Why should the crystal oscillator not be placed on the edge of the PCB
In the test of a driving recorder, it was found that the radiation exceeded the standard during operation when it was powered by an external adapter. The specific exceedance frequency points are concentrated in 84MHz, 144MHz and 168MHz. By analyzing the test data, we need to delve into the causes of these radiation excesses and propose corresponding solutions to ensure that the electromagnetic compatibility of the equipment meets the standards.
Radiation source analysis
After the analysis of the radiation source, we found that the product of the car recorder is only a PCB board, which is equipped with a 12MHz crystal oscillator. The frequency point that exceeds the standard is exactly the frequency doubling value of 12MHz. Further analysis of components that may cause excessive EMI radiation, locking to the screen and camera. Among them, the clock frequency of the LCD is 33MHz, and the MCLK of the camera is 24MHz. After checking and testing, after removing the camera, the exceedance frequency still exists, but when we shield the 12MHz crystal, the exceedance phenomenon is improved, especially the radiation at the 144MHz frequency is significantly reduced. It can be preliminarily judged that the 144MHz superstandard is related to the 12MHz crystal oscillator. The layout structure of the PCB board is as follows:
The principle of radiation generation
By analyzing the PCB layout, it can be found that the 12MHz crystal is located right on the edge of the PCB board. When the product is placed in the test environment of radiation emission, a certain capacitive coupling will be formed between the high-speed device of the product and the reference site in the laboratory, resulting in parasitic capacitance. This parasitic capacitor will lead to the phenomenon of common mode radiation, and the larger the parasitic capacitor, the stronger the common mode radiation.
The parasitic capacitance is essentially the electric field distribution between the crystal and the reference ground. When the voltage between the crystal and the reference ground is constant, the more the electric field distribution between the two, the greater the electric field intensity, and the parasitic capacitance will also increase. Because the crystal is located at the edge of the PCB, the electric field distribution between it and the reference ground is more extensive than when it is located in the middle of the PCB, resulting in increased parasitic capacitance, further enhancing the common-mode radiation.
As can be seen from the figure, when the crystal oscillator is located in the middle of the PCB or away from the edge of the PCB, most of the electric field is limited between the crystal oscillator and the working ground due to the complete working ground plane inside the PCB. This means that the electric field is mainly concentrated inside the PCB, and the electric field distributed to the external reference ground floor is significantly reduced, thus effectively reducing the intensity of radiation emission. Therefore, placing the crystal oscillator in the central area of the PCB or away from the edge can significantly improve the problem of excessive radiation.
The treatment measures are as follows: Move the crystal oscillator to the inside of the PCB so that it is at least 1cm away from the edge of the PCB. At the same time, copper foil is laid on the PCB surface within 1cm around the crystal oscillator, and the surface copper foil is connected to the ground plane of the PCB through the hole. This optimization measure effectively improves the electric field distribution around the crystal oscillator, thus reducing the radiation emission. The improved test results show that the radiation emission in the spectrogram is significantly reduced and the effect is obviously improved.
Reflection and enlightenment
Capacitive coupling between high-speed printed wire or device and reference ground floor is often one of the main causes of EMI problems. When the sensitive printing line or device is arranged on the edge of the PCB, it is easier to produce immunity problems. Therefore, in the design, the sensitive components should be avoided at the edge of the PCB.
If these devices or printed wires must be arranged on the edge of the PCB due to design requirements, the following measures can be taken: Arrange a work ground wire next to the printed wire, and connect the work ground wire to the working ground plane of the PCB by adding multiple holes. This approach can effectively reduce the impact of capacitive coupling on EMI, improve immunity, and enhance the electromagnetic compatibility of the design.
