Q. What factor in a oscillation circuit will be a cause of module malfunction?
Q. Please let me know a cause and measures for oscillation stop.
Q. Please let me know a cause and measures for oscillation frequency difference.
Q. If small oscillation amplitude will be a cause of trouble in oscillation circuit?
Q. It seems that oscillation frequency drift by temperature is not normal. What is a cause?
Immediate measures should be taken if module malfunction is confirmed in your development, pre-production and mass production stage. It is also required to investigate a cause to take effective measures.
We show supposed causes and measures for module malfunction caused by oscillation circuit.
In a oscillation circuit, below 4 points would be a cause of module malfunction.
You will be able to investigate a root cause and measures to check above points on malfunction module.
The module, operating based on timing clock, will not function if oscillation circuit does not work.
Please refer section "Q. Please let me know a cause and measures for oscillation stop" below.
Transaction timing error in IC or data transimiting / receiving error will occur if actual oscillation frequency is out of frequency range required by the module.
Please refer section "Q. Please let me know a cause and measures for oscillation frequency difference." below.
Clock pulse may not be generated if oscillation amplitude is insufficient. It will be a cause of malfunction of IC.
Please refer section "Q. If small oscillation amplitude will be a cause of trouble in oscillation circuit?" below.
Unusual temperature characteristics of oscillation frequency may occur if actual drive level of crystal resonator is greater than the maximum drive level specified. It might be a cause of problem for IC function.
Please refer section "Q. It seems that oscillation frequency drift by temperature is not normal. What is a cause?" below.
Please check oscillation frequency or waveform to make sure driving of oscillation circuit or frequency you want.
Please refer how to check oscillation frequency or waveform below.
How to check actual oscillation frequency
How to check oscillation waveform
If you confirm that oscillation circuit does not work, please check below points to make sure the cause.
Some ICs require to be programmed or reset to enable oscillation circuit.
Please check if any required setting is done.
Please make sure that C-MOS inverter for oscillation circuit works as inverting amplifier.
Feedback resistor (Rf) is important as bias resistor to make inverting amplifier with C-MOS inverter.
Please check Rf is integrated in IC or added on oscillation circuit.
If inverting amplifier works, DC voltage level at input / output of C-MOS inverter should be 1/2 voltage applied to C-MOS inverter.
Please check "R1" and "fL" are within specification.
Network analyzer or impedance analyzer is necessary to check those.
If characteristics of crystal resonator is normal, it is possible that insufficient oscillation margin causes the oscillation stop.
Please refer "What is oscillation margin" and "How to check oscillation margin" below.
How to check oscillation margin
We propose over 5 times of oscillation margin based on R1 specification.
We explain how to make oscillation margin large below.
Oscillation margin will be high with small external capacitance.
Lower external capacitance makes high impedance at output of C-MOS inverter.
Mentioned high impedance leads high negative resistance of oscillation circuit and increases oscillation margin.
After you change external capacitance small, please check actual oscillation frequency is within frequency range you want. Actual oscillation frequency becomes high with small external capacitance.
Oscillation amplitude is attenuated by damping resistor. Oscillation margin can be increased to change damping resistance small because of higher negative resistance.
After you change damping resistance small, please check drive level is within resonator's spec, because drive level becomes high with smaller damping resistance.
Reasons below will be considered if actual oscillation frequency is way shift from nominal frequency.
It is important actual drive level of crystal resonator 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.
| Measure 1: | 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. |
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| Measure 2: | 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. |
Oscillation frequency of the crystal resonator is sorted with load capacitance specified in its specification.
Therefore, actual oscillation frequency may be different from nominal frequency of the crystal resonator if actual load capacitance is different from load capacitance specified in the specification.
You can adjust this frequency difference by measures below.
| Measure 1: | 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. |
|
| Measure 2: | Change resonator which is specified different load capacitance |
| To apply crystal resonator specified with large load capacitance, actual oscillation frequency becomes high. Ex.) You need 30MHz frequency and applied a crystal resonator 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 resonator specified 30MHz with 8pF as load capacitance. By this change, actual oscillation frequency is 5ppm low from 30MHz, and you can adjust frequency difference. |
It is possible that oscillation circuit works at not around nominal frequency of resonator.
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.
*Unbuffer type
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 resonator.
A. If oscillation amplitude is excessively small, it might be cause of malfunction on module.
Please refer how to measure oscillation amplitude below.
How to check oscillation waveform
If you need to confirm how large amplitude is required for IC, please check it by below.
| Measure 1: | To change damping resistance small |
|
To change damping resistance small, oscillation margin becomes high due to less attenuation of amplitude. After you change damping resistance small, please check actual drive level of crystal resonator is within the spec, because drive level will be large due to small damping resistance. |
|
| Measure 2: | To change external load capacitance small |
| To change external load capacitance small, oscillation amplitude becomes large. Please pay attention that excessively small capacitance will cause of small amplitude. Please also check actual oscillation frequency because it will be high due to small load capacitance. |
|
| Measure 3: | To change external load capacitance at input side of inverter small |
| Oscillation amplitude at input side of inverter becomes large due to high impedance. As same as measure 2, please pay attention to excessive small capacitance and frequency shift. |
Below factors are considered for abnormal temperature characteristics of oscillation frequency.
Unusual temperature characteristics of oscillation frequency might be confirmed if drive level is over specified value in crystal resonator'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.
| Measure 1: | 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. |
|
| Measure 2: | 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. |
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 resonator's characteristics is normal, please check temperature characteristics of components used in oscillation circuit.
Oscillation frequency will shift if external load capacitance or stray capacitance is changed by temperature variation. Please check temperature characteristics of them.
