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Conception électronique
Application de l'oscillateur à cristaux et conception rationnelle des PCB
Conception électronique
Application de l'oscillateur à cristaux et conception rationnelle des PCB

Application de l'oscillateur à cristaux et conception rationnelle des PCB

2021-12-28
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Author:pcb

Application of crystal oscillator and reasonaB /le PCB board Conception

We often compare the crystal oscillator to the heart of the digital circuit. C'est parce que tous les circuits numériques fonctionnent sans le signal d'horloge. The crystal oscillator directly controls the entire system. Si l'oscillateur à cristaux ne fonctionne pas, the entire system will be paralyzed, so the crystal oscillator is the decision The prerequisites for the digital circuit to start working. The crystal oscillators we often talk about are quartz crystal oscillators and quartz crystal resonators. Ils sont tous fabriqués à partir de l'effet piézoélectrique du cristal de quartz. Applying an electric field on the two electrodes of a quartz crystal will cause mechanical deformation of the crystal. Au contraire., if mechanical pressure is applied on both sides of the crystal, an electric field will be generated on the crystal. Moreover, these two phenomena are reversible. Using this feature, Appliquer une tension AC des deux côtés du cristal, Les Wafers produisent des vibrations mécaniques. At the same time, an alternating electric field is generated. This kind of vibration and electric field are generally very small, but at a certain frequency, the amplitude will increase significantly. C'est une résonance piézoélectrique., which is similar to the common LC circuit resonance.

PCB Board

As the heart of digital circuits, how does crystal oscillator play a role in smart products? For smart home products such as air conditioners, Rideaux, security, Surveillance, etc., wireless transmission modules are needed. They send modules from one end to the other through protocols such as Bluetooth, Wifi ou ZigBee, or directly control by mobile phones, and crystal oscillators are The components in the wireless module affect. The stability of the whole system, Sélectionnez donc l'oscillateur à cristaux utilisé dans le système, determines the success or failure of the digital circuit.

En raison de l'importance des oscillateurs à cristaux dans les circuits numériques, we need to be careful when using and Conceptioning:
1) There is a quartz crystal inside the crystal oscillator. When it is subjected to external impact or falling, Il est facile de provoquer une rupture continue des cristaux de quartz, which will cause the crystal oscillator to fail to vibrate. Alors..., the reliable installation of the crystal oscillator should be considered when Conceptioning the circuit.
2) Pay attention to the welding temperature during manual welding or machine welding. The crystal oscillator is sensitive to temperature, the temperature should not be too high during welding, and the heating time should be as short as possible? Reasonable crystal oscillator layout can suppress system radiation interference.

1. Problem description
This product is a field camera, L'intérieur est divisé en cinq parties: panneau de commande, sensor board, camera, SD memory card and battery. Le boîtier est un boîtier en plastique. The small board has only two interfaces: DC5V external power interface and USB interface for data transmission. After radiation test, it is found that there is a problem of harmonic noise radiation around 33MHz.

2. Analyze the problem
The shell structure of this product is made of a non-shielding material. Lors de l'essai de l'ensemble de la machine, seuls les câbles électriques et USB sont sortis du boîtier. Is the interference frequency radiated by the power cord and USB cable? Therefore, a few steps were tested separately:
(1) Only add a magnetic ring to the power cord, the test result: the improvement is not obvious;
(2) Only add a magnetic ring to the USB cable, the test result: the improvement is still not obvious;
(3) A magnetic ring is added to both the USB cable and the power cable. The test result: the improvement is obvious, and the overall interference frequency is reduced. It can be seen from the above that the interference frequency is brought out from the two interfaces. It is not the problem of the power interface or the USB interface, Mais la fréquence d'interférence interne est couplée à ces deux Interfaces. It cannot be solved by shielding only one interface. problem. Après mesure en champ proche, it was found that the interference frequency comes from a 32.Oscillateur à cristaux 768khz du tableau de commande, which produces strong spatial radiation, so that the surrounding traces and GND are coupled with 32.768KHz harmonic noise, Ensuite, quel câble est le câble d'alimentation du connecteur USB . The problem with this crystal is caused by the following two problems:
a. The crystal oscillator is too close to the edge of the board, which will easily cause the crystal oscillator to radiate noise.
b. There is a signal line under the crystal oscillator, Cela conduit facilement au bruit harmonique de l'oscillateur à cristaux couplés à la ligne de signal.
c. Le filtre est placé sous l'oscillateur à cristaux, and the filter capacitor and matching resistor are not arranged according to the signal flow direction, Cela rend le filtre moins efficace.

3. Solution
According to the analysis, the following countermeasures are drawn:
(1) The filter capacitor and matching resistor of the crystal should be placed close to the CPU chip first, away from the edge of the board;
(2) Remember not to place the ground in the crystal placement area and the projection area below;
(3) The filter capacitors and matching resistors of the crystal are arranged according to the signal flow direction, and they are placed neatly and compactly near the crystal;
(4) Place the crystal close to the chip, and the wiring between the two should be as short and straight as possible.

4. Conclusion
Nowadays, De nombreux oscillateurs à cristaux de système ont une fréquence d'horloge élevée et une forte énergie harmonique d'interférence. The interference harmonics are not only conducted from the two traces of its input and output, Mais il y a aussi des radiations de l'espace. If the layout is unreasonable, Ça pourrait être très puissant.. The problem of noise radiation is difficult to solve by other methods, Donc l'oscillateur à cristaux et la ligne de signal d'horloge.
Precautions for PCB board Conception of crystal oscillator
(1) The coupling capacitor should be placed as close as possible to the power pin of the crystal oscillator, and the order of placement: according to the direction of the power supply, Capacité maximale à minimale, and the
capacitance value of the capacitor is close to the power pin.
(2) The shell of the crystal oscillator must be grounded, which can radiate outward from the crystal oscillator and also shield the interference of external signals to the crystal oscillator.
(3) Do not wire under the crystal oscillator, ensure that the ground is completely laid, Ne pas câbler à moins de 300 mm de l'oscillateur à cristaux, so as to prevent the crystal oscillator from interfering with the performance of other wiring, Équipement et couches.
(4) The trace of the clock signal should be as short as possible, Et la largeur de ligne devrait être plus grande. Find a balance between the length of the wiring and the distance from the heat source.
(5) Do not place the crystal oscillator on the edge of the PCB board. Pay special attention to this point when Conceptioning the PCB board.