Drive level and Aging of Crystal Resonators
DRIVE LEVEL
The Drive level is an indication of the degree to which the quartz resonator dissipates power.
The frequency stability of the quartz resonator is related to the drive level, that is,
the frequency stability varies with the drive level. Especially when the drive level is too large,
the frequency stability will be significantly deteriorated.
The drive level is generally expressed in terms of power dissipation mV.
There are rated values for different types of quartz resonators.
The drive level of the quartz resonator working in the oscillation circuit is not allowed to exceed this value,
otherwise the various indicators of the quartz resonator, especially the frequency stability,
will not meet the specified indicators. Even when the drive level is too high, the vibration of the chip is too strong,
which may "shatter" the chip.
In fact, the frequency of all crystal components varies with the drive level to some extent, and generally speaking,
the frequency of the crystal will increase slightly with the drive level.
Excessive drive levels can distort the temperature characteristics of the resonator and activate parasitic modes.
Excessive excitation makes the crystal heat and stress too much, resulting in irreversible frequency drift.
At very low drive levels (microwatts or less), the resonant resistance of the crystal element
can be much higher than at the nominal excitation level, making start-up of the oscillator difficult.
This effect is exacerbated after a period of non-operating storage, which is the drive level dependence (DLD).
Therefore, the drive level that the crystal actually uses in the circuit should not be too large or too small.
The following are commonly used values for the recommended drive level.
2mw、1mw、0.1mw（100μw）、0.05mw（50μw）、0.02mw（20μw）、0.01mw（10μw）、0.001 mw(1μw)
AGING
Under normal circumstances, the crystal oscillator will age after a period of use.
The aging of the crystal oscillator refers to the frequency drift of the vibration frequency of the crystal over time.
The rate of frequency drift can be determined by the maximum rate of change within a specified time limit to express.
In the early stage of crystal use, the aging is mainly affected by the stress release inside the component,
and the frequency gradually increases. In the later stage, affected by the adsorption of the electrode film,
the frequency changes in the decreasing direction according to the logarithmic relationship,
and the amount of change gradually decreases with the change of time.
Molecules of contaminants and residual gases can deposit on the wafer and oxidize the crystal electrodes.
The higher the oscillation frequency and the thinner the wafer used, the greater the effect.
So, what are the reasons for the aging of the crystal oscillator?
The aging of crystal oscillators is caused by various reasons such as stress, pollutants, residual gases,
structural process defects and other problems during the production of crystals.