Quartz crystal clocks

Because of its piezoelectric properties, synthetic CC-quartz is used for frequency control in electrical oscillators and filters and in electromechanical transducers. When mechanically stressed in the correct direction, CC-quartz develops an electric polarization. The opposite is also tme an applied electric field gives rise to a mechanical distortion in the crystal. Thin sections of quartz are cut to dimensions that produce the desired resonance frequency when subjected to an alternating electric field the vibrating crystal then reacts with the driving circuit to produce an oscillation that can be narrowly controlled. Quartz is ideal for this application because it is hard, durable, readily synthesized, and can be tuned to high accuracy, for example, quartz crystal clocks can be made that are stable to one part in 109. 

Q can be increased in appropriately designed circuits this is utilized in several instruments (radios, in quartz crystal oscillators, quartz clocks, etc.) that are "tuned" to detect resonant transitions. 

Centuries ago, time was measured by the gnomon, the clepshydra, weights and gears (eventually controlled by an escapement), incense sticks, hourglasses, and then finally mechanical clocks, pendula, and self-winding watches (mechanically wound, self-winding, or by now controlled by a quartz crystal oscillator tank circuit). 

When an atom makes a transition from a high-energy quantum state to a lower energy state, electromagnetic radiation with a definite frequency and a definite period is emitted. When properly detected, this frequency, or period, becomes the ticking of an atomic clock, just as the crystal vibration frequency and the swinging frequency are the inaudible ticks of a quartz clock and a pendulum clock. The frequency emanating from the atom, however, is much less influenced by environmental factors such as temperature, pressure, humidity, and acceleration than are the frequencies from quartz crystals or pendula. Thus, atomic clocks hold inherently the potential for reproducibility, stability, and accuracy. 

The natural resonance frequency of a piezoelectric crystal may be used as a frequency standard. Quartz is the material of choice. Quartz crystal resonators provide highly stable crystal-controlled clocks and watches (constant to 1 part in 10 ) and control fixed frequencies... 

Quartz docks eventually replaced the mechanical docks around the middle of the 20th century. A quartz dock or watch makes use of the piezodectric property of quartz crystal A quartz crystal, when subjected to a mechanical pressure, creates an electric fidd. The inverse is also tme—that is to say, the shape of the crystal changes when it is subjected to an dectric fidd. These principles are used to design clocks that make the crystal vibrate and generate an electric signal of constant fisquency. 

The twentieth century saw two major advances in time measurement the development of quartz clocks, which used electric circuits to generate constant electrical vibrations in quartz crystals, and the invention of atomic clocks, which take advanti e of the natural resonance frequency of atoms to create... 

Household Clocks. The majority of clocks, watches, and small electronic circuits used in everyday life are built on oscillators that use quartz crystals to generate a consistent pulse. Quartz crystals (whether natural or synthetic) have the property known as piezoelectricity, which means that the crystals expand and contract—or vibrate—when they receive an electric force. The combination of a quartz crystal with a battery that applies and then reverses an electric voltage produces a regular oscillation, the frequency of which depends on the size of the crystal and the form into which it is cut. A quartz oscillator found in an ordinary household clock will probably have a quality fector (Q) of about 10 to 10 and be accurate to about a few seconds per month— perfectly adequate for everyday use. 

Electronics and Computers. Every computer contains a small, built-in quartz-based oscillator that serves as an internal marker of time intervals for the machine. The central processing unit uses this clock to determine the intervals at which its microprocessor is directed to complete instructions, as well as for purposes such as scheduling automatic processes and time-stamping events. It is also important for computers that are sending and receiving information over a network or the Internet to be highly synchronized with each other for data to be transmitted accurately. Since the piezoelectric qualities of quartz crystals change with temperature, however, computer clocks tend to drift as the machinery inside them heats up with use. For this reason, most computer... 

