Crosstalk and Protection

1. What is crosstalk

Crosstalk is the noise on the line caused by the coupling between two signal lines, the mutual inductance and mutual capacitance between the signal lines.Capacitive coupling induces coupling current, while inductive coupling induces coupling voltage.The parameters of the PCB layer, the distance between the signal lines, the electrical characteristics of the driving end and the receiving end, and the wire termination method have a certain influence on the crosstalk.

Crosstalk is one of the most fundamental phenomena in signal integrity, and its impact is especially severe when the trace density on the board is high.We know that the linear unrelated system satisfies the superposition theorem. If there is signal transmission on the victim line, the noise caused by crosstalk will be superimposed on the signal on the victim line, thereby distorting the signal.

2. How is crosstalk caused?

Crosstalk is caused by electromagnetic coupling, which is divided into capacitive coupling and inductive coupling.

Crosstalk refers to the undesired effects of electromagnetic coupling on adjacent transmission lines when a signal propagates on a transmission line

voltage noise.

Capacitive coupling is due to the voltage change on the interferer (Aggressor) on the interfered object (Victim)

Electromagnetic interference caused by induced current;

The inductive coupling is the electromagnetic interference caused by the induced voltage on the interfered object caused by the magnetic field generated by the current change on the interference source.Therefore, when a signal passes through a conductor, two different types of noise signals are induced on adjacent conductors: capacitively coupled signals and inductively coupled signals.

Inductive coupling:

Capacitive coupling:

3. What are the methods to reduce crosstalk?

When two networks are close together, the current change of one network will cause the current change of the other network, that is, crosstalk occurs.That is, the electromagnetic field coupling between the two networks is generated.Crosstalk occurs only when the rising and falling edge currents change.

Methods to reduce crosstalk include:

1. Select devices with slow-changing edge signals.

2. Select devices with small output swing and current.

3. In order to reduce the coupling between lines on the PCB, you can take:

(1) Increase the distance between the power layer and the signal layer;

(2) Increase the spacing between adjacent signal layers;

(3) Reduce the length of the parallel line;

(4) Increase the line spacing suppression;

(5) Ground wire isolation

(6) Adopt matching technology on the victim line;

(7) The key signal line goes STRIPLINE

Stripline: The stripline that goes on the inner layer (stripline/double stripline) and is buried inside the PCB, as shown in the figure below

The blue part is the conductor, the green part is the insulating dielectric of the PCB, and the stripline is the ribbon wire embedded between the two layers of conductors.

Because the stripline is embedded between two layers of conductors, its electric field is distributed between the two conductors (planes) that wrap it, and it will not radiate energy and will not be disturbed by external radiation.But since it is surrounded by dielectric (dielectric constant greater than 1), the signal transmission speed in the stripline is slower than in the microstrip line!