Crosstalk analysis in high speed PCB design

At present, the increasingly fine semiconductor process makes the transistor size smaller and smaller, so the signal transition edge of the device becomes faster and faster, which leads to the problem of signal integrity and electromagnetic compatibility in the field of high-speed digital circuit system design. . Signal integrity problems mainly include transmission line effects such as reflection, delay, ringing, overshoot and undershoot of signals, and crosstalk between signals. Signal crosstalk is the most complex, involving many factors, complicated calculation and difficult to control. So today's electronic product design urgently needs new ideas, processes, methods and techniques that are different from traditional design environments, design processes and design methods.

EDA technology has developed a suite of high-speed PCB and board-level system design analysis tools and methodologies that cover all aspects of high-speed circuit design analysis: static timing analysis, signal integrity analysis, EMI/EMC design, and ground-reflex analysis , power analysis and high speed routers. It also includes signal integrity verification and Sign-Off, design space detection, interconnection planning, interconnection of electrical rules constraints, and technical methods such as expert systems. It also provides the possibility to solve signal integrity problems with high efficiency. Signal integrity analysis and design is the most important high-speed PCB board-level and system-level analysis and design tools, playing an increasingly important role in hardware circuit design. Here we will discuss signal crosstalk in signal integrity problems.

Crosstalk solution

An undesired noise voltage signal between signals due to mutual coupling of electromagnetic fields is called signal crosstalk. Crosstalk beyond a certain value may cause the circuit to malfunction and cause the system to malfunction. The problem of solving crosstalk problems can be considered from the following aspects:

a. Reduce the conversion rate of the signal edge if possible

Generally, when the device is selected, the slower device is selected while satisfying the design specifications, and different kinds of signals are mixed, because the rapidly converted signal has a potential crosstalk risk to the slowly converted signal.

b. Use shielding measures

Providing a packet for high-speed signals is an effective way to solve the crosstalk problem. However, the package will cause an increase in the amount of wiring, making the originally limited wiring area more crowded. In addition, the ground shield should achieve the intended purpose, the grounding point spacing on the ground is critical, generally less than twice the length of the signal change. At the same time, the ground line also increases the distributed capacitance of the signal, so that the impedance of the transmission line increases and the signal edge becomes slower.

c. Reasonable layer and wiring

Reasonably set the wiring layer and wiring pitch, reduce the parallel signal length, shorten the distance between the signal layer and the plane layer, increase the signal line spacing, and reduce the parallel signal line length (within the critical length range). These measures can effectively reduce Crosstalk.

d. Set different wiring layers

Setting different wiring layers for signals of different speeds and setting the plane layer reasonably is also a good way to solve crosstalk.

e. Impedance matching

If the impedance of the near-end or far-end termination of the transmission line matches the impedance of the transmission line, the amplitude of the crosstalk can be greatly reduced.

The purpose of crosstalk analysis is to quickly discover, locate, and resolve crosstalk problems in PCB implementations. The general simulation tools are independent of the simulation analysis and PCB layout environment in the environment. After the wiring is finished, the crosstalk analysis is performed, the crosstalk analysis report is obtained, the new wiring rules are derived and rewiring, and then the analysis is corrected.

Through simulation analysis, we can see that the actual crosstalk results are different and the gap is very large. Therefore, a good tool should not only be able to analyze crosstalk, but also be able to apply crosstalk rules for routing. In addition, general wiring tools are limited to physical rule driving, and wiring for controlling crosstalk can only be constrained by physical rules such as setting line width and line spacing, and maximum parallel line length. The Signal Integrity Analysis and Design Tool Set ICX can support true electrical rule-driven routing. Simulation analysis and routing are done in one environment. Electrical rules and physical rules can be set during simulation, and automatically calculated while wiring. Signal integrity factors such as overshoot, crosstalk, etc., and automatically correct the wiring based on the calculated results. This wiring speed is fast and truly meets the actual electrical performance requirements.