How to solve the radiation exceeding standard? The layout of the crystal oscillator on the PCB is a technical activity

For a driving recorder, an external adapter should be added during the test. When the machine is powered on and tested, it is found that it exceeds the standard. The specific frequency points are 84MHZ, 144MH, and 168MHZ. It is necessary to analyze the reasons for the excessive radiation and give corresponding countermeasures.

1. Problem description:

For a driving recorder, an external adapter should be added during the test. When the machine is powered on and tested, it is found that it exceeds the standard. The specific frequency points are 84MHZ, 144MH, and 168MHZ. It is necessary to analyze the reasons for the excessive radiation and give corresponding countermeasures. The radiation test data is as follows:

How to solve the radiation exceeding standard? The layout of the crystal oscillator on the PCB is a technical activity

2. Analysis of radiation source:

This product has only one PCB with a 12MHZ crystal on it. Among them, the over-standard frequency points are exactly the frequency multiplier of 12MHZ, and analyzing the screen and camera of the machine that is prone to EMI radiation exceeding the standard, it is found that the LCD-CLK is 33MHZ, and the camera MCLK is 24MHZ;

It is found by exclusion that after removing the camera, the over-standard point still exists, but by shielding the 12MZH crystal, the over-standard point is reduced, so it is judged that the 144MHZ over-standard point is related to the crystal, and the PCB layout is as follows:

How to solve the radiation exceeding standard? The layout of the crystal oscillator on the PCB is a technical activity

Three, the principle of radiation generation:

It can be seen from the PCB layout that the 12MHZ crystal is just arranged on the edge of the PCB. When the product is placed in the test environment of radiation emission, the high-speed device of the tested product and the reference ground in the laboratory will form a certain capacitive coupling, resulting in Parasitic capacitance, resulting in 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 more the electric field is distributed between the two, the greater the electric field strength between the two, and the larger the parasitic capacitance. The electric field distribution of the crystal at the edge of the PCB and in the middle of the PCB is as follows:

How to solve the radiation exceeding standard? The layout of the crystal oscillator on the PCB is a technical activity

Schematic diagram of the electric field distribution between the crystal at the edge of the PCB and the reference ground plane

How to solve the radiation exceeding standard? The layout of the crystal oscillator on the PCB is a technical activity

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

As can be seen from the figure, when the crystal oscillator is arranged in the middle of the PCB, or far 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, in the Inside the PCB, the electric field distributed to the reference ground plane is greatly reduced, resulting in lower radiated emissions.

Fourth, the treatment measures

Move the crystal oscillator inward so that it is at least 1cm away from the edge of the PCB, and apply copper within 1cm of the surface of the PCB from the crystal oscillator. At the same time, connect the copper on the surface layer to the PCB ground plane through vias. The modified spectrogram of the test results is shown below, from which it can be seen that the radiated emission has been significantly improved.

How to solve the radiation exceeding standard? The layout of the crystal oscillator on the PCB is a technical activity

V. Thinking and Inspiration

Capacitive coupling between high-speed traces or devices and the reference ground plane can cause EMI problems, and sensitive traces or devices placed on the edge of the PCB can cause immunity problems.

If the design must be arranged on the edge of the PCB due to some other reasons, then a working ground wire can be placed on the edge of the printed line, and more vias can be added to connect the working ground wire to the working ground plane.

The Links:   NL6448AC33-18K PAM2421AECADJR