Field Cancellation
Figures 1, 2, and 3 illustrate the pattern of magnetic fields surrounding one microstrip trace on a typical PCB (printed-circuit board). The trace appears in cross-sectional view as a small black rectangle. The line below the trace represents a solid plane layer within the board, which may contain other signal layers not shown. Current on the trace flows into the page in all three cases.
Figure 1 shows the pattern of magnetic lines of force resulting from the current on the trace, without considering current on the reference plane. This pattern of lines looks like a nested set of concentric circles. As you move to a distance r away from the trace, the field intensity falls off as 1/r. An infinite straight wire suspended in free space generates this same familiar pattern.
In a singled-ended system, outgoing current on the trace stimulates an equal and opposite current returning to its source through the power and ground system. Figure 2 depicts the magnetic field resulting from that current. At frequencies higher than approximately 1 MHz on a typical PCB, current returning to its source flows along the top surface of the solid reference plane, parallel to the outbound trace. The current bunches strongly right underneath the signal trace, with a distribution that falls off rapidly as you move away from the trace on either side. The pattern of magnetic fields from this distribution makes football-like shapes. Where each line of force intercepts the solid plane, it “bends” in proportion to the profile of current flowing in the sheet at that location. These lines of force circulate in a direction opposite those in Figure 1.
When you superimpose the two patterns, the fields in the bottom half of the diagram cancel perfectly, assuming a reference plane of infinite extent and perfect conductivity (Figure 3). The field intensity above the trace does not cancel perfectly, but it substantially cancels, vastly reducing both crosstalk and EMI (electromagnetic interference) on that side. The field intensity increases only in the small yellow region directly under the trace, at which point it approximately doubles. The net benefit is fantastic. There is no simpler or more effective means of combatting crosstalk and EMI than a solid reference plane.
I love the field-cancellation benefit, but who knew that fields do what they do so well? James Clerk Maxwell, in his seminal text, A Treatise on Electricity and Magnetism, published in 1891 by Clarendon Press, explains in volume two, article 654, that current flows in the solid reference plane in reaction to the current flowing in the trace. The reference-plane currents, called eddy currents, form in a pattern that neutralizes the component of the magnetic field normal to the reference plane. The solution in Figure 3 attains that goal, perfectly canceling the vertical component of the magnetic field at the surface of the solid reference plane. Were some portion of the field to behave differently, it would set up eddy currents in the plane that would counteract the miscreant field, forcing it back into the shape shown in Figure 3.