Visualize Differential Crosstalk
Figure 1 depicts two 100-Ω differential pairs of PCB
(printed-circuit-board) traces. A solid-plane layer appears at the
bottom of the figure. An upper solid plane exists somewhere above the
figure. This plot shows only a subset of the full stripline cross
section.
A pattern of thin, colored lines represents the magnetic field resulting
from current in the left-most pair. Technically, the lines prescribe
contours of constant 2-D magnetic scalar potential. The magnetic field
is most intense where the lines fall closest together and least intense
on the right side of the diagram where the lines spread far apart. The
field intensity proximate to the conductors appears very intense.
Pixelation effects in the figure may create what look like moiré
interference patterns in the dense region. The moiré patterns are not
real. The field patterns proximate to the conductors form a set of
concentric curves near the surface of each tiny conductor.
Figure 1—The number of lines of magnetic flux passing between A and B indicates crosstalk.
You can estimate the crosstalk picked up by the
victim pair, wires A and B, based on the field patterns in this diagram.
Your estimate will not be perfect because the mere presence of wires A
and B will distort somewhat the high-frequency magnetic-field pattern,
but the general principle remains quite useful. Simply count the number
of magnetic-field lines that pass between wires A and B to estimate the
crosstalk picked up between them.
There are 96 magnetic-field lines between the two conductors on the left
side. You can’t see those lines, but I know how many there are because I
wrote the code that generated this figure. Three of the lines pass
through the flux window drawn between the centerlines of victim traces A and
B. In this type of diagram, the near-end crosstalk ratio is simply the ratio of 3 to 96—about 3 That’s
all there is to estimating crosstalk. Just count how many lines, out of
the total, pass between the differential traces of interest.
Experts in electric- and magnetic-field calculations may recognize that
my procedure takes into account only magnetic-field effects, ignoring
capacitive coupling. That's OK. The procedure works because, in a stripline
configuration, inductive- and capacitive-coupling effects are nearly
always exactly equal. Compute one, and you have the other.
Now, let's change something. Grab traces A and B and move them both to the
left by about one-half a trace width. In response to that action, two
new magnetic lines of force fall within the victim’s flux window.
Crosstalk increases to 5, almost double the previous result. Obviously,
the
spacing between the aggressor pair and the victim pair greatly affects
crosstalk.
Next, put the traces back to their original positions and try something
different. This
time, leave trace A in its original position while you bring B to the
left by one-half a trace width. Notice that the line density near B is
quite sparse. After you slide B into its new position,
the number of flux lines penetrating the window does not change. Three flux lines still penetrate. Crosstalk hardly changes. In a more
accurate plot, you would surely observe some small effect, but I think
you get the picture: the spacing between the wires of a differential-stripline
pair only mildly affects crosstalk.
In this example, the spacing between wires A and B has little effect
because the crosstalk effect is not well-balanced. The aggressive field
affects wire A much more strongly than it affects wire B. Differential
systems can reject noise only if it affects both wires equally. In a nonsymmetric situation,
like this, differential-noise cancellation does not occur.
I hope Figure 1 helps you visualize why differential PCB traces do not
significantly reduce crosstalk from other traces. To reduce intertrace
crosstalk, you must enforce spacing rules between the aggressor and the
victim, much as you would with single-ended signals.
Let me wrap up this article with one more figure, this time showing the magnetic-field pattern associated with broadside-coupled pairs.
Figure 2—Crosstalk in this broadside-coupled scenario is about the same as that shown in Figure 1.