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Calibration & Calibration Tolerance

Calibration of a crystal unit is the adjustment the frequency of vibration to the design frequency called out by the specification. Calibration, sometimes called Set-On, is accomplished by either adding or removing mass from the quartz plate by adding or removing metal from the electrode. Adding mass will lower the frequency and removing mass will raise the frequency and both automated and manual manufacturing processes exist to make the adjustment.

For most AT Cut crystals, calibration is specified and performed at 25 deg. C or Room Temperature (at or close to the Inflection Point). In the case of crystals designed for OCXO application, calibration takes place at the operating temperature of the oven which is set for the temperature of the Upper Inversion Point.

Calibrating the frequency of a crystal involves a tolerance. Much like asking ten carpenters to cut a piece of wood to 6 inches in length, the results would be ten pieces of wood with lengths in a range centered about a nominal, six inches. Some would be a bit long, some a bit shorter than six inches but all close to six inches. The level of required precision (calibration specification) will include a tolerance, say 6 inches +/- 1/16th inch. In the case of a crystal the Calibration Specification will include a design frequency and a tolerance expressed in hertz, percent or PPM. Examples expressing the same frequency and tolerance would be:

10 MHz. +/- 100 Hz. @ 25 deg. C
10 MHz. +/- .001% @ 25 deg. C
10 MHz. +/- 10 PPM @ 25 deg. C

As the degree of accuracy specified increases, the calibration process becomes more difficult with a reduced yield. Stringent accuracy specifications are cost/price drivers. Examples of accuracy are as follows:

VERY HIGH PRECISION: +/- 2 PPM @ 25 deg. C*
HIGH PRECISION: +/- 5 PPM @ 25 deg. C*
PRECISION: +/- 10 PPM @ 25 deg. C*
SEMI-PRECISION: +/- 20 PPM @ 25 deg. C
LOW PRECISION: +/- 30 PPM @ 30 deg. C
NON-PRECISION: +/- 100 PPM @ 25 DEG. C

* Due to the non-linearity of Delta F vs. Load Capacitance, calibration
accuracy =/< 10 PPM is not recommend. Load capacitances =/> 20 pF are
recommended.

Resonator Reactance vs. Frequency

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