PCB Tech - High-speed PCB crosstalk analysis and its minimization

1 Introduction

With the increasing complexity and performance of electronic products, the density of printed circuit board s and the frequency of related devices are constantly increasing. Maintaining and improving the speed and performance of the system has become an important issue for designers. The signal frequency becomes higher, the edges become steeper, the size of the printed circuit board becomes smaller, and the wiring density increases, etc., which significantly increase the influence of crosstalk in high-speed PCB design. The crosstalk problem exists objectively, but exceeding a certain limit may cause false triggering of the circuit and cause the system to fail to work normally. The designer must understand the mechanism of crosstalk and apply appropriate methods in the design to minimize the negative effects of crosstalk.

2. The generation and changing trend of high-frequency digital signal crosstalk

Crosstalk refers to the undesirable noise voltage signal generated by the mutual coupling of electromagnetic fields between adjacent signals when the signal propagates on the transmission line, that is, the energy is coupled from one line to another.

As shown in Figure 1, for ease of analysis, we describe the crosstalk model of two adjacent transmission lines according to the discrete equivalent model. The characteristic impedance of the transmission lines AB and CD is Z0, and the terminal matching resistance R=Z0. If the driving source at point A is the interference source, the wire network between A and B is called the Aggressor line, and the wire network between C and D is called the Victim line, which is interfered. The crosstalk near the driving end of the interference source network is called near-end crosstalk (also called backward crosstalk), and the crosstalk near the receiving end of the interference source network is called far-end crosstalk (also called forward crosstalk). Crosstalk mainly originates from the mutual inductance Lm and mutual capacitance Cm formed between two adjacent conductors.

2.1 Inductive coupling

In Figure 1, first consider only the inductive coupling caused by the mutual inductance Lm. The magnetic field of the signal transmitted on the line A to B induces a voltage on the line C to D. The magnetic coupling acts like a transformer. Since this is a distributed transmission line, the mutual inductance also becomes a series of transformers distributed in two adjacent Parallel transmission lines. When a voltage step signal moves from A to B, each transformer distributed on the interference line will sequentially induce an interference spike to appear on the interfered network. The voltage noise superimposed by the mutual inductance on the interfered network is proportional to the change of the driving current on the interfered network. The calculation formula for noise generated by mutual inductance is

It is worth noting that the polarity of the mutual inductance coupling of each section of the coupling transformer is different. The interference energy induced to the interfered network is forward and backward in sequence, but the polarity is opposite, going to C and C and respectively along the transmission line CD. Go at point D.

As shown in Figure 2, the forward interference energy in the direction of C is proportional to the incident voltage and each mutual inductance component Lm. Because all the forward interference energy reaches point C almost at the same time, the forward interference energy and the mutual inductance of the two transmission lines The total amount is proportional, the longer the parallel length of the transmission line, the greater the total amount of mutual inductance generated, and the forward interference energy also increases; however, the backward interference energy toward point D is different from the forward interference energy toward point C Yes, although the total coupling area of the two is the same, the interference component induced by each mutual inductance transformer reaches D in sequence, and the effective time of the backward interference energy is as long as 2Tp (Tp is the propagation delay). With the extension of the parallel length (that is, the increase of mutual inductance), the magnitude of the backward crosstalk will not change, but the duration will increase.

2.2 Capacitive coupling

Mutual capacitance is another mechanism that produces crosstalk. Mutual capacitance Cm will produce an induced current on the interfered network. This current is proportional to the rate of change of the voltage on the interference network. The noise calculation formula generated by mutual capacitance Cm is:

The coupling mechanism of the distributed coupling capacitor is similar to that of the distributed inductive coupling, the difference lies in the polarity of the coupling. As shown in Figure 3, the polarities of the forward and backward interference energy of mutual capacitive coupling are both positive.

2.3 The combined effect of mutual inductance and mutual capacitance

Generally, capacitive crosstalk and inductive crosstalk occur at the same time. From the literature [1], we can obtain the calculation formulas of the total crosstalk of the near-end and the far-end respectively, which are superimposed by capacitive coupling and inductive coupling respectively.

The total noise of near-end crosstalk is:

The total far-end crosstalk noise is:

Among them, Z0, C, l, Cm, Lm, L, and V0 are the characteristic impedance of the transmission line, the capacitance per unit length, the inductance per unit length, the coupling capacitance and coupling inductance between the two transmission lines, the parallel length of the two transmission lines, and the voltage peak value.

From the above two formulas, we can see that the total noise of far-end crosstalk is mutually subtracted due to the polarity relationship of capacitive and inductive coupling, that is, far-end crosstalk can be eliminated. In PCB layout, stripline circuit can show a good balance between inductive and capacitive coupling, and its forward coupling energy is very small; for microstrip line (Microstfip), the electric field related to crosstalk is large Part of the passage is air, not other insulating materials, so the capacitive crosstalk is smaller than the inductive crosstalk, resulting in a small negative number for the forward coupling. This is the reason why the interference of the far-end crosstalk is often ignored in the usual design, and the improvement of the near-end crosstalk is emphasized.

