Below factors are considered for abnormal temperature characteristics of oscillation frequency.
- Excessively high drive level
- Unusual characteristics of crystal unit
- Influence of temperature characteristics of oscillation circuit components
5-1. Excessively high drive level
Unusual temperature characteristics of oscillation frequency might be confirmed if drive level is over specified value in crystal unit's specification.
It is called "Dip" or "Activity dip". This phenomenon might happen with excessively high drive level.
As shown in figure below, temperature characteristics of AT-cut crystal is usually smooth third-order curve.
If there is activity dip, temperature characteristics of oscillation frequency will be distorted.
To avoid occuring activity dip, it is necessary to reduce drive level.
||To change damping resistance large
||By changing damping resistance large, output amplitude of inverting amplifier is attenuated and actual drive level becomes low.
By this change, oscillation margin will be down. So it is better to check if oscillation margin is over 5 times.
Also, you need to pay attention oscillation amplitude does not become excessively small.
||To change external load capacitance small
||By changing external load capacitance small, actual drive level becomes low due to high impedance of oscillation circuit.
In this case, actual oscillation frequency becomes high due to small load capacitance. So it is better to check if actual oscillation frequency is within frequency range you want.
5-2. Unusual characteristics of crystal unit
Please check temperature characteristics of load resonance frequency if it is within specification of frequency - temperature characteristics or not. Network analyzer or impedance analyzer is necessary for this measurement.
If crystal unit's characteristics is normal, please check temperature characteristics of components used in oscillation circuit.
5-3. Influence of temperature characteristics of oscillation circuit components
Oscillation frequency will shift if external load capacitance or stray capacitance is changed by temperature variation. Please check temperature characteristics of them.