Electronic Design - High-speed PCB design based on Cadence

1 Introduction

The proportion of high-speed PCB design in modern PCB design is increasing, and the design difficulty is getting higher and higher. Its solution not only requires high-speed components, but also requires the wisdom and careful work of the designer. It must be carefully studied and analyzed., Solve the existing high-speed circuit problems.

2 The basic content of high-speed PCB design

High-speed circuit design accounts for an increasing proportion of modern circuit design, and the difficulty of design is getting higher and higher. Its solution requires not only high-speed devices, but also the wisdom and careful work of the designer. The specific situation must be carefully studied and analyzed., Solve the existing high-speed circuit problems. Generally speaking, it mainly includes three aspects of design: signal integrity design, electromagnetic compatibility design, and power integrity design.

2.1 Signal integrity design

Signal integrity refers to the quality of the signal on the signal line. A signal with good signal integrity means that it has the value of the voltage level that must be reached when it is needed. Poor signal integrity is not caused by a certain factor, but by a combination of factors in board-level design. Especially in high-speed circuits, too fast switching speeds of the used chips, unreasonable layout of termination components, unreasonable circuit interconnection, etc. will all cause signal integrity problems. Specifically, it mainly includes crosstalk, reflection, overshoot and undershoot, oscillation, signal delay, etc.

2.2 Electro Magnetic Compatibility (Electro Magnetic Compatibility) design

Electromagnetic compatibility includes electromagnetic interference and electromagnetic tolerance, that is, excessive electromagnetic radiation and sensitivity to electromagnetic radiation. There are two types of electromagnetic interference: conducted interference and radiated interference. Conducted interference refers to the conduction of signals on one electrical network to another electrical network through a conductive medium in the form of electric current. The PCB mainly shows ground noise and power noise. Radiated interference means that the signal radiates out in the form of electromagnetic waves, thereby affecting another electrical network. In high-speed PCB and system design, high-frequency signal lines, chip pins, connectors, etc. may all become sources of radiation interference with antenna characteristics. The EMC design can be divided into four levels according to the importance of the design: device and PCB-level design, grounding system design, shielding system design, and filter design. Among them, the first two are the most important. Device and PCB-level design mainly includes the selection of active devices, the stacking of circuit boards, and the layout and routing. The design of grounding system mainly includes grounding method, ground impedance control, ground loop and shielding layer grounding, etc. In Cadence's simulation tool, the simulation parameters of electromagnetic interference can be set in the three directions of X, Y, Z distance, frequency range, design margin, compliance with standards, etc. This simulation is post-simulation and mainly checks whether it meets the design requirements. Therefore, when doing the preliminary work, we also need to design according to the theory of electromagnetic interference. The usual practice is to apply the design rules for controlling electromagnetic interference to every design. Each link realizes the rule-driven and control in each link.

2.3 Power integrity design

In high-speed circuits, the integrity of power and ground is also a very important factor, because the integrity of power and signal integrity are closely related. In most cases, the main cause of signal distortion is the power supply system. For example, too much ground bounce noise, improper decoupling capacitor design, poorly divided multiple power supplies or ground planes, unreasonable ground layer design, uneven current distribution, etc. will bring about power integrity problems and cause signal distortion. To the integrity of the signal. The main idea to solve the problem is to determine the power distribution system, divide the large-size circuit board into several small-size boards, determine the decoupling capacitor according to the ground bounce (ground bounce), and focus on the consideration of the entire PCB board. Aspects.

3 High-speed PCB design method

3.1 Traditional design method

Traditional design methods, before the final test, do not do any processing, basically rely on the designer's experience to complete. The problem can be found only when the prototype is tested and inspected, and the cause of the problem can be determined. In order to solve the problem, it is possible to design it from scratch again. Regardless of the development cycle or the development cost, this method that mainly relies on the designer's experience cannot meet the requirements of modern product development, let alone the high-complexity design of modern high-speed circuits. Therefore, it is necessary to use advanced design tools to qualitatively and quantitatively analyze and control the design process.

3.2 Cadence design method

More and more high-speed designs are now adopting a more effective method that is conducive to speeding up the development cycle. First, establish a set of physical design rules that meet the design performance indicators, and use these rules to restrict PCB layout and routing. Before the device is installed, the simulation design is carried out. In this kind of virtual test, designers can compare design indicators to evaluate performance. These key prerequisite factors are to establish a set of physical design rules for performance indicators, and the basis of the rules is based on model-based simulation analysis and accurate prediction of electrical characteristics. Therefore, simulation analysis at different stages is very important. . Cadence software has developed its own design process for high-speed PCB design. Its main idea is to use good simulation analysis and design to prevent problems, and try to solve all possible problems before PCB production. Compared with the traditional design process on the left, the main difference is the addition of control nodes in the process, which can effectively control the design process. It integrates schematic design, PCB layout and high-speed simulation analysis, and can solve the problems related to electrical performance in all aspects of the design. By analyzing many factors such as timing, signal noise, crosstalk, power supply structure, and electromagnetic compatibility, it is possible to optimally design the signal integrity, power integrity, electromagnetic interference and other issues of the system before placing and routing.

4 Conclusion

In the specific design process, the designers of all parts are required to work together horizontally, and all stages of the design are required to be comprehensively considered in the vertical direction. Design and simulation are run through the entire design process to achieve the controllability of the process and the quantification of specific indicators. . Only in this way can an efficient design be achieved. High-speed PCB design is a very complex system engineering. Only with the help of those that can not only calculate the physical and electrical characteristics of each component used in the design, the influence and interaction, but also must be automatically extracted from the designed PCB. EDA software tools with powerful functions, such as the establishment of models, and the simulators that provide dynamic characterization of actual design operations, can more comprehensively solve the above problems of signal integrity, electromagnetic interference, and power integrity.