In the actual design, the relevant parameters of the PCB (such as thickness, dielectric constant, etc.), as well as line length, line width, line spacing, the position of the transmission line and the ground plane, and the current flow direction will affect c, l, Cm, Lm, L, The size is determined by the signal frequency and the rise/fall time of the device.

Here we will not do a quantitative analysis of the influence of these parameters on the crosstalk. For the relationship between these parameters and the degree of the influence on the crosstalk, please refer to other relevant references for details.

2.4 The changing trend of crosstalk

The magnitude of mutual inductance and mutual capacitance affects the magnitude of crosstalk, thereby equivalently changing the characteristic impedance and propagation speed of the transmission line. Similarly, the geometry of the transmission line largely affects the changes in mutual inductance and mutual capacitance, so the characteristic impedance of the transmission line itself also has an impact on these parameters. In the same medium, the coupling between a relatively low-impedance transmission line and the reference plane (ground plane) is stronger, and the coupling between the relatively low-impedance transmission line and the adjacent transmission line will be weaker, so the low-impedance transmission line has a smaller impedance change caused by crosstalk.

3 Several effects caused by crosstalk

In high-speed, high-density PCB design, a complete ground plane is generally provided, so that each signal line basically only interacts with its nearest signal line, and the cross-coupling from other distant signal lines is negligible. Nevertheless, in an analog system, when high-power signals pass through low-level input signals or when components with higher signal voltages (such as TTL) are close to components with lower signal voltages (such as ECL), very high resistance is required. Crosstalk capability. In PCB design, if not handled correctly, crosstalk has the following two typical effects on the signal integrity of high-speed PCBs.

3.1 False triggering caused by crosstalk

Signal crosstalk is an important part of the signal integrity problems faced by high-speed design. The functional error of digital circuits caused by crosstalk is the most common one.

Figure 4 is a typical transmission of error logic in adjacent networks caused by crosstalk pulses. The signal transmitted on the interference source network passes through the coupling capacitor, causing a noise pulse on the interfered network and the receiving end, resulting in an undesired pulse being sent to the receiving end. If the intensity of this pulse exceeds the trigger value of the receiving end, an uncontrollable trigger pulse will be generated, causing confusion in the logic function of the next-level network.

3.2 Timing delay caused by crosstalk

In digital design, the timing issue is an important consideration. Figure 5 shows the timing problems caused by crosstalk noise. The lower part of the figure is the two kinds of noise pulses generated by the interference source network (Helpful Figure 5 Delay glitch and Unhelpful glitch caused by crosstalk noise). When the noise pulse (helpful glitch) is superimposed on the interfered network, it will cause the interference network signal transmission. The delay is reduced; similarly, when the noise pulse (Unhelpful glitch) is superimposed on the interfered network, it increases the delay of the normal transmission signal of the interfered network. Although this kind of crosstalk noise that reduces the network transmission delay is helpful to improve the PCB timing, in actual PCB design, due to the uncertainty of the interference source network, this delay is uncontrollable, so this kind of crosstalk The delay caused must be suppressed.

4. Minimize crosstalk

Crosstalk is ubiquitous in high-speed and high-density PCB design, and the impact of crosstalk on the system is generally negative. In order to reduce crosstalk, the most basic thing is to make the coupling between the interference source network and the interfered network as small as possible. It is impossible to completely avoid crosstalk in high-density and complex PCB design. However, in the system design, the designer should choose an appropriate method to minimize crosstalk without affecting other performance of the system. Combined with the above analysis, the solution to the crosstalk problem is mainly considered from the following aspects:

If the wiring conditions permit, increase the distance between the transmission lines as much as possible; or reduce the parallel length between adjacent transmission lines as much as possible (cumulative parallel length), and it is best to route the lines between different layers.

The signal layer (without planar layer isolation) of the adjacent two layers should be perpendicular to the routing direction, and try to avoid parallel routing to reduce crosstalk between layers.

In the case of ensuring signal timing, try to choose devices with low conversion speed as much as possible to slow down the rate of change of the electric field and magnetic field, thereby reducing crosstalk.

When designing the stack, under the condition of satisfying the characteristic impedance, the dielectric layer between the wiring layer and the reference plane (power or ground plane) should be made as thin as possible, thus increasing the coupling between the transmission line and the reference plane and reducing adjacent Coupling of transmission lines.

Since the surface layer has only one reference plane, the electric field coupling of the surface layer wiring is stronger than that of the middle layer, so signal lines that are more sensitive to crosstalk should be placed in the inner layer as much as possible.

Through termination, the impedance of the far-end and near-end terminals of the transmission line can be matched with that of the transmission line, which can greatly reduce the amplitude of crosstalk.

5. Concluding remarks

Digital system design has entered a new stage. Many high-speed design issues that used to be of secondary importance now have a critical impact on system performance. Signal integrity issues including crosstalk have brought about changes in design concepts, design processes, and design methods. In the face of new challenges, the most important thing for crosstalk noise is to find out those networks that have a real impact on the normal operation of the system, instead of blindly suppressing crosstalk noise on all networks. This is also in line with limited wiring resources. contradictory. The crosstalk discussed in this article is of great significance for solving the crosstalk problem in high-speed and high-density circuit design.