The frequency difference, positive or negative, (expressed in Hz) which should be added to the nominal frequency when setting the oscillator at the specified temperature (usually 25°C). The purpose is to minimize the frequency deviation over the entire operating temperature range. Each oscillator will have it's own offset frequency marked on the case. In the following example Fig 1., the dotted line shows that if the oscillator is set to nominal frequency at 25°C, the frequency deviates from -1.7 ppm to +0.3 ppm over the temperature range. By setting the frequency at 25°C to the specified offset frequency, the overall frequency deviation is reduced to ±1.0 ppm.
Specified a stability vs. temperature range, for example ±1 ppm over -40°C to 85°C, this means a total peak deviation for 2 ppm over the temperature range, not referenced to the frequency at any specific temperature. If a reference, such as room temperature, is desired with a maximum allowed deviation of ±1 ppm from that reference, the specification should clearly state ±1 ppm over -40°C to 85°C referenced to the frequency @25°C. The oscillator frequency is often deliberately offset a room temperature to minimize the largest deviation from nominal frequency over the whole temperature range. Furthermore, it should be noted that the frequency vs. Temperature characteristics is not linear, perturbations in the crystal characteristics (activity dips) make it virtually impossible to guarantee exceptional stability on a per degree basis in TCXOs.
Frequency adjustment range
This range indicates the width of frequency that can be adjusted by a trimmer built in the oscillator.
If the frequency of a TCXO is measured at one temperature and if this temperature is changed and them returned to the original temperature and the frequency is measured again, the two frequencies will not be exactly the same. The difference between the two frequencies is called "thermally induced hysteresis". The phenomena has two components, static and dynamic. The static component is present even if the unit is allowed to stabilize at temperature for a long time. whereas the dynamic effect is a function of the rate of change of temperature and is a transient effect. The effect on a typical TCXO frequency / temperature characteristic is shown beside. Typical values are less than 0.2ppm.
Ageing is the slow change of the oscillator frequency with time, The primary causes of mass transfer and stress relaxation mechanisms in the crystal unit can be reduced: