PCB Blog - Research on Crosstalk Analysis and Control in High Speed PCB Board Design

In today's rapidly developing PCB board design field, high speed and miniaturization have become a trend. How to keep and improve the speed and performance of the system while reducing the size of the electronic system has become an important issue for designers. EDA technology has developed a complete set of design analysis tools and methodologies for high-speed PCB and board-level systems, these technologies cover all aspects of high-speed circuit design analysis: static timing analysis, signal integrity analysis, EMI/EMC design, ground bounce analysis, power analysis, and high-speed routers. At the same time, it also includes signal integrity verification and sign-off, design space detection, interconnection planning, interconnection synthesis constrained by electrical rules, and the proposal of technical methods such as systems also provide the possibility to solve signal integrity problems efficiently and better. Here, we will discuss the method of analyzing signal crosstalk in signal integrity problems and its control.

1. The mechanism of crosstalk signal generation

Crosstalk means that when a signal is transmitted on the transmission channel, it has an undesired effect on adjacent transmission lines due to electromagnetic coupling, and a certain coupling voltage and coupling current are injected into the interfered signal. Excessive crosstalk may cause false triggering of the circuit, resulting in the system not working properly. In the circuit shown in Figure 1, the gate between AB is called the Aggressor Line, and the gate between CD is called the Victim Line. As soon as the aggressor changes state, we can observe the pulse crosstalk at the victim. Signal transmission on the transmission channel causes two different types of noise signals on adjacent transmission lines: capacitively coupled signals and inductively coupled signals. Capacitive coupling is the electromagnetic interference caused by the change of the voltage (Vs) on the interference source (Aggressor) on the interfered object (Victim) causing the induced current (i) to pass through the mutual capacitance Cm, while the inductive coupling is due to the interference source. The magnetic field generated by the change of the current (Is) causes the electromagnetic interference caused by the induced voltage (V) on the disturbing object through the mutual inductance (Lm).

2. The effect of current flow on crosstalk

Crosstalk is directional, and its waveform is a function of the direction of the current. Here we look at signal simulations in two cases. The first case is that the currents of the interference source wire mesh and the interfered object wire mesh are in the same direction, and the second case is that the currents of the interference source wire mesh and the interfered object wire mesh are in opposite directions (that is, the one located at point B is the driving source, and the one located at point A is the driving source. point is the load). Both AB and CD line nets are added with 20MHz signals. It can be seen from the simulation results that the peak value of far-end crosstalk (357.6mm) when the current flow is in the opposite direction is greater than the peak value of the far-end crosstalk (260.5mm) when the current flow is in the same direction. At the same time, it can be seen from Figure 4 that when the current flow of the interferer changes, the crosstalk polarity of the interfered source also changes. This shows that the magnitude and polarity of the crosstalk are related to the current flow of the signal on the corresponding interference source. The far-end crosstalk at point D is generally greater than that at near-end point C. Therefore, in crosstalk suppression, the far-end crosstalk at point D is usually used as the key factor to consider when examining the peak crosstalk voltage of the line network.

3. Signal source frequency and edge flip rate

The higher the frequency of the interferer signal, the greater the crosstalk amplitude on the interfered object. We simulated the crosstalk on the interfered object when the signal frequency f1 on the interferer network AB in Figure 1 took different frequency values, respectively. For the crosstalk waveforms when the signal frequencies are different, the waveform frequencies indicated by the arrows marked "1" and "2" are "500MHz" and "100MHz" respectively. It can be seen from the simulation results that the crosstalk voltage on the interfered object is proportional to the frequency of the interference source signal. When the frequency of the interference source is greater than 100MHz, necessary measures must be taken to suppress the crosstalk. At the same time, it can also be seen from Figure 5 that when the frequency of the interference source is as large as 500MHz, it is obvious that the crosstalk of the near-end point C of the interfered object is greater than the crosstalk of the far-end point D, which indicates that the capacitive Coupling has surpassed inductive coupling and has become the main interference factor. In this case, not only the far-end crosstalk must be handled well, but also the near-end crosstalk, which is often overlooked, must be handled carefully. In addition, let's analyze another factor that has a great influence on crosstalk, which is the edge flip rate of the signal. edge) has a greater impact on crosstalk, and the faster the edge changes, the greater the crosstalk. Since devices with large edge flip rates are more and more widely used in the design of modern high-speed digital circuits, such devices, even if their signal frequencies are not high, should be carefully routed to prevent excessive crosstalk from being generated.

4. The influence of the line spacing P and the parallel length L of the two lines on the size of the crosstalk

Under the condition that the distance between the two lines and the parallel length is unchanged, the crosstalk of the object to interfere (marked with "1") is detected; the second case is to increase the distance between the two lines to 10mils under the premise that the parallel length of the two lines is unchanged. Then detect the crosstalk mark "2" of the interfered object; the third case is to increase the parallel length of the two lines to 2.6inches mark "3" under the condition that the distance between the two lines is unchanged and then detect the crosstalk of the interfered object. It can be seen from the simulation results that when the distance between the two lines is increased (P is changed from 5mils to 10mils), the crosstalk is significantly reduced, and when the parallel length of the two lines is lengthened (L is changed from 1.3inches to 2.6inches), the crosstalk significantly increased. It can be seen from this that the magnitude of the crosstalk voltage is inversely proportional to the distance between the two lines and proportional to the parallel length of the two lines, but it is not complete multiple relationships. When the wiring space is small or the wiring density is large, when wiring in an actual high-speed circuit, in order to prevent the crosstalk of the high-frequency signal lines to the adjacent signal lines, which may cause false triggering of the gate level, the wiring resources allow Under certain conditions, the line spacing (except differential lines) should be opened as close as possible and the parallel length of two or more signal lines should be reduced. , which can not only save tense wiring resources but also effectively suppress crosstalk.

5. The effect of the ground plane on crosstalk

Multi-layer PCB boards generally include several signal layers and several power layers, and multiple signal layers and power layers are stacked to form standard microstrip transmission lines and strip transmission lines. There is generally a power supply plane adjacent to the microstrip transmission line and the strip transmission line, and the corresponding signal layer and the power supply layer are filled with dielectric. The thickness of this dielectric layer is an important factor affecting the characteristic impedance of the transmission line. When it becomes thicker, the characteristic impedance of the transmission line becomes larger, and when it becomes thinner, the characteristic impedance of the transmission line becomes smaller. The thickness of the dielectric layer between the transmission line and the ground plane has a great influence on the crosstalk. For the same wiring structure, when the thickness of the dielectric layer is doubled, the crosstalk increases significantly. At the same time, for the same thickness of the dielectric layer, the crosstalk of the strip transmission line is smaller than that of the microstrip transmission line. It can be seen that the influence of the ground plane on the transmission lines of different structures is also different. Therefore, in high-speed PCB routing, using strip transmission lines can achieve better crosstalk suppression than using microstrip transmission.

6. Control of crosstalk

It is impossible to eliminate crosstalk, we can only control the crosstalk within a tolerable range. Therefore, we can take the following measures when designing the PCB: 1) If the wiring space allows, increase the distance between the lines; 2) When counting the layers, reduce the distance between the signal layer and the ground layer under the condition that the impedance requirements are met. 3) Design key high-speed signals as differential line pairs, such as high-speed system clocks; 4) If two signal layers are adjacent, perform wiring in orthogonal directions to reduce the number of layers between layers. Coupling; 5) Design high-speed signal lines as strip lines or embedded microstrip lines; 6) When routing, reduce the length of parallel lines, and can route in jog mode; 7) In the case of meeting system design requirements, try to use the low-speed device on PCB board.