DIFFERENTIAL SIGNALING - Advanced Signal Integrity for High-Speed Digital Design - page 299

Note that the differential crosstalk (case 2) is less than the identically spaced
single-ended crosstalk (case 1), but greater than a single-ended case that occupies
the same board space as two differential pairs (case 3). This scenario is typical
for modern printed circuit boards, but is not necessarily always true. The benefit
of differential signaling for crosstalk is dependent on the specific geometries and
dielectric properties of the design.
7.3 VIRTUAL REFERENCE PLANE
Another advantage of differential signaling is that the complementary nature of
the electric and magnetic fields creates a virtual reference plane that provides a
continuous return path for the current. Figure 7-7 shows the field patterns for a dif-
ferential pair driven in the odd mode. Note that halfway between the conductors
there exists a plane that is normal to the electric fields and tangent to the magnetic
fields. These conditions are consistent with Section 3.2.1, which describes the
boundary conditions of a perfectly conducting plane. Therefore, for a differential
pair, there exists a virtual reference plane between the conductors . The existence
of the virtual reference plane is extremely helpful for cases when a nonideal ref-
erence exists that helps preserve signal integrity. Some common examples of a
nonideal reference include connector transitions, via fields, layer transitions, and
routing over a slot in the reference plane. In Chapter 10 we describe in detail the
Electric field is perpendicular
to the virtual plane
Electric field
Magnetic field is tangent to the
virtual plane
Magnetic field
Virtual reference plane
Figure 7-7 For an odd-mode (or differential) signal, the fields orient so that an ideal
virtual reference plane existed between the conductors. When the physical plane is inter-
rupted, the virtual plane provides a continuous reference for a differential signal and helps
preserve signal integrity.