Most of the quartz products are used as an electrical circuit component for frequency selection and/or frequency control, so the frequency-temperature characteristic of the devices is the most important parameter. This parts per million (ppm) level stability of frequency-temperature characteristic is another merit of quartz frequency device that LCR discret component oscillation circuitry can not be achieved in mass-production scale. For the usually used quartz crystal cuts, their frequency-temperature characteristics are shown in (Fig. 1).

﹝Fig.1﹞Frequency-temperature characteristics of various quartz cuts.

AT-cut is the most popular crystal cut in the quartz devices for MHz applications. (Fig. 2) shows the orientation of AT plat from top view of + X-axis.

( Fig.2) Orientation of AT plat

(Fig. 3) shows the frequency-temperature characteristics of the AT-cut crystal operating in thickness-shear mode, with the cut angle deviation as a parameter. It is shown that AT-cut quartz has excellent frequency stability over a wide temperature range since the first-and second-order of the temperature coefficients go to zero in this range and the temperature coefficient is only dominated by a third-order function of the temperature deviation.
△fs (i) = A1 (Ti –25)3 + A2 (Ti –25)2 + A3 (Ti –25) + A4

(Fig. 3) AT - cut frequency-temperature characteristics.