Reasons below will be considered if actual oscillation frequency is way shift from nominal frequency.
- Actual drive level of crystal unit is over its maximum value specified.
- Actual load capacitance differs from specified value in the specification
- Oscillation is not normal
3-1. Actual drive level of crystal unit is over its maximum value specified.
It is important actual drive level of crystal unit is within drive level specification.
Excessive drive level may lead higher oscillation frequency or larger R1.
Please refer how to measure drive level below.
If you want to adjust the drive level lower, you may take measures below.
||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.
3-2. Actual load capacitance differs from specified value in the specification
Oscillation frequency of the crystal unit is sorted with load capacitance specified in its specification.
Therefore, actual oscillation frequency may be different from nominal frequency of the crystal unit if actual load capacitance is different from load capacitance specified in the specification.
You can adjust this frequency difference by measures below.
||Adjust external load capacitance
||To change external load capacitance large, actual oscillation frequency becomes low.
Please pay attention that oscillation margin will be low if external load capacitance is large.
Oscillation amplitude may be small by large external load capacitance.
||Change crystal unit which is specified different load capacitance
||To apply crystal unit specified with large load capacitance, actual oscillation frequency becomes high.
Ex.) You need 30MHz frequency and applied a crystal unit specified 30MHz as nominal frequency with 6pF as load capacitance.
But you confirm actual oscillation is 30ppm low from 30MHz.
It seems that load capacitance on actual board is larger than 6pF. So you change crystal unit specified 30MHz with 8pF as load capacitance. By this change, actual oscillation frequency is 5ppm low from 30MHz, and you can adjust frequency difference.
3-3. Oscillation is not normal
It is possible that oscillation circuit works at not around nominal frequency of crystal unit.
It is called "irregular oscillation", and it might happen if C-MOS inverter is not unbuffer type.
It is possible to reduce the chance of irregular oscillation by adjusting damping resistance and external load capacitance.
To solve this problem fundamentally, it is necessary to apply IC which has unbuffer type C-MOS inverter.
When you find irregular oscillation, please contact IC manufacturer to confirm if C-MOS inverter is unbuffer type or not.
If IC you consider is not unbuffer type, please consider to change IC to substitutional one which has unbuffer type C-MOS inverter.
In oscillation circuit constracted with C-MOS inverter, inverter which has single C-MOS, it is called "unbuffer type", is better.
The inverter which is built with multi-C-MOS or schmitt trigger type are not suitable for oscillation circuit because they will be able to start undesired oscillation without crystal unit.