PCB Blog - Analyze and control crosstalk in high speed PCB board design

This article will introduce methods to suppress and improve signal crosstalk in signal integrity analysis, as well as design strategies for signal crosstalk control driven by electrical rule-driven high speed PCB board layout technology. With the large-scale improvement of system design complexity and integration, electronic system designers are engaged in circuit design above 100MHZ, and the operating frequency of the bus has reached or exceeded 50MHZ, and some even exceeded 100MHZ. At present, about 50% of the design The clock frequency exceeds 50MHz, and nearly 20% of the designs have a main frequency of more than 120MHz. When the system works at 50MHz, transmission line effects and signal integrity problems will occur; when the system clock reaches 120MHz, unless high-speed circuit design knowledge is used, otherwise PCB boards designed based on traditional methods will not work. Therefore, high-speed circuit design technology has become a design method that electronic system designers must take. Controllability of the design process can only be achieved by using the design techniques of high-speed circuit designers.

At present, the increasingly sophisticated semiconductor technology makes the transistor size smaller and smaller, so the signal transition edge of the device is getting faster and faster, which leads to the increasingly serious problems 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, time delay, ringing, signal overshoot and undershoot, and crosstalk between signals. Signal crosstalk is complex, involving many factors, complex calculations and difficult to control. Therefore, today's electronic product design urgently needs new ideas, processes, methods and technologies that are different from traditional design environments, design processes and design methods. EDA technology is to use the computer as a tool. The designer uses the hardware description language VHDL to complete the design file on the EDA software platform, and then the computer automatically completes the logic compilation, simplification, segmentation, synthesis, optimization, layout, wiring and simulation, until Adapted compilation, logic mapping and programming for a specific target chip. The emergence of EDA technology has greatly improved the efficiency and operability of circuit design and reduced the labor intensity of designers. Using EDA tools, electronic designers can design electronic systems from concepts, algorithms, protocols, etc., and a large amount of work can be done by computer, and the entire process of electronic products from circuit design, performance analysis to IC layout or PCB layout can be designed. is automatically processed on the computer. The concept or category of EDA is now widely used. Including in machinery, electronics, communications, aerospace, chemical industry, minerals, biology, medicine, military and other fields, there are applications of EDA. At present, EDA technology has been widely used in major companies, enterprises and institutions, and scientific research and teaching departments. For example, in the aircraft manufacturing process, from design, performance testing and characteristic analysis to flight simulation, EDA technology may be involved.

Crosstalk Solutions
Crosstalk: It is the coupling between two signal lines, the mutual inductance and mutual capacitance between the signal lines that cause the noise on the line. Capacitive coupling induces coupling current, while inductive coupling induces coupling voltage. The parameters of the PCB layer, the distance between the signal lines, the electrical characteristics of the driving end and the receiving end, and the wire termination method all have a certain influence on the crosstalk. With the development of science and technology, the price of computers is getting lower and lower, the performance is getting better and better, the transmission speed of the local area network is getting faster and faster, and the transmission medium of the local area network has also shifted from coaxial cable to twisted pair and optical fiber. The original CAT1, CAT3, CAT5 have developed to the current CAT5E, CAT6, CAT6A, CAT7. Although the performance of twisted pair has been continuously improved, there is a parameter that has been accompanied by twisted pair like a ghost, and it is accompanied by twisted pair. Development, this parameter is becoming more and more important. Undesirable noise voltage signals due to mutual coupling of electromagnetic fields between signals are called signal crosstalk. If the crosstalk exceeds a certain value, the circuit may malfunction and the system will not work properly. Solving the problem of crosstalk can be considered from the following aspects:
1) Reduce the conversion rate of the signal edge if possible. Usually, when selecting the device, try to select a slow device while meeting the design specifications, and avoid mixing different types of signals, because the fast-changing signal pair Slowly changing signals have a potential crosstalk hazard.
2) Adopt shielding measures: It is an effective way to solve the problem of crosstalk to provide grounding for high-speed signals. However, cladding leads to an increase in the amount of wiring, making the otherwise limited wiring area more crowded. In addition, in order to achieve the expected purpose of the ground wire shielding, the distance between the ground points on the ground wire is very critical, generally less than twice the length of the signal change. At the same time, the ground wire will also increase the distributed capacitance of the signal, which increases the impedance of the transmission line and slows down the signal edge.
3) Reasonable setting of layers and wiring: reasonable setting of wiring layers and wiring spacing, reducing the length of parallel signals, shortening the distance between the signal layer and the plane layer, increasing the spacing of signal lines, and reducing the length of parallel signal lines (within the key length range), these measures can effectively reduce the crosstalk.
4) Setting different wiring layers: Setting different wiring layers for signals of different rates and setting the plane layer reasonably is also a good way to solve crosstalk.
5) Impedance matching: If the near-end or far-end terminal impedance of the transmission line matches the impedance of the transmission line, the magnitude of crosstalk can also be greatly reduced. The purpose of crosstalk analysis is to quickly find, locate and solve crosstalk problems in PCB implementation. In general simulation tools and environments, the simulation analysis and the PCB layout environment are independent of each other. After the wiring is completed, the crosstalk analysis is performed to obtain the crosstalk analysis, and new wiring rules are deduced and re-wired, and then analyzed and revised. It can be seen from the simulation analysis that the actual crosstalk results are not the same, 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 routing tools are only driven by physical rules, and the routing for controlling crosstalk can only be constrained by physical rules such as setting the line width and line spacing, and the length of parallel lines. Using the signal integrity analysis and design tool set ICX can support true electrical rule-driven routing. The simulation analysis and routing are completed in one environment. Electrical rules and physical rules can be set during simulation, and automatic calculation is performed during routing. Signal integrity factors such as overshoot and crosstalk are automatically corrected according to the calculated results. This wiring is fast and truly meets the actual electrical performance requirements.

