Typical tuning fork crystals are usually cut to center the nominal frequency at 25°C. Above and below 25°C, the frequency will decrease with a parabolic characteristic, as shown in Figure 2. The frequency shift is given by Figure 1, where f0 is the target frequency at T0 (typically 32768 Hz at 25°C) and the PPM is the temperature tolerance coefficient given by the crystal data sheet.
Equation 1. Effect of Temperature
Variation
Figure 1. Typical Temperature vs. Frequency
Characteristics of a Crystal