Case analysis! Why can't crystal oscillator be placed on edge of PCB?

For driving recorder, you need to add an external adapter during test. When machine is turned on and tested, it is found to exceed standard. Specific frequencies: 84MHz, 144MHz and 168MHz. It's necessary. analyze cause of excessive radiation and take appropriate countermeasures. The radiation test data is as follows:

Emission source analysis

This product has only one PCB with 12MHz crystal. Among them, over-standard frequency points are 12MHz multipliers, and analysis of screen and camera of machine prone to electromagnetic radiation exceeding standard shows that LCD-CLK is 33MHz, and MCLK camera is 24MHz; by elimination, it was found that after removal of chamber, point of excess of standard still exists. However, due to shielding of 12 MHz crystal, overshoot point is reduced, so 144 MHz overshoot point is considered to be related to crystal, and PCB layout is as follows:

Principle of radiation generation

You can see from PCB layout that 12 MHz crystal is simply placed on edge of PCB. When a product is placed in a radiation test environment, a certain capacitance is formed between high-speed device of product under test and reference ground in laboratory. The connection creates parasitic capacitance, resulting in common mode radiation. capacitance lies in distribution of electric field between crystal and reference ground. When voltage between them is constant, greater electric field distribution between them, greater electric field strength between them and greater parasitic capacitance.The electric field distribution of crystal, when crystal is at edge of PCB and in middle of PCB, looks like as follows:

It can be seen from figure that when crystal oscillator is located in middle of PCB or far from edge of PCB, due to presence of a working ground plane (GND) on PCB, most of electric field is controlled between crystal oscillator and working ground, then there is an electric field inside printed circuit board, distributed over reference ground plane, is significantly reduced, resulting in a reduction in radiated interference.

Remedy

Move crystal oscillator inward so that it is at least 1 cm away from edge of PCB, and apply copper to surface of PCB within 1 cm of crystal oscillator, and at same time Time of passage of copper on surface layer through through hole and grounding of printed circuit board. The planes are connected. The spectrogram of modified test result is as follows, it can be seen from figure that emitted radiation has improved significantly.

Thinking and enlightenment

High-speed printed lines or capacitive coupling between devices and reference ground plane will cause EMI problems, and sensitive printed lines or devices placed on edge of PCB will cause immunity problems.

If design needs to be placed on edge of PCB for other reasons, then work ground wire can be placed on edge of printed line, and additional vias can be added to connect work ground wire. with working ground plane.