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Piezo electric effect
In their investigations, the brothers Pierre & Jacques Curie found that electrical charges were produced by mechanical stresses applied to various naturally occuring crystals. This phenomenon is called the "piezo electric effect", being derived from the Greek word "to press"

Conversely it was found that the crystal was deformed by the application of an electrical charge, later the application of an alternating alectrical field introduced mechanical vibrations.

The quartz crystal behaves as an electrical resonance circuit with very low attenuation


Quartz
Quartz was found to be one of the naturally occuring crystaline substance that exhibited the "piezo - electric effect " and being very stable, both chemically and mechanically, it became of great interest to the early electronic experimenters. The chemical description of Quartz is Silicon dioxide SiO2.

Although approximately 14% of the earth's crust consists of SiO2, it occurs relatively infrequently in usable crystaline form with the necessary purity and without physical defects, cracks etc. Raw quartz of quality suitable for the production of quartz crystal units is found Brazil and Madagascar.

For this reason continuous attempts have been made to produce quartz synthetically ; that is to grow by recrystalisation on to quartz seed plates. These processes are now commercially successful.

Quartz chippings are dissolved in an alkaline solution in steel autoclaves at approximately 400 and pressure of 10,000N/cm2, the desired direction of growth is ensured by the careful preorientation of the quartz seed plates.

The growth rate is normally approximately 1mm/day. Slow growth rates result in a more homogeneous material since fewer foreign atoms are being incorporated into the crystal lattice.

Synthetic quartz is now grown in suitable length and cross sections to enable the subsequent processing steps to be carried out in the most efficient way, with minimum cutting losses. Grades available today are of very high purity and mechanical quality, do that synthetic quartz is used almost exclusively for the production of quartz crystal units.


Generalizer formulas



Equivalent circuit & parameter define

The electrical properties of the oscillator crystal as a function of the frequency can be depicted in the vicinity of the resonance frequency by an equivalent circuit circuit diagram.

The oscillating mass is symbolized by the dynamic inductance L1, whereas the elasticity is represented by the dynamic capacitance C1.

Overtone resonance, as well as spurious resonances can be depicted by additional, parallel connected series resonance circuits.


Frequency VS. temperature curve graph of AT-cut crystal units




Basic crystal oscillator circuit