Crystal oscillators, or crystal oscillators, play a vital role in modern electronic design, producing precise clock signals and are the core components that keep digital circuits operating in sync. During the printed circuit board design process, the placement of the crystal oscillator, especially whether it should be avoided at the edge of the PCB, is a topic worth exploring in depth. The evaluation will be based on the following points of view.
1. Stability considerations
The core role of the crystal oscillator is to provide a highly stable and accurate frequency reference, which is the "heart" of the system clock. Therefore, ensuring the stability of their work is the primary consideration. Therefore, the crystal oscillator should not be placed on the edge of the PCB, which will reduce the risk of interference from external physical factors. For example, mechanical vibration or shock force, do not underestimate this insignificant force, which may lead to a serious degradation of equipment performance or functional failure (such as communication equipment, precision measuring instruments, etc.).
2. Electromagnetic Compatibility (EMC)
Electromagnetic compatibility is a measure of the ability of an electronic device to work properly in an electromagnetic environment without causing interference to other devices, and it is also an indicator of its ability to resist interference. When the crystal oscillator is working, it not only generates electromagnetic radiation, but also receives electromagnetic waves from the surrounding area. If it is placed on the edge of the board, the distance between the crystal oscillator and the outside world is shortened, and it is more susceptible to external electromagnetic interference (EMI), such as wireless signals, power line noise, etc., which may cause the frequency drift of the crystal oscillator and affect the accuracy of the system's timing. In addition, the crystal oscillator at the edge of the board may also become a source of electromagnetic interference to the outside world.
Schematic diagram of the electric field distribution between the crystal oscillator at the edge of the PCB and the reference ground plane
When the crystal oscillator is located in the middle of the PCB or 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 will be greatly reduced, resulting in a reduction in radiated emissions, which is better in line with the requirements of EMC regulations.
3. Signal integrity and routing optimization
In high-speed digital circuits, signal integrity refers to the ability of a signal to be transmitted without distortion and to be received correctly. The quality of the connection between the crystal oscillator and the IC associated with the system clock directly affects the quality of the clock signal. If the crystal oscillator traces are too long, this not only increases the delay of signal transmission, but also may introduce undesirable effects such as reflection and crosstalk, and reduce signal integrity. The ideal routing strategy is to keep the distance between the crystal oscillator and the associated components as short as possible, and the traces are simple and straightforward to reduce signal attenuation and distortion.
4. Considerations of manufacturing and maintainability
From a manufacturing perspective, PCB edge locations can pose a challenge for automated assembly processes such as SMT placement. Components in the edge area are more susceptible to mechanical positioning deviations, which increases assembly difficulty and reduces production efficiency. In addition, edge locations are not conducive to manual repair or troubleshooting, especially in mass production, where any factors that affect manufacturing efficiency need to be carefully considered.
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