The AT-cut crystal unit changes its oscillating frequency expressed by as a cubic curve in respond to changes in temperature.

The way in which the oscillation frequency changes when crystal unit is mounted on a circuit board depends mainly on the temperature characteristics of the crystal unit itself, but the temperature characteristics of the circuit board also have a slight effect.

The following graphs show the oscillation frequency/temperature characteristics of a single crystal unit and the oscillation frequency/temperature characteristics when the crystal unit is mounted on a circuit board. (Figs. 1 and 2)

Fig.1 Oscillation frequency/temperature characteristics of a single crystal unit	Fig. 2 Oscillation frequency/temperature characteristics when the crystal unit shown in FIG.1 is mounted on a circuit board.

The difference between the above two graphs is the oscillation frequency/temperature characteristics of the circuit board alone. In this case, the characteristics are shown in the graphs below. (Fig. 3)

Fig.3 Oscillation frequency/temperature characteristics of circuit board alone (Difference between Fig. 1 and 2)