Frequency accuracy is an important parameter when selecting a reference clock for any application. In general frequency accuracy is the difference in the measured value of the crystal/XO/TCXO/OCXO frequencies from the ideal expected value.
Following factors contribute to the accuracy measurement:
Frequency of a crystal/XO/TCXO/OCXO is measured after placing the device in a temperature-controlled chamber and then varying the temperature from -40°C to 85°C. Then the frequency accuracy at a particular temperature is calculated in ppm as follows:
Facc = Frequency accuracy at particular temperature
Ft = Frequency at particular temperature
Fref = Frequency at 25°C
This Ref freq can be calculated using any of the following two techniques (this is usually specified in the datasheet for the device):