Frequency Drift
Thermal sensitivity metrics quantify the change in the resonant frequency of an oscillator for every degree of temperature variation. The temperature coefficient of frequency provides a standardized value used to predict how much a clock signal will drift as the operating environment warms or cools. This value is expressed in parts per million per degree Celsius.
It is a critical specification for timing crystals used in cellular networks and GPS receivers. A lower coefficient indicates a more stable and reliable timing source.
Material Expansion
Changes in the physical dimensions of a resonator alter its natural vibration frequency. The temperature coefficient of frequency is largely determined by the thermal expansion of the material and the change in its elastic modulus. Quartz is chosen for many applications because it has specific cuts that offer a very low coefficient.
Parts Per Million
Precision measurement of the drift requires a high stability frequency counter and a temperature chamber. The temperature coefficient of frequency is calculated by recording the frequency at ten degree intervals. This data is used to create a compensation table for the software.
Turnover Point
Some materials exhibit a parabolic relationship between frequency and temperature. The point where the temperature coefficient of frequency reaches zero is called the turnover point. Designers aim to place this point at the expected operating temperature of the device.
