This article will analyze the causes of signal crosstalk in high-speed PCB board design, as well as the methods to suppress and improve. In today’s rapidly developing electronic design field, high speed and miniaturization have become the inevitable trend of design. At the same time, factors such as the increase of signal frequency, the reduction of the size of the circuit board, the increase of the wiring density, and the reduction of the interlayer thickness caused by the increase of the number of board layers will cause various signal integrity problems. Therefore, it is necessary to consider the signal integrity problem when designing the high-speed board, master the theory of signal integrity, and then guide and verify the design of the high-speed PCB board. Among all signal integrity problems, crosstalk is very common. Crosstalk can occur inside chips, as well as on circuit boards, connectors, chip packages, and cables.
1. The generation of PCB board crosstalk
Crosstalk refers to the interference on adjacent transmission lines due to electromagnetic coupling when a signal is transmitted on a transmission channel. Excessive crosstalk may cause false triggering of the circuit, resulting in the system not working properly. A changing signal (such as a step signal) propagates from A to B along the transmission line, and a coupled signal occurs on the transmission line C to D. When the changing signal returns to a stable DC level, the coupled signal also ceases to exist. Therefore, crosstalk only occurs in the process of signal hopping, and the faster the signal changes, the greater the crosstalk generated. Crosstalk can be divided into capacitive coupling crosstalk (due to the voltage change of the interference source, induced current is induced on the object to be interfered, resulting in electromagnetic interference) and inductive coupling crosstalk (due to the current change of the interference source, induced voltage is caused on the object to be interfered, thereby causing electromagnetic interference). Among them, the crosstalk signal generated by the coupling capacitor can be divided into forward crosstalk and reverse crosstalk Sc on the victim network, and these two signals have the same polarity; the crosstalk signal generated by the coupled inductor is also divided into forward crosstalk and reverse crosstalk Sc. The two signals have opposite polarities. Both mutual capacitance and mutual inductance are related to crosstalk, but need to be considered separately. When the return path is a wide, uniform plane, like most coupled transmission lines on a circuit board, the amount of capacitive and inductive coupling currents is about the same. At this time, it is necessary to predict the amount of crosstalk between the two. If the medium of the parallel signal is fixed, that is, in the case of a stripline, then the forward crosstalk caused by the coupled inductance and capacitance is approximately equal and cancels each other out, so it is only necessary to consider the reverse crosstalk. If the medium of the parallel signal is not fixed, that is, in the case of a microstrip line, the forward crosstalk caused by the coupling inductance is greater than the forward crosstalk caused by the coupling capacitance with the increase of the parallel length, so the crosstalk of the inner parallel signal is higher than that of the surface layer. The crosstalk of parallel signals is small.
2. Analysis and suppression of PCB crosstalk
The whole process of high-speed PCB board design includes steps such as circuit design, chip selection, schematic design, PCB board layout and wiring. During design, it is necessary to find and address crosstalk in different steps to suppress it and reduce interference.
3. Calculation of PCB crosstalk
Calculating crosstalk is complex. Factors affecting the amplitude of the crosstalk signal include the degree of coupling between traces, the spacing of traces, and the termination of traces. The current distribution along the microstrip traces on the forward and return paths is shown in Figure 2. The current distribution between traces and planes (or between traces and traces) is co-impedance, resulting in mutual coupling due to current spreading. When the traces are far apart from the plane, the loop area between the forward and return paths increases, increasing the circuit inductance proportionally.
4. Analysis of PCB crosstalk
Using EDA tools to simulate PCB board crosstalk can help quickly identify and solve crosstalk issues. Simulation in high-speed design includes schematic simulation before routing and PCB board simulation after routing. It can be used to predict and eliminate crosstalk problems early, thus effectively constraining layout and changes in stack up, optimizing clocking, critical signal topology, and termination before board layout.
