**The Basic Principles of High-Speed PCB Signal Line Routing**
(1) **Proper Selection of Layer Count**: High-frequency circuits typically require high integration and dense routing, which makes multilayer boards essential. Multilayer boards are an effective method for minimizing interference. By carefully choosing the number of layers, it is possible to significantly reduce the PCB size, optimize the use of inner layers for shielding, achieve better grounding, reduce parasitic inductance, shorten signal transmission paths, and decrease signal crosstalk and interference. All of these factors contribute to the stable performance of high-frequency circuits. Research indicates that noise on a 4-layer board made of the same material is 20dB lower than on a comparable double-sided board. However, increasing the number of layers also complicates the manufacturing process and raises costs.
(2) **Minimize Lead Bends Between High-Speed Circuit Component Pins**: For high-frequency circuit routing, it is ideal to keep leads as straight as possible. If bends are necessary, 45° angles or rounded traces should be used, as these minimize external radiation and mutual coupling of high-frequency signals.
(3) **Minimize Lead Lengths Between High-Frequency Circuit Component Pins**: One of the most effective ways to reduce signal degradation is to minimize the distance between pins in high-speed circuits. This can be achieved by prioritizing the routing of critical high-speed signals before performing automatic routing.
(4) **Reduce Via Usage Between High-Frequency Circuit Component Pins**: Reducing via usage in component connections is crucial. Each via introduces approximately 0.5pF of distributed capacitance, and minimizing the number of vias can lead to a significant increase in signal speed and overall performance.
(5) Be mindful of cross interference when signal lines are routed in parallel at close distances: If parallel routing cannot be avoided, placing a large area of ground trace on the opposite side of the signal line can significantly reduce the interference. Parallel routing within the same layer is often unavoidable, but the routing directions of two adjacent layers should be perpendicular to each other. In high-frequency circuit designs, it is ideal to route horizontally and vertically on adjacent layers. When parallel routing within the same layer cannot be avoided, a large ground plane can be placed on the opposite side of the PCB to mitigate interference. This approach is typically used in double-sided boards. In multilayer boards, the internal power layer can serve this purpose. A copper-clad PCB not only enhances high-frequency immunity but also provides significant advantages for heat dissipation and increases the overall strength of the PCB. Additionally, using tin-plated grids on the PCB mounting holes in the metal chassis can improve the mechanical strength and ensure a solid electrical connection, while also allowing the chassis to form an effective common ground.
(6) Implement ground plane shielding for particularly sensitive signal lines or critical components. Proper shielding of clock signals and other key units is highly beneficial for the stability of high-speed systems.
(7) Signal routing should avoid forming loops or current loops.
(8) A high-frequency decoupling capacitor should be placed near each integrated circuit block.
**Ground Wire Design**
In electronic systems, proper grounding is one of the most effective methods for controlling interference. By appropriately combining shielding techniques, most interference issues can be resolved. Grounding in electronic systems typically involves system ground, chassis ground (shield ground), digital ground (logic ground), and analog ground. When designing ground traces, consider the following four key points:
1) **Choosing Between Single-Point and Multi-Point Grounding**: In circuits with low-frequency signals (typically below 1 MHz), the inductance of traces and components has minimal impact, while the impact of ground loop formation can be significant. In such cases, single-point grounding is recommended. For signals operating above 10 MHz, the impedance of the ground trace increases significantly. To reduce this impedance, multi-point grounding should be used as close to the signal source as possible. For frequencies between 1-10 MHz, if single-point grounding is used, the length of the ground trace should not exceed 1/20th of the signal wavelength; otherwise, multi-point grounding should be applied.
2) **Separate Digital and Analog Grounds**: When both high-speed digital circuits and analog circuits are present on the same PCB, they should be physically separated as much as possible. The ground traces for each should not be shared, and they should be connected to separate power supply grounds. Additionally, try to maximize the grounding area for the analog circuits to minimize noise coupling.
3) **Increase Ground Trace Width**: Thin ground traces can cause voltage fluctuations due to current changes, leading to unstable timing signals and reduced noise immunity. Therefore, it is crucial to make the ground traces as thick as possible to handle the expected current. Ideally, the ground trace width should be at least 3mm to support 3 times the maximum allowable current for the PCB.
4) **Use Closed Ground Loops for Digital Circuits**: When designing the ground system for PCBs consisting entirely of digital circuits, a closed-loop ground design is recommended. This configuration significantly enhances noise immunity by reducing the potential difference across the ground plane, which is especially important in designs with many integrated circuits or power-hungry components. A closed loop minimizes ground bounce and improves the overall noise resistance of the system.
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