Quartz Clock - Explanation

Explanation

Chemically, quartz is 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 speaker or microphone is required to use it in a resonator. Similar 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. 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, because 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 that quartz can resonate with less equipment and better temperature stability, steel resonators disappeared in a few years. Later, scientists at NIST (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 "howls" with the exact frequency of interest. When the circuit starts up, even a single shot can cascade to bringing the oscillator at the desired frequency. If the amplifier is too perfect, the oscillator will 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 clocks and watches, the frequency is usually 32,768 Hz, and the crystal is cut in a small tuning fork shape on a particular crystal plane. This frequency is a power of two, just high enough so most people cannot hear it, yet low enough to permit inexpensive counters to derive a 1 second pulse. A 15-bit binary digital counter driven by the frequency 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.

Although 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 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 unpractical for consumer quartz clock and wrist watch movements.

The crystal planes and tuning of a consumer grade clock crystal are designed for minimal temperature sensitivity in terms of their effect on frequency and operate best at about 28 °C. At that temperature the crystal oscillates at its fastest. A higher or lower temperature will result in a -0.035 parts per million/°C2 (slower) oscillation rate. So a ±1 °C temperature deviation will account for a (1)2 x -0.035 = -0.035 parts per million (ppm) rate, 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 x -0.035 ppm = 100 x -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 best performance. Regular wearing 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 in its most accurate temperature range.