The primary advantage of an AT Cut crystal is the relatively small change in frequency over a wide temperature range. The change in frequency over temperature is usually expressed in PPM/degree Celsius and is referred to as frequency stability or temperature coefficient.
The change in frequency follows a series of 3rd degree S-Curves per the attached Beckman Chart. There is an infinite number of Curves on a Beckman Chart but only a few are shown. The angle at which the quartz plate is cut from the quartz bar , Cut-Angle determines the Frequency vs. Temperature of a crystal unit. Adjustment of the Cut-Angle allows the crystal design engineer to select the desired temperature coefficient for the application. The cutting process, being a mechanical process, has inherent issues associated with tolerances due to tool condition, wear and measurement accuracy. The net effect is a nominal temperature coefficient with a tolerance.
Other items that affect a crystals temperature coefficient of stability over temperature are drive level, quartz plate geometry , orientation in reference to the mounting structure and the interface to the mounting structure. These issues must be addressed in the design and manufacturing phases to assure the desired frequency vs. temperature of the finished resonator. Confirmation of the temperature coefficient is performed through temperature testing of the resonator in an automated system that measures and records the frequency as the temperature is slewed over the design temperature range. The slew rate is typically in 10 degree increments and the crystals are allowed to soak at the temperature for a period of seven minutes. Slew rate and soak time are adjustable within the automated test system.
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