A sensor that is especially interesting and instructive is made from a quartz crystal microbalance, or QCM. This device is based on the piezoelectric characteristics of quartz. When quartz is mechanically deformed, an electrical potential difference develops across its surface. Furthermore, when a voltage is impressed across the faces of a quartz crystal, the crystal deforms., A crystal connected in an appropriate electrical circuit oscillates at a frequency that is characteristic of the mass and shape of the crystal and that is amazingly constant as long as the mass of the crystal is constant. This property of some crystalline materials is called the piezoelectric effect and forms the basis for the QCM. Moreover, the characteristic constant frequency of the quartz crystal is the basis for modern high-precision clocks, time bases, counters, timers, and frequency meters, which in turn have led to many highly accurate and precise analyTical instrumental svsiems. 

Quartz is a piezoelectric material, which vibrates when a voltage is applied. Quartz crystals became used as a time standard with the first quartz clock in 1927. Quartz crystals for watches have been mass-produced since the 1970s. The cut and shape of the actual quartz crystal determine the frequency of the quartz oscillator. 

Velocity control is based on utilizing two transducers to calculate the ram velocity. Aposition transducer, such as a rectilinear potentiometer measures the ram position while a digital clock based on an oscillating quartz crystal keeps the interval time and the microprocessor calculates velocity as distance divided by time. The calculated velocity is compared against the servo input velocity to generate an error signal on which the controller acts to make a correction. 

Joseph W. Horton and Warren A. Marrison of Bell Laboratories built the first clock based on a quartz crystal oscillator in 1927. By the 1940s, quartz clocks had replaced pendulums as primary laboratory standards. Quartz crystals resonate at a nearly constant frequency when an electric current is applied. Uncertainties of < 100 usec/day (=1 X 10 ) are possible, and low-cost quartz oscillators are found in electronic circuits and inside nearly every wristwatch and wall clock. 

Quartz crystal oscillators are by far the most common time and frequency standard. An estimated 2 bilhon (2 x 10 ) quartz oscillators are manufactured armtrally. Most are small devices built for wristwatches, clocks, and electronic circuits. However, they are also fotmd inside test and measurement eqrripment, such as cormters, signal generators, and oscilloscopes, and interestingly enough, inside every atomic oscillator. 

It should be noted that, whereas ferroelectrics are necessarily piezoelectrics, the converse need not apply. The necessary condition for a crystal to be piezoelectric is that it must lack a centre of inversion symmetry. Of the 32 point groups, 20 qualify for piezoelectricity on this criterion, but for ferroelectric behaviour a further criterion is required (the possession of a single non-equivalent direction) and only 10 space groups meet this additional requirement. An example of a crystal that is piezoelectric but not ferroelectric is quartz, and ind this is a particularly important example since the use of quartz for oscillator stabilization has permitted the development of extremely accurate clocks (I in 10 ) and has also made possible the whole of modern radio and television broadcasting including mobile radio communications with aircraft and ground vehicles. 

Piezoelectric crystals, notably quartz, are used to control or limit the operating frequency of electrical circuits. A well-known example is their use in quartz clocks . The fact that a dielectric body vibrating at a resonant frequency can absorb considerably more energy than at other frequencies provides the basis for piezoelectric wave filters. The equivalent circuit for a piezoelectric body vibrating at frequencies close to a natural frequency is given in Fig. 6.3. At resonance the impedance due to L, and C falls to zero and, provided that Rx is small, the overall impedance is small. 

Quartz (silicon dioxide) crystals are used for piezoelectric crystals for radiofrequency control oscillators and digital watches and clocks. 

The device can be used to replace the tuned circuit in an oscillator by providing the resonant frequency or it can be coupled to the oscillator circuit, which is tuned approximately to the crystal frequency. In this type, the crystal prevents frequency drift. The device is widely used in quartz clocks and watches. 

If a quartz plate is subjected to an alternating electric field, the reverse piezoelectric effect causes it to expand and contract at the field frequency. If this field frequency is made to coincide with the natural elastic frequency of the crystal, the plate resonates the direct piezoelectric effect then augments the applied electric field. This is the basis of the crystal oscillator and the quartz clock. See also CRYSTAL MICROPHONE CRYSTAL PICK-UP. 

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