5. Crosstalk suppression
Efforts to suppress crosstalk include reducing load impedance, increasing distance between coupling wires, isolating wires with a ground wire, inserting a ground wire between adjacent signal wires, and avoiding signal sharing loops. Continuously improving design experience and utilizing relevant software can enhance design efficiency and competitiveness in high-speed PCB board design. Signal integrity issues, including crosstalk, are crucial in completing successful designs. Adopting various methods throughout the design cycle can help ensure quick and efficient completion of designs, saving time and avoiding redundant work on PCB boards.
1. The generation of PCB board crosstalk
Crosstalk refers to the interference on adjacent transmission lines due to electromagnetic coupling when a signal is transmitted on a transmission channel. Excessive crosstalk may cause false triggering of the circuit, resulting in the system not working properly. A changing signal (such as a step signal) propagates from A to B along the transmission line, and a coupled signal occurs on the transmission line C to D. When the changing signal returns to a stable DC level, the coupled signal also ceases to exist. Therefore, crosstalk only occurs in the process of signal hopping, and the faster the signal changes, the greater the crosstalk generated. Crosstalk can be divided into capacitive coupling crosstalk (due to the voltage change of the interference source, induced current is induced on the object to be interfered, resulting in electromagnetic interference) and inductive coupling crosstalk (due to the current change of the interference source, induced voltage is caused on the object to be interfered, thereby causing electromagnetic interference). Among them, the crosstalk signal generated by the coupling capacitor can be divided into forward crosstalk and reverse crosstalk Sc on the victim network, and these two signals have the same polarity; the crosstalk signal generated by the coupled inductor is also divided into forward crosstalk and reverse crosstalk Sc. The two signals have opposite polarities. Both mutual capacitance and mutual inductance are related to crosstalk, but need to be considered separately. When the return path is a wide, uniform plane, like most coupled transmission lines on a circuit board, the amount of capacitive and inductive coupling currents is about the same. At this time, it is necessary to predict the amount of crosstalk between the two. If the medium of the parallel signal is fixed, that is, in the case of a stripline, then the forward crosstalk caused by the coupled inductance and capacitance is approximately equal and cancels each other out, so it is only necessary to consider the reverse crosstalk. If the medium of the parallel signal is not fixed, that is, in the case of a microstrip line, the forward crosstalk caused by the coupling inductance is greater than the forward crosstalk caused by the coupling capacitance with the increase of the parallel length, so the crosstalk of the inner parallel signal is higher than that of the surface layer. The crosstalk of parallel signals is small.
2. Analysis and suppression of PCB crosstalk
The whole process of high-speed PCB board design includes steps such as circuit design, chip selection, schematic design, PCB board layout and wiring. During design, it is necessary to find and address crosstalk in different steps to suppress it and reduce interference.
3. Calculation of PCB crosstalk
Calculating crosstalk is complex. Factors affecting the amplitude of the crosstalk signal include the degree of coupling between traces, the spacing of traces, and the termination of traces. The current distribution along the microstrip traces on the forward and return paths is shown in Figure 2. The current distribution between traces and planes (or between traces and traces) is co-impedance, resulting in mutual coupling due to current spreading. When the traces are far apart from the plane, the loop area between the forward and return paths increases, increasing the circuit inductance proportionally.
4. Analysis of PCB crosstalk
Using EDA tools to simulate PCB board crosstalk can help quickly identify and solve crosstalk issues. Simulation in high-speed design includes schematic simulation before routing and PCB board simulation after routing. It can be used to predict and eliminate crosstalk problems early, thus effectively constraining layout and changes in stack up, optimizing clocking, critical signal topology, and termination before board layout.
5. Crosstalk suppression
Efforts to suppress crosstalk include reducing load impedance, increasing distance between coupling wires, isolating wires with a ground wire, inserting a ground wire between adjacent signal wires, and avoiding signal sharing loops. Continuously improving design experience and utilizing relevant software can enhance design efficiency and competitiveness in high-speed PCB board design. Signal integrity issues, including crosstalk, are crucial in completing successful designs. Adopting various methods throughout the design cycle can help ensure quick and efficient completion of designs, saving time and avoiding redundant work on PCB boards.
