As we all know, there is such an image metaphor in the electronics industry: if the MCU is likened to the "brain" of the circuit, then the crystal oscillator is undoubtedly the "heart". Similarly, the requirements of the circuit for "crystal crystal oscillators" (hereinafter referred to as "crystal oscillators") are also the same as those for one's heart. The most needed is stable and reliable. The role of the crystal oscillator in the circuit is to provide the basic frequency signal for the system. If the crystal oscillator does not work, the MCU will stop and the entire circuit will not work. However, many engineers lack sufficient attention and understanding of the crystal oscillator, but once they have problems, they are helpless and lack ideas and solutions to solve the problem.
First, the crystal oscillator can not be ignited
1. The material parameter selection error causes the crystal oscillator to not vibrate
For example, an MCU needs to match 32.768KHz of 6PF, and the result is 12.5PF, which results in no vibration.
Solution: Replace the specifications that meet the requirements. Please check with the original MCU or us if necessary.
2. The internal wafer is broken or damaged, resulting in no vibration.
Damage to the transportation process, or falling or impact during use causes damage to the crystal wafer inside the crystal, which causes the crystal to not vibrate.
Solution: Replace the crystal with a good one. Usually need to pay attention to: the use of foam in the transportation process is thicker, to avoid damage in the middle; avoid falling, heavy pressure, impact, etc. during the process, once the above conditions are prohibited from reuse.
3, the oscillation circuit does not match, causing the crystal to not vibrate
Three indicators affecting the oscillating circuit: frequency error, negative impedance, and excitation level.
The frequency error is too large, causing the actual frequency to shift to the nominal frequency and causing the crystal to not vibrate.
Solution: Choose the right PPM value for the product.
If the negative impedance is too large or too small, the crystal will not vibrate.
Solution: If the negative impedance is too large, the value of the external capacitors Cd and Cg can be increased to reduce the negative impedance. If the negative impedance is too small, the values ​​of the external capacitors Cd and Cg can be adjusted to increase the negative. Sexual impedance. In general, the negative impedance value should satisfy not less than 3-5 times the nominal maximum impedance of the crystal.
If the excitation level is too large or too small, the crystal will not vibrate.
Solution: Adjust the excitation level of the oscillator circuit to the crystal output by adjusting the size of Rd in the circuit. In general, the smaller the excitation level, the better the processing power consumption, and the stability of the oscillating circuit and the lifetime of the crystal oscillator.
4. Impurities or dust on the crystal wafer inside the crystal oscillator will also cause the crystal oscillator to not vibrate.
One of the processes of the crystal oscillator is a wafer-plated electrode, that is, a gold layer or a silver electrode is plated on the wafer, which requires completion in a 10,000-class clean room. If dust particles in the air are attached to the electrode, or if gold residue remains on the electrode, it will also cause the crystal to not vibrate.
Solution: Replace the new crystal. When selecting a crystal oscillator supplier, it is necessary to consider the manufacturer's equipment, workshop environment, process and process capability, which is related to the quality of the product.
5. The crystal oscillator leaks and causes no vibration.
During the process of the crystal oscillator, the internal vacuum is required to be filled with nitrogen. If the pressure seal is poor, the crystal gas is not leaky, or the crystal oscillator is exposed during the welding process due to the mechanical stress of the product during the process of cutting the foot. Poor air tightness; both will cause the crystal oscillator to not vibrate.
Solution: Replace the crystal with a good one. It is necessary to standardize the operation during the process and welding process to avoid product damage caused by misoperation.
6. When the temperature is too high or the time is too long during welding, the internal electrical performance index of the crystal oscillator is abnormal and the crystal oscillator is not vibrated.
Taking the 32.768KHz in-line type as an example, it is required to use a solder having a melting point of 178 ° C. The temperature inside the crystal oscillator exceeds 150 ° C, which may cause deterioration of the crystal characteristics or no vibration. When soldering the pins, it is within 5 seconds at 280 °C or within 10 seconds at 260 °C. Do not solder directly at the root of the lead. This will also result in deterioration of the crystal characteristics or no vibration.
Solution: The welding process must be standardized, and the welding time and temperature should be set in accordance with the requirements of the crystal. If you have any questions, please contact us to confirm.
7. Improper storage environment causes deterioration of crystal oscillator performance and causes no vibration
Prolonged use or storage under conditions of high temperature or low temperature or high humidity may cause deterioration of the electrical properties of the crystal oscillator, which may result in no vibration.
Solution: Use and store as much as possible under normal temperature and humidity conditions to avoid crystal or circuit board moisture.
