High-speed signals have become the cornerstone of modern PCB design, reflecting their critical role in a wide range of applications. As a PCB engineer, gaining expertise in high-speed signal design is a continuous process that involves both hands-on experience in real-world projects and a commitment to keeping up-to-date with the latest advancements and best practices in the field.

### Classification of High-Speed Signals

High-speed signals can be categorized in several ways depending on their physical interface and logic levels:

– **Physical Interface Classification**: Common high-speed signal types based on physical connectors include USB, RJ45, S-Video, VGA, DVI, HDMI, PCIe, PCI, and SAS/SATA. Each of these standards has specific requirements in terms of signal integrity, transmission speed, and compatibility.

– **Logic Level Classification**: High-speed signals can also be classified by their logic levels, such as LVDS (Low-Voltage Differential Signaling), CML (Current Mode Logic), and PECL (Positive Emitter Coupled Logic). These classifications define the electrical characteristics of the signals and influence how they are routed and managed on the PCB.

### High-Speed Signal PCB Design Process

Designing a PCB that can handle high-speed signals involves a structured process to ensure signal integrity and performance. A critical step in this process is the pre-simulation analysis of high-speed signals.

1. **Pre-Simulation Analysis**:

The design process begins with the division of the hardware circuit modules and a preliminary layout of the PCB structure. At this stage, a simulation is conducted to evaluate whether the key high-speed signals meet the required quality standards. If the simulation reveals signal integrity issues, it may be necessary to revise the hardware module architecture or even reconsider the overall system design.

On the other hand, if the simulation indicates that the signal quality is acceptable, the focus shifts to refining the layout. The next steps involve creating the general module layout plan and designing the high-speed circuit board’s signal topology. This includes establishing appropriate design rules to ensure that high-speed signals are routed with minimal interference and degradation.

### Conclusion

In high-speed PCB design, the importance of simulation and signal analysis cannot be overstated. It is essential to both understand the different types of high-speed signals and apply the appropriate design methodologies to ensure optimal performance. Through careful planning, continuous learning, and the application of design best practices, PCB engineers can ensure that high-speed signals are effectively managed, leading to reliable and high-performing electronic products.

### 2. PCB Layout Design

#### 3. PCB Wiring Design

When designing a PCB layout, if discrepancies arise between the actual board layout and the previously simulated design rules, it is essential to re-simulate and assess whether high-speed signal integrity meets the required standards. A common issue is using overly strict design rules, which can lead to excessive signal loss on high-speed traces. This loss may result in insufficient signal amplitude at the receiving end, causing the circuit to fail.

### High-Speed Signal PCB Design and Processing Principles

To ensure optimal high-speed signal performance, follow these core design principles:

1. **Layer Selection**: Prioritize layers that have ground (GND) on both sides when routing high-speed signals. This provides better shielding and improves signal integrity.

2. **Minimize Signal Path Length**: Always aim to reduce the total trace length of high-speed signals to minimize signal degradation and reflection.

3. **Via Restrictions**: Limit the number of vias for high-speed signals. Ideally, a signal should only switch layers once, with a GND via added to maintain signal integrity.

4. **Connector Signal Design**: If a high-speed signal at one end of the connector is not adjacent to a GND pin, a GND via should be added in the layout to improve the return path and reduce noise.

5. **Connector Routing**: For high-speed signals in connectors, ensure the traces are routed symmetrically from the center of the connector, helping to balance signal integrity and reduce noise coupling.

6. **Signal Pair Matching**: Maintain careful control over the length of paired high-speed signals, accounting for tolerances. When necessary, include a PIN delay to ensure proper signal timing.

7. **Layer Separation for Send/Receive Signals**: Whenever possible, route transmitting and receiving signals on separate layers. If constrained by space, increase the distance between signals on the same layer to minimize cross-talk.

8. **Clearance from High-Voltage Signals**: Maintain a minimum distance of 180 mils from 12V traces and 65 mils from clock signals to prevent interference and ensure clean signal transmission.

### Knowledge Required for High-Speed Signal PCB Design

PCB design engineers should master the following key areas to effectively design for high-speed signals:

1. **Signal Integrity Fundamentals**:

– **Transmission Line Theory**: Understand the behavior of signals as they propagate along PCB traces.

– **Impedance Control**: Properly manage trace impedance to match the source and load impedances, ensuring minimal reflection.

– **Reflection and Crosstalk Management**: Apply design techniques to prevent signal reflection and minimize crosstalk between traces.

2. **Power Integrity**:

– **Power Supply Noise Theory**: Understand the sources of noise in power supply systems and their impact on high-speed signals.

– **Filtering Techniques**: Design effective filters to mitigate power supply noise and stabilize signal quality.

3. **PCB Materials**:

– **Copper Foil Characteristics**: Gain knowledge of the electrical properties of copper used in PCB manufacturing, such as conductivity and thickness.

– **Board Substrate Properties**: Understand the electrical characteristics of PCB substrates (e.g., FR4, Rogers) and their impact on signal transmission.

4. **Signal Topology**:

– **Common Bus Architectures**: Familiarize yourself with common signal routing topologies such as point-to-point, daisy-chain, and star topologies.

– **PCB Routing Techniques**: Understand the principles behind different routing methods and how they affect signal performance and integrity.

By integrating these principles and concepts, PCB designers can create reliable, high-performance high-speed circuits. Effective simulation and layout planning, along with a deep understanding of signal and power integrity, are critical to achieving success in high-speed PCB design.