Signal integrity design for crosstalk control

There are two general rules for high-speed PCB design: physical rules and electrical rules. The so-called physical rules refer to some design rules that the design engineer specifies based on physical dimensions, such as a line width of 4Mil, a line-to-line spacing of 4Mil, and a parallel trace length of 4Mil. The electrical rules refer to design rules regarding electrical characteristics or electrical properties, such as wiring delay control between 1 ns and 2 ns, total crosstalk on a PCB line is less than 70 mV, and so on.

The high-speed router can be further explored by defining physical and electrical rules. Currently, the physical rules (physical rule-driven) high-speed routers on the market include AutoActive RE routers, CCT routers, BlazeRouter routers, and Router Editor routers. In fact, these routers are physical rule-driven autorouters. That is to say, these routers can only automatically meet the physical size requirements specified by the design engineer, and cannot be directly driven by high-speed electrical rules.

Electrical rules directly driven by the high speed router is very important to ensure high-speed design signal integrity, design engineers are always the first to get electrical rules and design specifications are also electrical rules, in other words, our design must ultimately meet the electrical rules It is not a physical rule, and the final physical design meets the electrical rules of the design to be the most essential. Physical rules are only a conversion of component manufacturers or design engineers themselves to electrical rules. We always expect this conversion to be equivalent and one-to-one. This is not the case.

For example, LVDS chips are used to achieve high-speed (up to 777.76Mbps) and long-distance (up to 100M) data transmission. Since the signal swing of LVDS technology is 350mV, the usual design specifications always require the total signal line. The crosstalk value should be less than or equal to 20% of the signal swing, that is, the total amount of crosstalk is 350mV×20%=70mV. This is the electrical rule, where 20% of the percentage depends on the noise margin of the LVDS, which can be found in the reference manual. obtain.

For IS_Synthesizer, the design engineer only needs to specify the crosstalk value on the LVDS signal line, and the wiring can be automatically adjusted and refined to ensure that the electrical performance requirements are met. In the routing process, all surrounding signal lines are automatically considered. The impact of LVDS signals. For physics-based routers, some imaginary analysis and considerations are needed first. Design engineers always think that the crosstalk between signals depends only on the length of the parallel lines between the parallel signals, so it can be used in high-speed circuits. Do some imaginary analysis in the front-end environment of the design. For example, you can assume that the length of the walking line is 2.5 mils, and then analyze the crosstalk between them. This value may not be 70 mV, but you can further adjust and walk the line according to the conclusions obtained. The length, if the length of the parallel line is a certain value, such as 7mil, the crosstalk between the signals is basically 70mV, then the design engineer believes that as long as the length of the differential pair and the length of the walking line is controlled to 7mil range Within such a requirement to meet such electrical characteristics (signal crosstalk value is controlled to within 70mV), the design engineer gets the physical rules of such a high-speed PCB design in the actual physical PCB layout, and the conventional high-speed router can ensure Meet the requirements of this physical size.

There are two problems here: First, the conversion of rules is not the same. First, the crosstalk between signals is not uniquely determined by the length of the traces between parallel signals, but also depends on the direction of the signal and the position of the parallel segments. And a variety of factors such as matching, and these factors may be difficult to predict, or even impossible to fully consider before the actual physical implementation. Therefore, after such a conversion, it is not possible to ensure that the original electrical rules can be satisfied while satisfying these physical rules. This is also one of the reasons why the above-mentioned high-speed routers still do not work properly when the rules are met. Secondly, it is almost impossible to consider multiple effects at the same time when these rules are converted. For example, when considering signal crosstalk, it is difficult to simultaneously consider the influence of all relevant signal lines around. These two aspects determine that high-speed routers based on physical rules will have great problems in the design of high-speed, high-complexity PCB systems, and the high-speed PCB routers driven by real-based electrical rules are better solved. This is a problem.

High-speed PCB board-level, system-level design is a complex process, and signal integrity issues, including signal crosstalk, lead to changes in design concepts, design ideas, design flows, and design tools. Ensuring the rapid discovery of problems in high-speed system design, solving problems, and guiding the prevention of problems in new designs has become the mainstream of today's high-speed system design.