During the PCB design process, the placement of the crystal oscillator is a detail that is often overlooked yet critically important. Many may wonder: Is it acceptable to place the crystal oscillator near the edge of the PCB board? Next, we will delve into this issue through a practical case study, using facts to illustrate the severe consequences that improper crystal oscillator placement can bring.

There was once a completed circuit board design that utilized a 12MHz crystal. The designer placed it at the edge of the PCB. After the board was manufactured and assembled, radiation exceeded standards during power-on testing. This result left the design team puzzled. After careful investigation and analysis, they finally identified the root cause.

In a radiation test environment, parasitic capacitance inevitably forms between high-speed components inside the product and the external reference ground. When the crystal oscillator is placed at the edge of the PCB, it acts like opening a "gateway" for the electric field. Since the crystal oscillator generates high-frequency signals during operation, the electric field formed by these signals can easily distribute to the external reference ground via the "channel" of parasitic capacitance. This outward diffusion of the electric field significantly enhances common-mode radiation. Common-mode radiation is a common form of electromagnetic interference. It propagates outward through wires or space in the form of common-mode currents, causing interference to surrounding electronic devices. Simultaneously, it can lead to the product's own radiation exceeding standards, failing to meet relevant Electromagnetic Compatibility (EMC) regulations.

In stark contrast, if the crystal oscillator is properly placed in the middle of the PCB, the situation is entirely different. The middle area of a PCB typically has a complete and continuous ground plane, which serves as a robust "shield." When the crystal oscillator is located in the center, the electric field generated during its operation is tightly "contained" by this ground plane. Most of the electromagnetic energy is confined within the board, making it difficult to radiate outward, thereby significantly reducing radiation intensity. According to comparative data from actual tests, moving the crystal oscillator from the edge to the center of the board can even reduce radiation intensity by approximately half.

During PCB design, the principle for crystal oscillator placement must be kept in mind: Avoid placing it near the board edge whenever possible. Prioritize positioning it in the central area of the board. This not only effectively reduces the product's radiation levels and improves its electromagnetic compatibility but also minimizes interference with other electronic devices, ensuring stable and reliable operation in various complex environments.