Quartz clock

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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.

Contents 1 Explanation 1.1 Quartz 2 Mechanism 3 Accuracy 4 Chronometers 5 History 6 References 7 See also 8 Bibliography 9 External links

[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. 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 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 resonance 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 a quartz tuning fork, laser-trimmed or precision lapped to vibrate at 32,768 Hz. This frequency is equal to 2¹⁵ Hz. 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 (Quartz Analog), or a liquid crystal display (LCD Digital). Light-emitting diode (LED) displays for watches have become rare due to their very high battery consumption: few people have the patience to change a watch battery every month.

The basic formula for calculating the frequency of a quartz tuning fork as a function of its dimensions (quadratic cross-section) are as follows:^[1]

l = length = 3 mm (or 4 mm)

a = thickness = 0.3 mm

E = Young's modulus of quartz = 1x10¹¹ N·m⁻² = 100 GPa

ρ = density of quartz = 2500 kg·m^-3 (actually, 2650 kg·m^-3)

f_o = fundamental frequency = 3.52

f = frequency (Hz)

If we use the above numbers in the formula for a vibrating cantilever:

The above returns f ~ 34 kHz, which is approximately 2¹⁵, or 32,768 Hz.

[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 the wristwatch is worn on one's body to keep its temperature constant, then the corrected time will easily be accurate within 10 seconds per year^{[citation needed]}. This is more than adequate 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] 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 average of a set of time measurements.

[edit] History

The piezoelectric properties of quartz were discovered by Jacques and Pierre Curie in 1880. The first quartz crystal oscillator was built by Walter G. Cady in 1921. In 1923, D. W. Dye at the National Physical Laboratory in the UK and Warren Marrison at Bell Telephone Laboratories produced sequences of precision time signals with quartz oscillators. In 1927 the first quartz clock was built by Warren Marrison and J.W. Horton at Bell Telephone Laboratories.^[2][3] The next 3 decades saw the development of quartz clocks as precision time standards in laboratory settings; the bulky delicate counting electronics, built with vacuum tubes, limited their use elsewhere. In 1932 a quartz clock was able to measure tiny weekly variations in the rotation rate of the Earth.^[4] The National Bureau of Standards (now NIST) based the time standard of the US on quartz clocks between the 1930s and the 1960s, when it went to atomic clocks.^[5] The wider use of quartz clock technology had to await the development of cheap semiconductor digital logic in the 60s.

In 1967, the Centre Electronique Horloger (CEH) in Neuchâtel Switzerland developed the world's first analog quartz wristwatch - the famous Beta 21. ^[6]

In 1969, Seiko produced the world's first quartz wristwatch, the Astron. ^[7] The inherent accuracy and low cost of production has resulted in the proliferation of quartz clocks and watches since that time. By the 1980s quartz technology had taken over applications such as kitchen timers, alarm clocks, bank vault time locks, and time fuzes on munitions, from earlier mechanical balance wheel movements.

Quartz timepiece production has emerged from Asia, notably Hong Kong and Japan. Many traditional European clockmakers, particularly in Switzerland, Germany, France and Russia have continued to produce the less-accurate but still-popular geared timepieces.^{[citation needed]}

Quartz wristwatches are in high demand today as they are more accurate than their mechanical brothers; 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. As for the motion-powered wristwatches, they have a tiny rotor spinning in response to motion and generating electricity.^[8]

[edit] References

1. ^ Itoh H, Aoshima Y, Sakaguchi Y (2002). "Model for a quartz-crystal tuning fork using plate spring approximated to torsion spring adopted at the joint of the arm and the base". Frequency Control Symposium and PDA Exhibition, 2002. IEEE International: 145-151. doi:10.1109/FREQ.2002.1075871. 
2. ^ Marrison, W.A.; J.W. Horton (February, 1928). "Precision determination of frequency". I.R.E. Proc. 16: 137-154. 
3. ^ Marrison, Warren (1948). "The Evolution of the Quartz Crystal Clock". Bell System Technical Journal 27: 510-588. AT&T. 
4. ^ Marrison, 1948)
5. ^ Sullivan, D.B. (2001). Time and frequency measurement at NIST: The first 100 years. Time and Frequency Division, National Institute of Standards and Technology., p.5
6. ^ From the roots until today's achievements.... Federation of the Swiss Watch Industry. Retrieved on 2007-12-06.
7. ^ Electronic Quartz Wristwatch, 1969. IEEE History Center. Retrieved on 2007-08-31.
8. ^ Quartz wristwatches

[edit] See also

• Miyota 8215

[edit] Bibliography

• Cook, A. (2001). "Time and the Royal Society". Notes and Records of the Royal Society of London 55(1): 9–27. 
• Marrison, W. A (1948). "The Evolution of the quartz crystal clock". Bell System Technical Journal 27: 510-588. 

[edit] External links

• TimeZone.com article on the development of quartz watches
• The American Time Guide to Quartz Watches