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Electronic Design
High-speed PCB board design in some crosstalk problems
Electronic Design
High-speed PCB board design in some crosstalk problems

High-speed PCB board design in some crosstalk problems

2021-11-06
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Author:Downs

In today's rapidly developing electronic design field, high-speed and miniaturization have become an inevitable trend in design. At the same time, factors such as the increase in signal frequency, the smaller the size of the circuit board, the increase in wiring density, and the decrease in the thickness between layers caused by the increase in the number of layers, will cause various signal integrity problems. Therefore, it is necessary to consider signal integrity issues when designing high-speed board-levels, master the theory of signal integrity, and then guide and verify the design of high-speed PCBs. In all signal integrity problems, crosstalk is very common. Crosstalk may appear inside the chip, or on circuit boards, connectors, chip packages, and cables. This article will analyze the causes of signal crosstalk in high-speed PCB board design, as well as the methods of suppression and improvement.

The generation of crosstalk

pcb board

Crosstalk refers to the influence of electromagnetic coupling on adjacent transmission lines when a signal is transmitted on a transmission channel. Excessive crosstalk may cause false triggering of the circuit and cause the system to fail to work normally.

The changing signal (such as a step signal) propagates from A to B along the transmission line, and a coupled signal is generated on the transmission line C to D. When the changed signal returns to a stable DC level, the coupled signal will no longer 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, the induced current on the interfered object leads to electromagnetic interference) and inductive coupling crosstalk (due to the current change of the interference source, the induced voltage is induced on the interfered object and thus Cause 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, these two signals have the same polarity; the crosstalk signal generated by the coupling inductor is also divided into forward crosstalk and reverse crosstalk Sl, The two signals have opposite polarities.

Mutual capacitance and mutual inductance are both related to crosstalk, but they need to be considered separately. When the return path is a wide uniform plane, such as most coupled transmission lines on a circuit board, the amount of capacitive coupling current and inductive coupling current are approximately the same. At this time, the amount of crosstalk between the two must be accurately predicted. 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 the capacitor are roughly equal and cancel each other, so only the reverse crosstalk needs to be considered. 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 capacitor as the parallel length increases. Therefore, the crosstalk of the parallel signal in the inner layer is greater than that of the surface layer. The crosstalk of parallel signals is small.

Analysis and suppression of crosstalk

The entire process of high-speed PCB design includes steps such as circuit design, chip selection, schematic design, PCB layout and routing. It is necessary to find crosstalk in different steps and take measures to suppress it in order to reduce interference.

Calculation of crosstalk

The calculation of crosstalk is very difficult. There are three main factors that affect the amplitude of the crosstalk signal: the degree of coupling between the traces, the spacing of the traces, and the termination of the traces. Current distribution along the microstrip line on the forward and return paths. The current distribution between the trace and the plane (or between the trace and the trace) is common impedance, which will cause mutual coupling due to current diffusion, and the peak current density is located directly below the center of the trace and from the trace The two sides of the decay rapidly toward the ground.

When the distance between the trace and the plane is far apart, the loop area between the forward and return paths increases, so that the PCB circuit board inductance, which is proportional to the loop area, increases. The following equation describes the optimal current distribution that minimizes the entire loop inductance formed by the forward and return current paths. The current it describes also minimizes the total energy stored in the magnetic field around the signal trace.