Common Problems and Solutions in Crystal Oscillator Applications

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Common Problems and Solutions in Crystal Oscillator Applications [Copy link]

As we all know, there is such a vivid metaphor in the electronics industry: if the MCU is compared to the "brain" of the circuit, then the crystal oscillator is undoubtedly the "heart". Similarly, the requirements of the circuit for the "crystal oscillator" (hereinafter referred to as "crystal oscillator") are the same as the requirements of a person for the heart, and the most important thing is stability and reliability. 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, and once a problem occurs, they are helpless and lack ideas and methods to solve the problem.

Summary of the problem of crystal oscillator not oscillating

1. Wrong material parameter selection causes the crystal oscillator to not oscillate

For example: a certain MCU needs to match 6PF's 32.768KHz, but 12.5PF is used, resulting in no oscillation.

Solution: Replace the model with one that meets the requirements. If necessary, please confirm with the MCU manufacturer or us.

2. The internal crystal sheet is broken or damaged, resulting in failure to vibrate

Damage during transportation, or dropping or impact during use may cause damage to the crystal sheet inside the crystal oscillator, resulting in the crystal oscillator not vibrating.

Solution: Replace the crystal oscillator with a good one. What you need to pay attention to is: use thicker foam to wrap it during transportation to avoid damage during transportation; avoid falling, heavy pressure, collision, etc. during the process. If the above situation occurs, it is forbidden to use it again.

3. The crystal oscillator fails to oscillate due to mismatch of the oscillation circuit

Three indicators that affect the oscillation circuit: frequency error, negative impedance, and excitation level.

The frequency error is too large, causing the actual frequency to deviate from the nominal frequency, thus causing the crystal oscillator to fail to oscillate.

Solution: Choose products with appropriate PPM values.

Negative impedance that is too large or too small will cause the crystal oscillator to fail to oscillate.

Solution: If the negative impedance is too large, you can increase the value of the crystal oscillator's external capacitors Cd and Cg to reduce the negative impedance; if the negative impedance is too small, you can reduce the value of the crystal oscillator's external capacitors Cd and Cg to increase the negative impedance. Generally speaking, the negative impedance value should be no less than 3-5 times the nominal maximum impedance of the crystal oscillator.

Excessive or insufficient excitation level will cause the crystal oscillator to fail to oscillate.

Solution: Adjust the excitation level of the oscillator circuit to the crystal oscillator output by adjusting the size of Rd in the circuit. Generally speaking, the smaller the excitation level, the better. In addition to low power consumption, it is also related to the stability of the oscillator circuit and the service life of the crystal oscillator.

4. Impurities or dust on the crystal sheet inside the crystal oscillator may also cause the crystal oscillator to not vibrate.

One of the processes of crystal oscillator is to plate the electrode on the crystal sheet, that is, to plate a layer of gold or silver electrode on the crystal sheet, which requires to be completed in a Class 10,000 dust-free workshop. If dust particles in the air are attached to the electrode, or gold or silver residue remains on the electrode, the crystal oscillator will not vibrate.

Solution: Replace the crystal oscillator with a new one. When choosing a crystal oscillator supplier, you need to consider the manufacturer's equipment, workshop environment, technology and process capabilities, which are related to the quality of the product.

5. The crystal oscillator leaks and does not vibrate

The crystal oscillator is required to be evacuated and filled with nitrogen during the manufacturing process. If the pressure seal is poor, the crystal oscillator will have poor airtightness and leakage. Or the crystal oscillator may have poor airtightness due to mechanical stress caused by product cutting during the welding process. Both of these will cause the crystal oscillator to fail to oscillate.

Solution: Replace the crystal oscillator with a good one. Be sure to standardize the process and welding process to avoid product damage caused by misoperation.

6. The temperature is too high or the time is too long during welding, which causes the internal electrical performance indicators of the crystal oscillator to be abnormal and cause the crystal oscillator to fail to vibrate.

Taking the 32.768KHz plug-in type as an example, solder with a melting point of 178°C is required. If the temperature inside the crystal exceeds 150°C, it will cause the crystal characteristics to deteriorate or fail to oscillate. When soldering the pins, it should be within 5 seconds at 280°C or within 10 seconds at 260°C.

Do not solder directly at the root of the pin, as this will also cause the crystal oscillator characteristics to deteriorate or even fail to oscillate.

Solution: The welding process must be standardized, and the welding time and temperature must be set in accordance with the requirements of the crystal oscillator. If you have any questions, please contact us for confirmation.

7. Improper storage environment causes the electrical performance of the crystal oscillator to deteriorate and cause it to fail to oscillate.

Long-term use or storage under high temperature, low temperature or high humidity conditions will cause the electrical performance of the crystal oscillator to deteriorate and may cause it to fail to oscillate.

Solution: Use and store the device under normal temperature and humidity conditions as much as possible to prevent the crystal oscillator or circuit board from getting wet.

