Electronic Design - Application of crystal oscillator and reasonable PCB board design

Application of crystal oscillator and reasonable PCB board design

We often compare the crystal oscillator to the heart of the digital circuit. 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. The crystal oscillators we often talk about are quartz crystal oscillators and quartz crystal resonators. They are all made by using the 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, an electric field will be generated on the crystal. Moreover, these two phenomena are reversible. Using this feature, applying alternating voltage on both sides of the crystal, the wafer will produce mechanical vibration. 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. This is piezoelectric resonance, which is similar to the common LC circuit resonance.

As the heart of digital circuits, how does crystal oscillator play a role in smart products? For smart home products such as air conditioners, curtains, security, monitoring, etc., wireless transmission modules are needed. They send modules from one end to the other through protocols such as Bluetooth, WIFI or ZIGBEE, or directly control by mobile phones, 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, determines the success or failure of the digital circuit.

Due to the importance of crystal oscillators in digital circuits, we need to be careful when using and designing:
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, which will cause the crystal oscillator to fail to vibrate. Therefore, the reliable installation of the crystal oscillator should be considered when designing 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, which is internally divided into five parts: control board, sensor board, camera, SD memory card and battery. The outer shell is a plastic shell. 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. Only the power cord and USB cable lead out of the shell in the whole machine test. 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, but the internal interference frequency is coupled to these two interfaces. It cannot be solved by shielding only one interface. problem. After near-field measurement, it was found that the interference frequency comes from a 32.768KHz crystal oscillator of the control board, 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. 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, which easily causes the signal line to couple the harmonic noise of the crystal oscillator.
c. The filter device is placed under the crystal oscillator, and the filter capacitor and matching resistor are not arranged according to the signal flow direction, which makes the filtering effect of the filter device worse.

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, many system crystal oscillators have high clock frequency and strong interference harmonic energy. The interference harmonics are not only conducted from the two traces of its input and output, but also radiated from space. If the layout is unreasonable, it is likely to be very strong. The problem of noise radiation is difficult to solve by other methods, so the crystal oscillator and CLK signal line layozut.
Precautions for PCB board design 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, 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, 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, and do not wire within the 300mil range of the crystal oscillator, so as to prevent the crystal oscillator from interfering with the performance of other wiring, devices and layers.
(4) The trace of the clock signal should be as short as possible, and the line width should be larger. 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 designing the PCB board.