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.
Frequency
stability
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.
Thermal hysteresis
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
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:
