours, minutes, and seconds.
As the advent of solid-state digital electronics in the 1980s allowed them to be made more compact and inexpensive, quartz timekeepers became the world's most widely used timekeeping technology, used in most clocks and watches as well as computers and other appliances that keep time.
Explanation
Chemically, quartz is a specific form of a compound called silicon dioxide. Many materials can be formed into plates that will resonate. However, quartz is also a piezoelectric material: that is, when a quartz crystal is subject to mechanical stress, such as bending, it accumulates electrical charge across some planes. In a reverse effect, if charges are placed across the crystal plane, quartz crystals will bend. Since quartz can be directly driven (to flex) by an electric signal, no additional transducer is required to use it in a resonator. Similar crystals are used in low-end phonograph cartridges: The movement of the stylus (needle) flexes a quartz crystal, which produces a small voltage, which is amplified and played through speakers. Quartz microphones are still available, though not common.
Quartz has a further advantage in 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. A quartz plate's resonance frequency, based on its size, will not significantly rise or fall. Similarly, since its resonator does not change shape, a quartz clock will remain relatively accurate as the temperature changes.
In the early 20th century, radio engineers sought a precise, stable source of radio frequencies and started at first with steel resonators. However, when Walter Guyton Cady found in the early 1920s that quartz can resonate with less equipment and better temperature stability, steel resonators disappeared within a few years. Later, scientists at National Institute of Standards and Technology (then the U.S. National Bureau of Standards) discovered that a crystal oscillator could be more accurate than a pendulum clock.
The electronic circuit is an oscillator, an amplifier whose output passes through the quartz resonator. The resonator acts as an electronic filter, eliminating all but the single frequency of interest. The output of the resonator feeds back to the input of the amplifier, and the resonator assures that the oscillator runs at the exact frequency of interest. When the circuit is powered up, a single burst of shot noise (always present in electronic circuits) can cascade to bring the oscillator into oscillation at the desired frequency. If the amplifier were perfectly noise-free, the oscillator would not start.
The frequency at which the crystal oscillates depends on its shape, size, and the crystal plane on which the quartz is cut. The positions at which electrodes are placed can slightly change the tuning as well. If the crystal is accurately shaped and positioned, it will oscillate at a desired frequency. In nearly all quartz clocks and watches, the frequency is , and the crystal is cut in a small tuning fork shape on a particular crystal plane. This frequency is a power of two ( = 215), just high enough to exceed the human hearing range, yet low enough to keep electric energy consumption, cost and size at a modest level and to permit inexpensive counters to derive a 1-second pulse. The data line output from such a quartz resonator goes high and low times a second. This is fed into a flip-flop (which is essentially two transistors with a bit of cross-connection) which changes from low to high, or vice versa, whenever the line from the crystal goes from high to low. The output from that is fed into a second flip-flop, and so on through a chain of 15 flip-flops, each of which acts as an effective power of 2 frequency divider by dividing the frequency of the input signal by 2. The result is a 15-bit binary digital counter driven by the frequency that will overflow once per second, creating a digital pulse once per second. The pulse-per-second output can be used to drive many kinds of clocks. In analog quartz clocks and wristwatches, the electric pulse-per-second output is nearly always transferred to a Lavet-type stepping motor that converts the electronic input pulses from the flip-flops counting unit into mechanical output that can be used to move hands.
It is also possible for quartz clocks and watches to have their quartz crystal oscillate at a higher frequency than (= 215) Hz (high frequency quartz movements) and/or generate digital pulses more than once per second, to drive a stepping motor powered second hand at a higher power of 2 than once every second, but the electric energy consumption (drain on the battery) goes up because higher oscillation frequencies and any activation of the stepping motor costs energy, making such small battery powered quartz watch movements relatively rare. Some analog quartz clocks feature a sweep second hand moved by a non-stepped battery or mains powered electric motor, often resulting in reduced mechanical output noise.
Mechanism
In modern standard-quality quartz clocks, the quartz-crystal resonator or oscillator is cut in the shape of a small tuning fork (XY-cut), laser-trimmed or precision-lapped to vibrate at . 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 signal needed to drive the watch's second hand. In most clocks, the resonator is in a small cylindrical or flat package, about long. The resonator has become so common due to a compromise between the large physical size of low-frequency crystals for watches and the larger current drain of high-frequency crystals, which reduces the life of the watch battery.
The basic formula for calculating the fundamental frequency of vibration of a cantilever as a function of its dimensions (quadratic cross-section) is
where (rounded) is the smallest positive solution of the equation ; is the length of the cantilever; is its thickness along the direction of motion; is its Young's modulus; and is its density.
A cantilever made of quartz ( = = ; = ), with a length of and a thickness of , thus has a fundamental frequency around . The crystal is tuned to exactly 215 = , or runs at a slightly higher frequency with inhibition compensation (see below).
Accuracy The relative stability of the quartz 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 six parts per million (0.0006%) at : that is, a typical quartz clock or wristwatch will gain or lose 15 seconds per 30 days (within a normal temperature range of ) or less than a half second clock drift per day when worn near the body.
Temperature and frequency variation Though quartz has a very low coefficient of thermal expansion, temperature changes are the major cause of frequency variation in crystal oscillators. The most obvious way of reducing the effect of temperature on the oscillation rate is to keep the crystal at a constant temperature. For laboratory-grade oscillators, an oven-controlled crystal oscillator is used, in which the crystal is kept in a very small oven that is held at a constant temperature. This method is, however, impractical for consumer quartz clock and wristwatch movements.
The crystal planes and tuning of consumer-grade clock crystal resonators used in wristwatches are designed for minimal temperature sensitivity to frequency and operate best at a temperature range of about . The exact temperature where the crystal oscillates at its fastest is called the "turnover point" and can be chosen within limits. A well-chosen turnover point can minimize the negative effect of temperature-induced frequency drift, and hence improve the practical timekeeping accuracy of a consumer-grade crystal oscillator without adding significant cost. A higher or lower temperature will result in a −0.035 ppm/°C2 (slower) oscillation rate. So a ±1 °C temperature deviation will account for a (±1)2 × −0.035 ppm = −0.035 ppm rate change, which is equivalent to −1.1 seconds per year. If, instead, the crystal experiences a ±10 °C temperature deviation, then the rate change will be (±10)2 × −0.035 ppm = −3.5 ppm, which is equivalent to −110 seconds per year.
Quartz watch manufacturers use a simplified version of the oven-controlled crystal oscillator method by recommending that their watches be worn regularly to ensure the best time-keeping performance. Regular wearing of a quartz watch significantly reduces the magnitude of environmental temperature swings, since a correctly designed watch case forms an expedient crystal oven that uses the stable temperature of the human body to keep the crystal oscillator in its most accurate temperature range.
Accuracy enhancement Some movement designs feature accuracy-enhancing features or 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. Clocks that are sometimes regulated by service centers with the help of a precision timer and adjustment terminal after leaving the factory, also become more accurate as their quartz crystal ages and somewhat unpredictable aging effects are appropriately compensated.
Autonomous high-accuracy quartz movements, even in wristwatches, can be accurate to within ±1 to ±25 seconds per year and can be certified and used as marine chronometers to determine longitude (the East–West position of a point on the Earth's surface) by means of celestial navigation. When time at the prime meridian (or another starting point) is accurately enough known, celestial navigation can determine longitude, and the more accurately time is known the more accurate the latitude determination. At latitude 45° one second of time is equivalent in longitude to , or one-tenth of a second means .
Trimmer condenser Regardless of the precision of the oscillator, a quartz analog or digital watch movement can have a trimmer condenser. They are generally found in older, vintage quartz watches – even many of the cheaper ones. A trimmer condenser or variable capacitor changes the frequency coming from the quartz crystal oscillator when its capacitance is changed.Transistor Crystal Oscillator Circuit The frequency dividers remain unchanged, so the trimmer condenser can be used to adjust the electric pulse-per-second (or other desired time interval) output. The trimmer condenser looks like a small screw that has been wired into the circuit board. Typically, turning the screw clockwise speeds the movement up, and counterclockwise slows it down at about 1 second per day per turn of the screw. Few newer quartz movement designs feature a mechanical trimmer condenser and rely on generally digital correction methods.
Thermal compensation