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1. Problem elaboration Many engineers are encountering problems such as crystal oscillator on the circuit board, vibrating for a while, not vibrating for a while, or working normally after blowing with a hair dryer. 2. Cause of the problem If this kind of problem is encountered, first of all, it is necessary to check whether the frequency parameters of the crystal oscillator and the resistance of the crystal oscillator are within the required range, if it is within the required range; Secondly, it is necessary to check whether the welding temperature of the crystal oscillator will be high, causing the second damage of the crystal oscillator. Generally, the welding temperature is required to be about 250 degrees or lower, and not more than 300 degrees as high. If these are within the scope of the requirements, it is necessary to consider whether the whole microcontroller circuit problem. 3. Coping strategies If you encounter such problems, it is easy to detect the parameters of the crystal oscillator and adjust the welding temperature. If the problem of the microcontroller circuit can be solved by the solution provider, then it is necessary to see if the crystal oscillator has air leakage. This problem will also occur in humid weather, so it is recommended to replace the crystal oscillator with better quality.
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Generally, the ripple and noise requirements of the power supply are less than 1% of the output voltage. Due to the high-precision voltage reference inside the crystal oscillator, its ripple suppression and load adjustment rate are very excellent, and the accuracy can reach 3ppm/℃. The interference of the ripple within 1% of the power system to the crystal oscillator can be ignored.
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Since the main factor affecting the short-term stability of the crystal oscillator is temperature change, under the consideration of the overall layout of the design, try to avoid the crystal oscillator close to the chassis shell or close to components with large temperature changes such as fans, and should also be away from high-power RF devices such as RF power amplifiers. In the environment of vibration or acceleration changes, the force of the crystal should also be considered to ensure uniform stress distribution and take effective damping measures such as buffering.
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Answer: Crystal oscillator (Oscillator) does not need capacitor, crystal (Crystal) needs capacitor. The relationship between the actual frequency and the nominal frequency of the crystal oscillator: Fx = F0(1 + C1/(C0 + CL))^(1/2); And CL = Cg * Cd/(Cg + Cd)+ Cs; Where Cs is the stray capacitance, Cg and Cd are the two capacitors added externally to us, usually everyone has the same value, and their pairs are connected in series and the stray capacitance is the load capacitance CL of the crystal oscillator. There is no need to think about the specific formula. We can know from it that the reduction of load capacitance can make the actual frequency Fx larger. Only Cg and Cd can be changed. Through preliminary calculation, it is found that CL can be changed by 1pF and Fx can be changed by several hundred Hz. The original circuit uses two capacitors of 33pF, which are 16.5pF in parallel. Our patch capacitors are only 27pF,33pF and 39pF, so we choose 27pF and 39pF in parallel, and the capacitor is 15.95pF. After the capacitor is welded, the measurement is more than 200Hz larger than the original, falling within the design range. Conclusion: The two capacitors on the crystal oscillator circuit can be unequal. The oscillation frequency of the crystal oscillator can be fine-tuned by fine-tuning the value of the capacitor. However, if you measure several crystal oscillators, the frequency is large or small, and the offset is large, then the crystal oscillator is unqualified. The load capacitance mentioned in the datasheet of the crystal is the sum of the capacitance connected to the two pins, the internal parasitic capacitance and the capacitance introduced by the layout. If the matching is appropriate, the frequency offset will be smaller. Of course, the actual individual differences need to be adjusted appropriately. The specific calculation formula has been given above, but for the load capacitance of the crystal oscillator =[(Cd * Cg)/(Cd + Cg)]+ Cic + △C, where Cd,Cg is the capacitance connected to the two legs of the crystal oscillator and to ground respectively, Cic (internal capacitance of integrated circuit) + △C (capacitance on PCB). That is to say, if the load capacitance is 15pf, the two sides are connected to 27pf. Generally, A is the Cic mentioned in 6.5 ~ 13.5pF. Personally, I understand that it should be the parasitic capacitance between the chip leads inside the crystal. This capacitance is generally large at 7pf (the value given by the datasheet) and actually at 3 ~ 5pF. This value should be taken into account when calculating the values of the two external capacitors. The resistance value of the resistor connected between the two pins of the crystal is generally several megohms. The principle is also mentioned kinpoagilent. In actual batch product application, if there is no easy vibration, it is generally effective, but in most cases, it is not necessary to have it inside the IC. As to whether the crystal shell needs to be grounded during production, there is no difference from the perspective of mass production and shipment of our products. When visiting the crystal manufacturer, there is a product that specially places an insulating pad on the crystal shell to separate the crystal from the board. It is said that there is such a requirement from the customer. I wonder if grounding has a clear influence. We can discuss it. When some crystals are in use, a resistor will be connected in series on the crystal input pin XI, with a resistance value of tens to hundreds of ohms. Some people say that the input level is improved, and the frequency offset accuracy will be affected during actual debugging. I don't know that expert is familiar with this, please give me more advice. When designing, the bottom of the crystal and the input and output leads should generally be as complete as possible.
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