Signal Integrity Design for Crosstalk Control
High-speed PCB board design rules are usually divided into two types: physical rules and electrical rules. The so-called physical rules refer to certain design rules specified by the design engineer based on physical dimensions, such as the line width is 4Mil, the spacing between lines is 4Mil, and the length of parallel traces is 4Mil. The electrical rules refer to the design rules related to electrical characteristics or electrical performance, such as the wiring delay is controlled between 1ns and 2ns, the total amount of crosstalk on a certain PCB board line is less than 70mV, and so on. Once the physical and electrical rules are clearly defined, high-speed routers can be explored further. At present, the high-speed routers based on physical rules (physical rule-driven) on the market include AutoActive RE router, CCT router, BlazeRouter router and Router Editor router. In fact, these routers are all automatic routers driven by physical rules. That is, 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. High-speed routers that are directly driven by electrical rules are very important to ensure signal integrity in high-speed designs. Design engineers always have electrical rules and design specifications are also electrical rules. In other words, our designs must meet electrical rules and not physical rules. It is essential that the final physical design implementation meets the electrical requirements of the design. Physical rules are just a conversion of electrical rules by component manufacturers or design engineers themselves. We always expect this conversion to be equivalent and one-to-one. In reality this is not the case. Taking the use of LVDS chips to complete high-speed (up to 777.76Mbps) and long-distance (up to 100M) data transmission as an example, since the signal swing of LVDS technology is 350mV, the usual design specifications always require that the total line on the signal line be 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, and the 20% percentage depends on the LVDS noise tolerance, which can be obtained from the reference manual . For IS_Synthesizer, as long as the design engineer specifies the crosstalk value on the LVDS signal line, the wiring can be automatically adjusted and refined to ensure that the electrical performance requirements are met, and all surrounding signal lines will be automatically considered during the wiring process. effects of LVDS signals. For routers driven by physical rules, some hypothetical analysis and consideration are required first. Design engineers always think that the crosstalk between signals only depends on the length of parallel lines between parallel signals, so it can be used in high-speed circuits. Do some hypothetical analysis in the front-end environment of the design. For example, you can assume that the length of the parallel wiring is 2.5 mil, and then analyze the crosstalk between them. This value may not be 70mV, but you can further adjust the parallel wiring according to the conclusions obtained. If the crosstalk value between the signals is basically 70mV when the length of the parallel line is a certain value such as 7mil, then the design engineer thinks that as long as the length of the parallel line of the differential line is controlled within the range of 7mil Such electrical characteristics requirements can be met within the range (signal crosstalk value is controlled within 70mV), so the design engineer obtains such a physical rule for high-speed PCB board design during the actual physical PCB board layout and wiring. Conventional high-speed routers are all This physical size requirement can be guaranteed. There are two problems here: First, the regular conversions are not equivalent. First, the crosstalk between the signals is not determined by the length of the traces between the parallel signals, but also depends on the flow direction of the signals, the position of the parallel line segments, and There are various factors such as the presence or absence of matching, which can be difficult to predict or even fully account for before the actual physical implementation. Therefore, after such a conversion, it cannot be ensured that the original electrical rules can be met at the same time as these physical rules are met. This is also a very important reason why the above-mentioned high-speed routers still cannot work normally when the above-mentioned high-speed routers meet the rules. Secondly, it is almost impossible to consider multiple influences at the same time when these rules are converted. For example, when considering signal crosstalk, it is difficult to consider the influence of all surrounding related signal lines at the same time. These two situations determine that the high-speed router based on physical rules will have great problems in the design of high-speed and high-complexity PCB board systems, and the high-speed PCB board router based on electrical rules is better. Solved this problem. High-speed PCB board-level and system-level design is a complex process. Signal integrity issues including signal crosstalk bring changes in design concepts, design ideas, design processes and design methods. Ensuring that problems are quickly identified, resolved, and guided in new designs to prevent problems in high-speed system design has become mainstream in high-speed system PCB board design today.