Wafeek,Anthony,Augustine

INTRODUCTION

A quartz clock is a clock that functions by means of an electronic oscillator regulated by a quartz crystal to keep time. Quartz clocks are preferred over mechanical clocks because they are about an order of magnitude more accurate.  This is due to the unique quality of the quartz crystal oscillator that makes it to create a signal with very precise frequency. Warren Marrison and J.W. Horton of Bell Telephone Laboratories are known to be the first to build a quartz clock.[ 1]

Photo from wikipedia http://en.wikipedia.org/wiki/Quartz_clock

EXPLANATION

Quartz is a crystalline mineral, silicon dioxide, the principal component of sand which could be found in abundance on the earth's surface.  However, manufacturers use synthetic or man-made quartz crystals instead of natural ones because they have more consistent properties.  Many materials can be formed into plates that will resonate but quartz is the most preferred material for time keeping because of two special qualities that it has: its piezoelectric quality and its stability in every time, place and condition. 

Quartz crystal is piezoelectric, that is, it can vibrate when an electric charge is passed through the crystal plane and, conversely, it can generate voltage while vibrating.  Quartz is so stable that it can withstand all but the most extreme environmental condition without getting deformed.  Its shape remains relatively the same with temperature fluctuations.  This is why a quartz clock remains relatively accurate in every given temperature.

The quartz crystal in a quartz clock is designed in the form of a tuning fork.  Electrical charges from the battery of a clock passing across the crystal plane sets it in motion and then the clock’s circuitry measures the current fluctuations that represent the resonate motions of the tines.

The quartz tuning forks are made so accurate that they vibrate at 32,768 times per second give or take one six-hundredth.  In order to get the forks tuned to this frequency, the designers first add some deposits of gold at the ends of the tines to lower the fork’s vibration frequency.  Then a laser zaps tiny bits of the gold off until the desired frequency is reached.[1 ][3 ]

Photo from wikipedia http://en.wikipedia.org/wiki/Quartz_clock

Crystal oscillator

"A crystal oscillator is an electronic oscillator circuit that uses the mechanical resonance of a vibrating crystal of piezoelectric material to create an electrical signal with a very precise frequency. This frequency is commonly used to keep track of time (as in quartz wristwatches), to provide a stable clock signal for digital integrated  circuits, and to stabilize frequencies for radio transmitters and receivers . The most common type of piezoelectric resonator used is the quartz crystal, but other piezoelectric materials including polycrystalline ceramics are used in similar circuits.Quartz crystals are manufactured for frequencies from a few tens of kilohertz to tens of megahertz. More than two billion crystals are manufactured annually. Most are used for consumer devices such as wristwatches ,clocks,radios,computers, and cellphones. Quartz crystals are also found inside test and measurement equipment, such as counters,signal generators, and oscilloscopes."[4 ]

Photo from wikipedia http://en.wikipedia.org/wiki/Crystal_oscillator

MECHANISM

I. The quartz oscillator receives an electrical charge from an integrated circuit, which gets its power from the watch battery (or, in the case of a battery-less watch, the poser storage cell). The electricity makes the quartz vibrate, or oscillate, at the rate or 32,768 times per second. (Quartz crystals can be cut to vibrate at a huge range of frequencies. the bigger the piece, the slower it vibrates.)

II. As the quartz oscillates, it sends electrical pulses- at the same rate of 32,768 per second- back to the integrated circuit. A device called a "trimmer" regulates the quartz oscillations.

III. The IC "divides the electrical pulses repeatedly until they have been reduced to a single pulse each second. The circuit is, in effect, counting the pulses and returning to zero each time the count hits 32,768.

IV. If the watch is an analog model, the one-second impulses are transmitted to a stepping motor, which transforms them into mechanical pulses that drive a chain of gears and, ultimately, the watch hands. If it's a digital watch, the integrated circuit continues counting pulses as they add up to minutes, hours, and if the watch has a calendar display, days. In a digital watch the IC also provides the power to the display and contains the watch's setting functions. Because a digital watch has no gears or other moving parts, its construction is called "solid state." [2 ]  

The basic formula for calculating the fundamental frequency(f) of vibration of a cantilever as a function of its dimensions (quadratic cross-section) is:

where

• 1.875 the smallest positive solution of cos(x)cosh(x) = -1 
• l is the length of the cantilever
• a is its thickness along the direction of motion
• E is its Young's modulus
• and ρ is its density

A cantilever made of quartz(E = 10¹¹ N·m^−2^ = 100 Gpa and ρ = 2634 kg·m^−3 with a length of 3 mm and a thickness of 0.3 mm has thus a fundamental frequency of around 33 kHz. The crystal is tuned to exactly 2¹⁵ = 32,768 Hz or runs at a slightly higher frequency with inhibition compensation . [1 ]

REFERENCES

1. Wikipedia  http://en.wikipedia.org/wiki/Quartz_clock
2. Buchanan, Norma http://www.capetowncorp.com/whatsnew/quartz.html
3. Hammack, Bill http://www.engineerguy.com/videos/video-quartz-watch.htm
4. Wikipedia  http://en.wikipedia.org/wiki/Crystal_oscillator