Far-end and Near-end Cross Talk

Crosstalk is the unwanted coupling of energy between two or more adjacent lines.  The electromagnetic fields between two closely coupled lines interact with each other and will affect the behaviour of the signals on both lines.

Crosstalk figures estimations are commonly divided into two types near-end crosstalk and far-end crosstalk

1. Energy that is coupled from the actual signal line, the aggressor, onto a quiet passive victim line so that the transferred energy “travels back” to the start of the victim line. This is known as the backward or near-end crosstalk.
2. Energy that is coupled from the active signal line, the aggressor, onto a quiet passive victim line so that the transferred energy “travels forward” to the end of the victim line. This known as forward or far-end crosstalk.

Overview of crosstalk induced noise

If the victim line is not terminated at both ends in its characteristic impedance, the induced spurious signals can reflect at the ends of the line and travel in the opposite direction down the line. Thus, a reflected near-end crosstalk can end up appearing at the far end — and vice versa.

The crosstalk magnitude discussed in this article are the peak values and These equations only hold true when coupling is weak and the following assumptions apply:

1. The voltage and current on the aggressor line are unaffected by coupling to the victim line
2. The terminations of either the aggressor or victim lines have no effect
3. Propagation on either line is that of an uncoupled line.
4. The average attenuation along each line is the same.
5. The characteristic impedance of each line is that of an uncoupled line, Z0.

Near-end crosstalk (NEXT)

The magnitude of the near-end crosstalk is dependent upon the mutual capacitance and inductance between the two interacting lines and it will increase to a maximum amplitude as the coupling length increases.

Assuming two identical lines:

Where Cm is the mutual capacitance between lines per unit length, Lm is the mutual inductance between lines per unit length, C is the capacitance per unit length of either victim or aggressor and L is the inductance per unit length of either victim or aggressor.

The figure, Kb, which shows the maximum near-end crosstalk is a dimensionless ratio of voltages between the victim and aggressor lines.

Far-end crosstalk (FEXT)

Even and odd modes are the two main modes of propagation of the signal through a coupled transmission line pair.

Odd mode impedance is defined as impedance of a single transmission line when the two lines in a pair are driven differentially (with signals of the same amplitude and opposite polarity).

Even mode impedance is defined as impedance of a single transmission line when the two lines in a pair are driven with a common mode signal (the same amplitude and the same polarity).

The far-end crosstalk can be considered as an effect caused by the difference in velocity between the odd and even modes of propagation and thus a difference in edge arrival times at the end of the line — see Figures 2 and 3 below.

Far-end cross talk measurement setup

Far-end crosstalk caused by the superposition of even and odd mode waveform.

The far-end crosstalk coupling coefficient (Kf) can be calculated by:

Which can be written as:

The far-end crosstalk coupling coefficient (FEXT) is a unitless ratio of the maximum voltage perturbation caused on the victim line.

Far-end crosstalk increases with a sharper risetime, a longer coupling length and a higher Kf factor. In an ideal homogeneous stripline situation there will be no far-end crosstalk.

Frequency Dependent Crosstalk

For a more complete understanding of the crosstalk effect, the s-parameters should be examined.

S- parameters defines how a single frequency sine wave interacts with a “black box” device and affects the output signal appearing at different output ports. Crosstalk is the output on a port of a line different from the applied signal.

Using the s-parameter port numbering scheme, a signal is injected at port 1, the return loss is S11 and the transmitted signal is S21.

The near-end crosstalk is represented by S31, and the far-end crosstalk is represented by S41.

Crosstalk parameters from different geometric models and materials can be directly compared to determine their respective merits and identify trends.

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