High-Speed Circuit Crosstalk Issues

Suppression of Crosstalk Problems in High Speed Circuit PCB Design

With the rapid development of PCB design, its high speed and miniaturization have become a trend. On the one hand, due to the smaller size of the PCB circuit board, the wiring density is greatly increased; on the other hand, the signal frequency becomes higher and the edge becomes steeper. In this way, in the design of high-frequency circuit PCB circuit boards, the problem of crosstalk between signals cannot be ignored more and more. This is an important consideration in the design of high-frequency PCB circuit boards. Through the analysis of the crosstalk problem, the crosstalk problem can be quickly found, located and solved in the PCB design. So how does crosstalk arise? What factors are related? What is the effect on the PCB circuit? And how to control it?

1. The generation of crosstalk problem

Mutual inductance and mutual capacitance between signal transmission lines are two important factors that cause crosstalk problems. Signal transmission lines include printed lines, wires and cable bundles, etc. Crosstalk is the coupling of electrical signals from one transmission line to another. When the alternating current of the signal passes through the transmission line, a magnetic field is generated around it. When the electromagnetic fields generated by different transmission lines are superimposed and interact, crosstalk occurs.

In digital circuits, since it is mainly a pulse circuit, crosstalk occurs during the transition of the signal. The faster the signal changes, the greater the crosstalk will be.

As shown in Figure 1, the changing signal propagating from A to B along the transmission line produces a coupled signal on the transmission line CD. When the varying signal returns to a stable DC level, the coupled signal is no longer present.

Crosstalk can be divided into capacitive coupling crosstalk (Sc) and inductive coupling crosstalk (Sl).

Capacitive coupling crosstalk is when there is signal transmission on the interference line, due to the change of the signal edge voltage, in the area near the signal edge, the distributed capacitance on the interference line will induce a time-varying electric field, and the victim line is in this electric field , so the changing electric field induces a current in the victim wire. This results in capacitively coupled crosstalk. as shown in picture 2.

Inductive coupling crosstalk means that when the signal propagates on the interference line, due to the change of the signal current, a time-varying magnetic field will be generated by the action of the distributed inductance in the vicinity of the signal transition, and the changing magnetic field will be induced on the victim line. Noise voltage, and then form inductive coupling current, the resulting crosstalk is inductive coupling crosstalk. As shown in Figure 3.

2. Parameter factors affecting crosstalk

(1) The influence of the coupling length of the signal transmission line on crosstalk: the coupling length of the signal transmission line is different, and the degree of crosstalk generated is different. The far-end crosstalk is proportional to the length of the signal transmission line, the longer the coupling length, the greater the crosstalk. As for near-end crosstalk, only when the coupling length is less than the saturation length, the crosstalk increases with the increase of the coupling length, and when the coupling length is greater than the saturation length, the near-end crosstalk is a stable value.

(2) The effect of line spacing on crosstalk: line spacing is inversely proportional to crosstalk. When the line spacing is greater than or equal to 3 times the line width, the crosstalk is very small.

(3) The influence of signal rise time on crosstalk: In high-speed PCB design, the speed of signal rise time has a great influence on signal crosstalk. As the rise time becomes shorter, especially when the parallel trace length is less than the saturation length, the crosstalk voltage amplitude will decrease rapidly. Therefore, devices with fast edge rates are increasingly used in modern high-speed board designs.

(4) The effect of the thickness of the medium layer on the crosstalk: The crosstalk is inversely proportional to the thickness of the medium. The thinner the dielectric, the less crosstalk it will cause.

3. The influence of crosstalk on high-speed PCB circuits

Crosstalk is ubiquitous in high-speed and high-density PCB circuits. Each of its signal transmission line pairs interacts with its nearest signal line. In high-speed PCB design, it is necessary to correctly deal with the crosstalk problem of signal lines and improve the anti-interference ability of signal lines. Generally, crosstalk has the following two effects on high-speed PCB circuits.

(1) Crosstalk causes false triggering: Signal crosstalk is an important part of the signal integrity problems faced by high-speed PCB design. Functional errors in digital circuits caused by crosstalk are the most common one.

(2) Trigger delay caused by crosstalk: In digital circuit design, timing is an important consideration. Due to the existence of crosstalk, the timing delay is caused.

