Дизайн PCB - применение кварцевых генераторов и рациональное проектирование панелей PCB

применение кварцевых генераторов и рациональный выбор панель PCB проектировать

мы часто сравниваем кристаллический генератор с ядром цифровой схемы. This is because all the work of the digital circuit is inseparable from the clock signal. The crystal oscillator directly controls the entire system. If the crystal oscillator does notwork, the entire system will be paralyzed, so the crystal oscillator is the decision The prerequisites for the digital circuit to start working. кварцевый генератор - это кварцевый генератор и кварцевый резонатор.. They are all made by usчеренок piezoelectric effect of quartz crystal. Applying an electric field on the two electrodes of a quartz crystal will cause mechanical deformation of the crystal. On the contrary, if mechanical pressure is applied on both sides of the crystal, электрическое поле на кристалле. Moreover, Эти два явления обратимы. Using this feature, applying alternating voltage on both sides of the crystal, вафли производят механические колебания. At the same time, создавать переменное электрическое поле. This kind of vibration and electric field are generally very small, but at a certain frequency, the amplitude will increase significantly. Это пьезоэлектрический резонанс, which is similar to the common LC circuit resonance.

ядро цифровой схемы, how does crystal oscillator play a role in smart products? For smart home products such as air conditioners, curtains, безопасность, monitoring, сорт., wireless transmission modules are needed. They send modules from one end to the other through protocols such as Bluetooth, WIFI or ZIGBEE, или непосредственно через мобильный телефон, and crystal oscillators are The components in the wireless module affect. The stability of the whole system, so choosing the crystal oscillator used in the system, решить успех цифровых схем.

Due to the importance of crystal oscillators in digital circuits, we need to be careful when using and проектироватьing:
1) There is a quartz crystal inside the crystal oscillator. When it is subjected to external impact or falling, it will easily cause the quartz crystal to break and break, это приведет к вибрации кварцевого генератора. Therefore, the reliable installation of the crystal oscillator should be considered when проектироватьзакорачивать цепь.
2) Pay attention to the welding temperature during manual welding or machine welding. кварцевый генератор очень чувствительный к температуре, 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 фотоаппарат, внутреннее разделение на пять частей: панель управления, sensor board, camera, SD memory card and battery. корпус пластмассовый корпус. The small board has only two interfaces: DC5V external power interface and USB interface for data transmission. После радиационных испытаний, 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. при испытании машины в целом из оболочки выводятся только кабели питания и USB. 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, и общее снижение частоты помех. It can be seen from the above that the interference frequency is brought out from the two interfaces. Это не проблема интерфейса питания или USB - интерфейса, but the internal interference frequency is coupled to these two interfaces. It cannot be solved by shielding only one interface. problem. После измерения в ближней зоне, it was found that the interference frequency comes from a 32.кварцевый генератор на панели управления, which produces strong spatial radiation, so that the surrounding traces and GND are coupled with 32.768KHz harmonic noise, which is then radiated through the coupling of the interface USB line and the power line . The problem with this crystal is caused by the following two problems:
a. кварцевый генератор слишком близко к краю платы, which will easily cause the crystal oscillator to radiate noise.
b. There is a signal line under the crystal oscillator, Это легко вызывает гармонический шум кварцевого генератора с линией сигнала.
c. установка фильтра внизу кварцевого генератора, and the filter capacitor and matching resistor are not arranged according to the signal flow direction, Это снижает эффективность фильтрации.

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, соединение между двумя точками должно быть как можно короче и прямо.

4. Conclusion
Nowadays, Многие системные кварцевые генераторы имеют высокую тактовую частоту и энергию гармоник с сильными помехами. The interference harmonics are not only conducted from the two traces of its input and output, Но есть и радиация из космоса. If the layout is unreasonable, Он может быть очень сильным. The problem of noise radiation is difficult to solve by other methods, Таким образом, кристаллический генератор и линия тактовых сигналов.
Precautions for панель PCB проектировать of crystal oscillator
(1) The coupling capacitor should be placed as close as possible to the power pin of the crystal oscillator, и порядок расположения: по направлению питания, place the capacitance value from largest to smallest, and the
capacitance value of the capacitor is close to the power pin.
(2) The shell of the crystal oscillator must be grounded, Он может излучаться наружу от кварцевого генератора, а также может экранировать интерференцию внешних сигналов на кристаллический генератор.
(3) Do not wire under the crystal oscillator, ensure that the ground is completely laid, не устанавливайте линии в пределах 300 мм кварцевого генератора, so as to prevent the crystal oscillator from interfering with the performance of other wiring, оборудование и слой.
(4) The trace of the clock signal should be as short as possible, и ширина линии должна быть больше. 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 панель PCB. Pay special attention to this point when проектироватьing the панель PCB.