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Crosstalk: Why It's Difficult to Understand, Part 1
Wednesday, January 04, 2012 | Douglas G Brooks, PhD

Many people find crosstalk difficult to understand. Perhaps this is because there are a few unusual characteristics about it:

1:  It has two different fundamental causes

2:  These causes generate two different signals

3: These two signals flow in opposite directions

4:  These signals can interact with each other

5:  These two signals have significantly different shapes

6:  These shapes behave differently as a function of coupled length, and

7:  Neither shape resembles the “aggressor” signal that caused the crosstalk in the first place!

Aside from these few minor points, crosstalk is pretty easy to understand! So let’s see if we can make some sense out of all of this. First, to have crosstalk on our circuit boards we must have at least two traces. One of these carries a driving signal that is going to do the coupling. We call this the aggressor trace. The other trace(s) receives the coupled signal (crosstalk). We call this the victim trace. It is possible in practical circuits for a trace to be both an aggressor trace and a victim trace at the same time, although we usually analyze these effects separately.

The degree of coupling is related to the length over which the coupling occurs. Traces that are perpendicular to each other have a very short coupled length. Crosstalk is usually not an issue with traces that are routed perpendicular to each other. Therefore, crosstalk relates only to traces that are (more or less) parallel to each other and relatively close to each other. The length over which this is true is called the coupled length. The coupled length may be only a portion of the total length of a trace.

Two Fundamental Causes

Crosstalk is created when a signal on one wire or trace couples into another wire or trace. Since DC signals do not couple, we know right away that crosstalk is an AC effect. It is the electromagnetic field around the wire or trace that causes the coupling (see Note 1.)

The electromagnetic field has two components: An electric field and a magnetic field. These two fields are very closely related, but still different, with different effects. Think of the electric field as causing an electric, a charge, or a capacitive coupling effect. This effect is caused by the electric charge (the electrons) component of the signal. The magnetic field is created any time a current flows down a trace. A changing current causes a changing magnetic field. A changing magnetic field causes magnetic, or inductive, coupling into an adjacent trace (see Note 2.) It is the changing nature of the aggressor signal that is responsible for the crosstalk coupling.

So, we have two independent (crosstalk) signal components coupled into the victim trace, a charge component and a magnetic component. This takes care of point 1, above.

Two Different Signals

Recall that current is the movement (flow) of electrons (See Note 3.) An electron is a charged particle. Like charges repel and unlike charges attract. If you move a charged particle onto one plate of a capacitor, for example, a like charge is repelled from the other plate of the capacitor. Similarly, if you move a charged particle down a trace, like charges on an adjacent trace are repelled away from that charged particle. The like charges are repelled in all directions, but since they are constrained to the conductive trace, they move in both directions away from the particle that is repelling them.  These repelled charges become a current that flows in both directions away from the charge that is repelling them.

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