4. Suppression of crosstalk problems

Crosstalk is a problem that needs to be focused on in high-speed PCB design. Although it is impossible to eliminate crosstalk, it can still be suppressed within a tolerable range technically. The whole process of high-speed PCB design includes steps such as circuit design, chip selection, schematic design, PCB layout and wiring. It is necessary to find crosstalk in different steps and take measures to suppress it in order to reduce interference. .

The problem of controlling crosstalk can be considered from the following aspects:

4.1 Suppress crosstalk by controlling signals

The transition rate of the transmitted signal edge also has an effect on the suppression of crosstalk. The faster its conversion rate, the greater the impact on crosstalk. Therefore, when selecting devices, try to choose slow devices while meeting the design specifications, and avoid mixing different types of signals, because fast-changing signals have potential crosstalk hazards to slow-changing signals. Through the PCB circuit design, the impedance of the signal transmission line is matched. It is necessary to match the terminal impedance of the near end or the far end of the transmission line with the impedance of the transmission line as much as possible, so as to suppress the amplitude of the crosstalk, and then achieve the purpose of suppressing the crosstalk.

4.2 Adopt shielding measures

Providing package ground for high-speed signals is an effective way to solve the crosstalk problem. However, the ground wrap increases the amount of routing, which makes the limited routing area more crowded.

The shielding of the ground wire requires that the distance between the ground points meet certain requirements, which is generally less than twice the length of the signal change edge. At the same time, the ground wire will also increase the distributed capacitance of the signal, which will increase the impedance of the transmission line and slow down the signal edge.

4.3 Suppress crosstalk from product design

For sensitive internal circuits, it is necessary to prevent the injection of external interference signals; at the same time, it is also necessary to prevent crosstalk between internal noise circuits and other signal lines, especially crosstalk between I/O signal lines.

4.4 Suppress crosstalk through PCB wiring layer and wiring spacing

Through reasonable setting of wiring layers and wiring spacing, effectively shortening the length of parallel signal lines and increasing the spacing of signal transmission lines, crosstalk can be effectively suppressed.

Increasing the distance between printed lines can reduce capacitive coupling, and inserting a ground wire between printed lines is more effective in reducing capacitive crosstalk. It is relatively difficult to suppress inductive coupling. The number of loops should be reduced as much as possible, and the signal loops are prohibited from sharing the same wire. At the same time, because the crosstalk generated by capacitive coupling and inductive coupling increases with the increase of the load impedance of the disturbed line, so reduce the load to reduce the influence of coupling interference.

When conditions permit, try to increase the distance between traces and reduce the length of parallel traces. If necessary, you can use a fixed maximum parallel length push wiring method, that is, jog traces. This wiring method can effectively suppress crosstalk. As shown in Figure 4.

The signal layer adjacent to the ground wire should be equipped with low-level analog signal lines and high-speed digital signal lines, while the signal layer farther from the ground wire should be equipped with low-level signal lines and high-level analog signal lines.

Reduce parallel wiring, especially the wiring between the input end and the output end, and parallel wiring should be strictly prohibited. In this way, feedback coupling can be avoided, thereby effectively suppressing the occurrence of crosstalk.

In PCB design, the corner of the printed wire generally takes an obtuse angle of 135 degrees. The clock line should be adjacent to the ground layer, and the line width should be as large as possible. The line width of each clock line should be the same.

The line width of the power line and ground line should be increased as much as possible. Generally, the signal line width of digital circuits should be between 8mil and 10mil, and the line spacing should be between 6mil and 8mil. For devices with a pitch of 0.5mm, the wiring width should not be less than 12mil, and the high-speed signal lines should be designed as striplines or embedded microstrip lines.

If the two signal layers are adjacent, the wiring should be carried out in an orthogonal direction to reduce the coupling between layers. By termination, the far-end and near-end impedance of the transmission line can be matched with the transmission line, thereby reducing crosstalk.

In PCB design, a unified ground is generally used, and digital circuits and analog circuits are partitioned for layout and wiring. The digital ground and the analog ground should be separated, and the wiring cannot cross the partition gap, otherwise the crosstalk will be sharply enhanced.

Summarize

Crosstalk is ubiquitous in high-speed and high-density PCB design, and the influence of crosstalk on circuits cannot be ignored. In order to reduce crosstalk, the most effective way is to reduce bad signal coupling, reduce the possibility of crosstalk as much as possible in PCB design, and minimize the influence of crosstalk. The above is some experience combined with PCB design, and I have read some related professional books, and put forward some solutions to the crosstalk problem in high-speed and high-density PCB design, for colleagues in the future High-speed and high-density PCB design learn from.

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