What is Crosstalk?

In succession to the previous section on “Differential (Normal) Mode Noise and Common Mode Noise”, this time crosstalk is explained.

Crosstalk

Crosstalk is the transmission of signals and noise due to coupling between lines, and is also called interference. The term “crosstalk” itself is evocative of the age of analog telephony, and of “talking across lines”. If two wires are separate (including the thin film wiring on a PCB), then electrical signals and noise should not propagate between them, but if two lines are parallel, in particular, the stray (parasitic) capacitance and mutual inductance that exists between the two lines results in noise transmission. Hence crosstalk is regarded as inductive noise.

Coupling between lines can be capacitive (electrostatic) coupling due to the stray (parasitic) capacitance, or can be inductive (electromagnetic) coupling resulting from the mutual inductance. As a result of these, noise is induced. The following are illustrations of the two types of coupling and the simplest equivalent circuits in each case.

In both cases, equations are indicated for the noise voltage Vn that occurs in nearby wiring pattern 2 due to noise in wiring pattern 1, which is a nearby noise source. R is the resistance component, C is capacitance, M is mutual inductance, Vs is the noise source voltage, and Is is the noise source current.

Here, it should be understood that crosstalk occurs across parallel wires. It should also be noted that when the wires are perpendicular, the stray capacitance and mutual inductance are much smaller.