Low-cost 32.768 kHz crystal oscillators typically have a parabolic frequency dependency over temperature, as shown in Figure 1, for which the crystal manufacturers specify:
- A temperature coefficient (B) in ppm/°C2
- A turnover temperature (T0) in °C
Figure 1. Crystal Frequency Drift with Temperature
The temperature coefficient B is negative, meaning that the crystal oscillator slows down at cold and hot temperatures. The temperature coefficient has a tolerance; typically this is ±15%. The turnover temperature, T0, is typically 25°C, with a tolerance typically in the range ±5°C. The frequency drift at temperature T is given by:
As an example, assume B = -0.035 ppm/°C2 ±15%, T0 = 25°C ±5°C, and T = 85°C. This leads to a typical drift of: