Quartz clock

With a 1 MHz quartz clock, the contribution to the standard deviation a(i 0) is mainly that from the intensities N. For example, with fP+ = = 20 s, the time... 

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. 

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. 

Quartz clocks are accurate to about 0.0001 seconds per day. Such accuracy is more than adequate for everyday affairs. Suppose, however, the challenge is to measure the difference in clock rate between sea level and the top of Mount Everest The answer is that a clock on the top of Everest loses about 0.000030 second... 

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. 

For bus systems there are several standard or quasi-standard bus structures available. For high-speed connection you can use the popular automotive CAN bus (control area network) or the SPI interface (serial peripheral interface) often used to connect the sensor to the microcontroller inside the control unit For lower speeds the LIN bus (local interconnect network) is available. In both cases a microcontroller inside the sensor unit is necessary, which works with a stable quartz clock frequency. 

With a 1 MHz quartz clock, the contribution to the standard deviation o(i o) is mainly that from the intensities N. For example, with fp+ = t,. = 20 s, the time contribution to o(i o) is equal to 1/V2- 10- i o- This is negligible compared to the values calculated above. This cyclic method reduces the time needed to obtain a given accuracy in flipping ratio measurements particularly in the case of strong reflections. 

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. 

The 133 cesium clock is based on the two hyperfine levels of the spin of Its valence electron which may be In the ground state (singlet, opposing the spin direction of the nucleus) or excited by 6.1x10 J (doublet, parallel to the spin of the nucleus). The electromagnetic radiation that can achieve the transition has a frequency of 9,192,631,770 Hz (wavelength of %3.26 cm). The 133 cesium clock Is reproducible to 1-2 ms per year (1 part in lO ). The feedback arrangement that controls a quartz clock to this stability Is shown below (simplified schematic). 

Piezoelectricity is an effect of the generation of electric polarity in certain dielectric materials (i.e., electric insulators) in response to the applied mechanical pressure. The effect was discovered by Pierre and Jacques Curie in 1880. The polarization of the material is proportional to the applied stress. The piezoelectric effect has very important practical applications in transducers, i.e., devices which convert electric signals to mechanical signals, or the mechanical signals to electric signals, such as microphones, loudspeakers, gramophone needles, ultrasound generators, quartz clocks, etc [16,17]. 

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... 

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. 

The phenomenon utilized in the quartz clock is called the piezoelectric effect. When a crystal is subjected to alternate compressive and tensile strains, oscillating (+, -) electric charges appear on opposing faces of the crystal. Similarly, when a crystal is subjected to oscillating charges, it undergoes expansion and contraction. 

A still more accurate timepiece is the atomic clock. In this case, regulation depends upon the frequency of molecular vibration. The maximum accuracy of a gaseous ammonia clock is about 1,000 times that of the best quartz clock. Thus, the best atomic clock has an accuracy of about3 ps/. ... 

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. 

UTO The original mean solar time scale, based on the rotation of the earth on its axis. UTO was first kept with pendulum clocks. When quartz clocks became available, astronomers noticed errors in UTO due to polar motion and developed the UTl time scale. 

---