8. MCU quality problems, software problems, etc. cause the crystal oscillator to fail to oscillate

Solution: There are a lot of different MCUs on the market, such as new, refurbished, disassembled, and OEM products. If you don’t have certain industry experience or choose a regular supplier, you are likely to buy counterfeit products. This will easily cause problems in the circuit, resulting in the oscillation circuit not working. In addition, even if it is a genuine MCU, if there is a problem with the burning program, it may also cause the crystal oscillator to fail to oscillate.

9. EMC problem causes crystal oscillator to fail to oscillate

Solution: Generally speaking, metal packaged products are better than ceramic packaged products in terms of anti-electromagnetic interference. If the EMC of the circuit is large, metal packaged products should be used as much as possible. In addition, do not run signal lines under the crystal oscillator to avoid interference.

10. Other problems cause the crystal oscillator to fail to oscillate

Summary of other bad problems of crystal oscillator

1. The frequency deviation exceeds the normal value.

Solution: When the center frequency of the circuit is positive, it means that CL is too small, and the values of the crystal oscillator external capacitors Cd and Cg can be increased. When the center frequency of the circuit is negative, it means that CL is too large, and the values of the crystal oscillator external capacitors Cd and Cg can be reduced.

2. The crystal oscillator becomes hot during operation and gradually stops oscillating.

Excluding the influence of the working environment temperature, the most likely situation is that the excitation level is too large.

Solution: Reduce the excitation level DL and increase Rd to adjust DL.

3. The crystal oscillator gradually stops vibrating during operation. Touch it with your hand or heat the crystal oscillator pin with a soldering iron to start it working again.

Solution: This happens because the negative impedance in the oscillation circuit is too small. You need to adjust the values of the crystal oscillator's external capacitors Cd and Cg to meet the loop gain of the oscillation circuit.

4. The crystal oscillator is poorly soldered or the pins and pads are not tinned.

This situation usually occurs when the pins are oxidized or the pin plating falls off.

Solution: The storage environment of the crystal oscillator is very important. Store it at room temperature and humidity to avoid moisture. In addition, the coating on the crystal oscillator pins may fall off, which may be related to the process technology of the crystal oscillator manufacturer or SMT manufacturer, and further confirmation is needed.

5. The same product was tried with products from two different crystal oscillator manufacturers, and the results were different.

This situation is easy to understand. Different manufacturers have different materials, process technologies, etc., which will lead to some differences in specifications. For example, for the same +/-10ppm frequency deviation, A may be mostly positive, while B may be mostly negative.

Solution: Generally speaking, in this case, if it is a RF product, it is best to ask the crystal oscillator manufacturer to help do some circuit matching tests 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 oscillator shell falls off.

Sometimes the crystal shell will fall off after reflow soldering; sometimes the shell falls off because the crystal oscillator is hit by external force.

Solution: Before the SMT factory reflows the crystal oscillator, please fully confirm whether the furnace temperature curve meets the crystal oscillator's furnace requirements. Generally speaking, the datasheet provided by regular crystal oscillator manufacturers will provide reference values.

If the shedding is caused by external factors, try to avoid this situation.

7. Other adverse issues

Crystal oscillator design, suggestions during the process

1. When wiring the PCB, the crystal oscillator circuit should be as short and straight as possible and as close to the MCU as possible. Try to reduce the influence of stray capacitance in the oscillation circuit on the crystal oscillator.

2. When wiring the PCB, try not to run the signal line under the crystal oscillator to avoid electromagnetic interference to the crystal oscillator, which will cause the oscillation circuit to be unstable.

3. If your PCB is relatively large, try not to design the crystal oscillator in the middle, but rather closer to the edge. This is because if the crystal oscillator is designed in the middle, it will be affected by the mechanical tension caused by the deformation of the PCB, which may cause defects.

4. If your PCB is relatively small, it is recommended that the crystal oscillator be designed as close to the center as possible, rather than at the edge. This is because the PCB is small, and generally SMT reflow soldering is done with multiple panels. The mechanical tension generated during panel separation will affect the crystal oscillator and may cause defects.

5. When selecting the model and specifications of the crystal oscillator, engineers should try to confirm with large crystal oscillator manufacturers or professional agents to avoid choosing uncommon sizes or indicators, which will lead to fewer supply channels and long batch supply cycles, affecting production, and will also be passive in terms of price.

6. It is generally not recommended to use ultrasonic cleaning for circuit boards with crystal oscillators to avoid resonance and damage to the crystal oscillator, resulting in malfunctions.

Although the general crystal oscillator is relatively cheap and not so noticeable in the circuit, the crystal oscillator is now more and more valued by engineers. The most direct reason is that if the crystal oscillator is abnormal, it often drives engineers crazy and they are often helpless. Therefore, it is particularly important to choose a good crystal oscillator supplier.