8, MCU quality problems, software problems, etc. cause the crystal oscillator to not vibrate
Solution: At present, the MCU in the market is scattered with new products, refurbished goods, dismantled goods, and OEM goods. If there is no certain industry experience or choose a regular supplier, it is very easy to buy non-authentic. Such a circuit is prone to problems, resulting in an unstable operation of the oscillating circuit. In addition, even if it is a genuine MCU, if there is a problem with the programming program, it may cause the crystal oscillator to not start.
9, EMC problems cause the crystal to not vibrate
Solution: In general, metal-encapsulated products are superior to ceramic packaging products in terms of electromagnetic interference resistance. If the EMC on the circuit is large, metal packaging products should be used as much as possible. In addition, do not take the signal line under the crystal to avoid interference.
10, other problems cause the crystal to not vibrate
Second, the other problems of crystal oscillators are summarized:
1. The frequency offset exceeds the normal value.
Solution: When the center frequency of the circuit is positively biased, it indicates that the CL is too small, and the values ​​of the external capacitors Cd and Cg of the crystal oscillator can be increased. When the center frequency of the circuit is negatively biased, it indicates that the CL is too large, and the values ​​of the external capacitors Cd and Cg of the crystal oscillator can be reduced.
2. The crystal oscillator appears hot during work and gradually stops vibrating.
Excluding the effect of the working environment temperature on it, the most likely situation is that the excitation level is too large.
Solution: Reduce the excitation level DL and increase Rd to adjust the DL.
3. The crystal oscillator gradually appears to stop vibrating during work. Touch the hand or use a soldering iron to heat the crystal oscillator pin and start working again.
Solution: This happens because the negative impedance value in the oscillating circuit is too small, and the values ​​of the external capacitors Cd and Cg of the crystal oscillator need to be adjusted to achieve the loop gain of the oscillating circuit.
4, crystal vibration welding or pins, pads do not eat tin.
In this case, the pin is generally oxidized or the lead plating is peeled off.
Solution: The storage environment of the crystal oscillator is very important, and it can be stored under normal temperature and humidity to avoid moisture. In addition, the plating of the crystal oscillator pins may fall off, which may be related to the process technology of the crystal oscillator manufacturer or SMT manufacturer, and needs further confirmation.
5, the same product trials of two different crystal oscillator manufacturers' products, the results are not the same.
It is well understood that this situation is different. Different manufacturers' materials and process technologies are different, which will lead to some differences in specifications. For example, the frequency offset of +/-10ppm is also the most likely A is the positive bias, and the most likely B is the negative bias.
Solution: Generally speaking, in this case, if it is a radio frequency product, it is best to let the crystal oscillator manufacturer do some circuit matching test to ensure the best circuit matching. If it is a non-RF product, it is generally compatible if the indicators are the same.
6. The crystal casing is peeled off.
Sometimes the crystal oscillator will fall after the reflow soldering; some are due to the crystal oscillator being damaged by external force and other reasons.
Solution: Before the crystal oscillator is reflowed, please confirm whether the furnace temperature curve satisfies the requirements of the crystal oscillator. Generally, the reference value is provided in the datasheet provided by the regular crystal oscillator manufacturer.
If it is caused by external force, try to avoid this.
7, other bad problems
Third, the crystal design, the process of the recommendations
1. When wiring the PCB, the trace of the crystal oscillator circuit should be as short as possible and as close as possible to the MCU. Minimize the effect of stray capacitance in the oscillating circuit on the crystal.
2. When PCB layout, try not to walk the signal line under the crystal oscillator to avoid electromagnetic interference to the crystal oscillator, which leads to unstable oscillation circuit.
3. If your PCB board is large, try not to design the crystal oscillator in the middle. This is because the crystal design is affected by the mechanical tension generated by the deformation of the PCB in the middle position, and may be defective.
4. If your PCB board is small, it is recommended that the crystal design position be as far as possible in the middle, not at the edge position. This is because the PCB board is small, and the general SMT over-reflow soldering is multi-ply board. The mechanical tension generated during the sub-board will affect the crystal oscillator and may cause defects.
5. When selecting the model and specification parameters of the crystal oscillator, the engineer should try to confirm with the crystal oscillator manufacturer or professional agent to avoid the selection of the size or index is not commonly used, resulting in less supply channels and long batch supply cycle, which affects production. It will also be passive in price.
6. Circuit boards with crystal oscillators are generally not recommended for ultrasonic cleaning to avoid resonance and damage to the crystal.
Although the average crystal price is relatively cheap, it is not so eye-catching on the circuit, but the crystal oscillator is now more and more valued by engineers. The most direct reason is that if the crystal oscillator is abnormal, the engineers are often mad and often helpless. So choosing a good crystal supplier is especially important.
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