Dry information | Why can’t the crystal oscillator be placed on the edge of the PCB?

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When laying out the crystal oscillator, it is generally not allowed to be placed at the edge of the PCB. Today we will explain this with a practical example.

A dashcam needs to be tested with an external adapter. When the machine is powered on and tested, it is found that the radiation exceeds the standard. The specific frequencies are 84MHz, 144MHz, and 168MHz. It is necessary to analyze the reasons for the excessive radiation and give corresponding countermeasures. The radiation test data is as follows:

Figure 1: Radiation test data

1. Radiation source analysis

The product has only one PCB with a 12MHz crystal on it. The frequency points that exceed the standard are all multiples of 12MHz. When analyzing the screen and camera of the machine that are prone to EMI radiation exceeding the standard, it was found that the LCD-CLK is 33MHz and the camera MCLK is 24MHz.

Through the process of elimination, it is found that after removing the camera, the over-standard point still exists, and by shielding the 12MHz crystal, the over-standard point is reduced. Therefore, it is judged that the 144MHz over-standard point is related to the crystal. The PCB layout is as follows:

Figure 2: PCB layout diagram

2. Principle of radiation generation

It can be seen from the PCB layout that the 12MHz crystal is placed exactly on the edge of the PCB. When the product is placed in a radiation emission test environment, the high-speed device of the product under test will form a certain capacitive coupling with the reference ground in the laboratory, resulting in Parasitic capacitance leads to common mode radiation. The larger the parasitic capacitance, 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 two is constant, the electric field distribution between the two The more there are, the greater the electric field intensity between the two, and the greater the parasitic capacitance. The electric field distribution when the crystal is at the edge of the PCB and in the middle of the PCB is as follows:

Figure 3: Schematic diagram of electric field distribution between the crystal oscillator at the edge of the PCB and the reference ground plate

Figure 4: Schematic diagram of the electric field distribution between the crystal oscillator and the reference ground plane in the middle of the PCB

As can be seen from the figure, when the crystal oscillator is placed in the middle of the PCB or far away from the edge of the PCB, due to the existence of the working ground (GND) plane in the PCB, most of the electric field is controlled between the crystal oscillator and the working ground, that is, inside the PCB. The electric field distributed to the reference ground plane is greatly reduced, resulting in reduced radiation emission.

3. Treatment measures

Move the crystal inwards, making it at least 1cm away from the edge of the PCB, and apply copper to the surface of the PCB within 1cm of the crystal, and connect the surface copper to the PCB ground plane through vias. The modified test result spectrum is as follows, from which it can be seen that the radiation emission has been significantly improved.

4. Thoughts and inspirations

Capacitive coupling between high-speed printed lines or devices and the reference ground plane will cause EMI problems, and placement of sensitive printed lines or devices at the edge of the PCB will cause immunity problems.

If the design must be arranged at the edge of the PCB due to some other reasons, then you can lay another working ground line next to the printed line and add more vias to connect this working ground line to the working ground plane.

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