PCB News - High-speed PCB design crosstalk analysis

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. With the increasing complexity and performance of electronic products, the density of printed circuitboards 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 higher the signal frequency, the steeper the edge, the smaller PCB size, and the increased wiring density all make the influence of crosstalk in high-speed PCB design increase significantly. 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 principle of crosstalk and apply appropriate methods in the design to minimize the negative impact of crosstalk.

The cross-winding in high-speed PCB design can be caused by magnetic field coupling generated by mutual inductance, or electric field coupling generated by mutual capacitance. the crosstalk models of two coupled transmission lines. Near-end crosstalk refers to the crosstalk on the interfered line close to the interference line driver, and far-end crosstalk refers to the crosstalk on the interfered line close to the receiving end of the interference line.

Magnetic field (inductive) and electric field (capacitive) crosstalk model diagram

Inductive coupling is the interference caused by the induced voltage on the interfered object due to the magnetic field generated by the current change on the interference source. The magnetic field of the signal transmitted on the line ab in induces a voltage on the line cd. The interference line can be regarded as the primary side of the transformer, the interfered line can be regarded as the secondary side of the transformer, and the current generated by the interfered line is at the near end. Flow in the load resistance and the remote load resistance. Tp is the delay time of the transmission line, and Tr is the rise time of the drive signal. it can be seen that the far-end coupling produces a negative pulse with a pulse width of Tr, and the near-end coupling stores 2TP time expansion, and its amplitude remains unchanged, but the total area of their coupling crosstalk is the same. The total area of crosstalk coupling is proportional to LM (dIs/dt) and coupling length.

Capacitive coupling is the interference caused by the induced current on the interfered object due to the voltage change on the interference source. The waveform of each point caused by mutual capacitive coupling is shown in .The difference from mutual inductive coupling is that the remote coupling is a positive pulse. The coupling crosstalk area is proportional to CM[(dv/dt) and coupling length.

Inductive and capacitive co-coupling crosstalk is essentially the result of the superposition of two coupling crosstalks. It can be seen from that both inductive coupling and capacitive coupling crosstalk try to enhance their effect at the near end d (they have the same polarity at point d), while at the far end c try to cancel each other's effect (their polarities at point c on the contrary). The amplitude of the near-end crosstalk pulse is constant, and the pulse width is twice the propagation time Tp represented by the coupling region. The width of the far-end pulse is approximately the rise time Tr of the pulse on the interference line, and the amplitude increases with the increase of the coupling length. Under normal conditions, in a complete plane, the inductive and capacitive crosstalk voltages are basically the same. The stripline circuit in the PCB circuit has a good balance of inductive and capacitive coupling, and its far-end crosstalk is small; for microstrip For the line, most of the electric field related to crosstalk passes through the air instead of other insulating materials, so the capacitive crosstalk is smaller than the inductive string, which causes its far-end coupling to be a negative number. If crosstalk is the main problem, then arrange all sensitive traces as strip lines.

Mutual inductance and mutual capacitive coupling crosstalk waveform diagram

The effect of crosstalk on the system is generally negative. It is impossible to completely avoid crosstalk in high-density and complex PCB design. In order to reduce crosstalk, the basic thing is to make the coupling between the interference source network and the interfered network as small as possible. In the system design, we 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:

When the wiring conditions allow, 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), preferably wiring between different layers;

In the case of ensuring the signal timing, 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;

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

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

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

Crosstalk is a problem that cannot be ignored in high-speed PCB circuit design, and it is getting more and more attention. Digital-based system design has entered a new stage. Many high-speed design issues that were secondary in the past have now had 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 key to 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, which also contradicts limited wiring resources. of.