Corners and Bends
Corners and bends are unmodelable in the sense that SPICE does not have built-in mechanisms to deal with them. The handling of a corner in an individual trace is a bit more complicated than the handling of a bend in a differential pair. In the lower gigahertz frequencies, the impact of a corner in a trace is often so much less than the variability of the materials themselves—the manufacturing variables—that corners in traces can often be ignored.

As shown in Figure 7-10 below, a square corner adds a small amount of capacitance; the amount can be calculated with a field solver—not by SPICE. The added capacitance is sometimes the combined effect of added physical area and the reactance of ephemeral modes produced by the corner discontinuity. The usual way of modeling a corner in SPICE is to add a small discreet capacitor at that point in the transmission line, add a small L-C-L segment, or ignore it.

Figure 7.10. Capacitance Added by a Corner

Bends are another issue altogether. Here we are using the word "bend" to refer to a differential pair's corner. The handling of a bend has been already described, but more details may be useful. The main thing that a bend does, but is not recognized by SPICE, is increase emissions.

Conversion of some signal to common-mode by a bend can cause loss due to radiation, and SPICE doesn't know about radiation. Usually this is not a major cause of signal loss, but it can be a major cause of emissions.

Planar Waveguide
The region between two metal planes can be described as a planar waveguide. Examples of this type of waveguide, shown in Figure 7.11 below, exist on most circuit boards in the form of the region between power and ground planes, or the region between multiple ground planes.

Figure 7.11. A Planar Waveguide

Signals can be injected into this waveguide by applying a current that traverses the distance from one plane to the other. In other words, when the signal passes through a via that traverses this region, as in Figure 7.12 below, the signal couples into the planar waveguide, as in Figure 7.13 below. SPICE doesn't know about this. Though this phenomenon can couple resonances to the signal and cause signal problems, it also can usually be easily controlled.

Figure 7.12. A Via Traversing a Planar Waveguide.

Figure 7.13. Signal Injected into a Planar Waveguide

The cancellation of radiation by the presence of a complementary signal, the other side of the differential pair, significantly reduces the coupling to this waveguide. Radiation into this waveguide is a significant factor in the total loss of a via, and, in the case of a single-ended signal, is the major cause of signal loss. The way to deal with vias is to calculate an appropriate L-C model through use of a field solver and import that model into SPICE.

When a planar waveguide has a lot of vias and holes in it, energy leaks out or is extracted fast enough that its probability of becoming resonant is low. However, if the waveguide has few holes and vias, it can become resonant. This can cause a problem. The mechanism that couples your signal's energy into the waveguide also couples energy out of the waveguide.

The mechanism that converts part of your differential signal to common mode also converts common mode to differential energy. The end result of this chain is that if you have a plane that goes resonant, you are likely to see that resonance in the differential characteristics of the link.

This is yet another reason why you need to take care to minimize mode conversions by maintaining symmetry within your differential pair, as much as you can.

Plane-Splits
As with many other things examined above, SPICE does not know about plane splits, shown in Figure 7.14 below. Signal integrity and the emissions impact will require a different tool if you are concerned. Emissions apply primarily to the common-mode component of a signal.

The signal-integrity impact can be modeled by treating the gap as a pi-network of capacitors. But the value of the capacitors is found through use of a field solver. This is not too severe a problem if this rule is followed: never cross a plane-split with any high-speed signal.

Figure 7.14. A Signal Crossing a Plane Split

On the other hand, the impact of a narrow, say 10-mil wide gap, on the signal integrity of a differential signal is not going to be very significant. If the particular design is such that emissions is not a big concern, crossing plane splits is not too big a concern either.

One approach that can be used is to assign some penalty to crossing a plane-split and use that as a criterion to help minimize the number of plane-split crossings. That is, tell the architects and the layout guys that each plane split is equivalent to reducing the overall useful trace length by an inch. With features such as plane splits and vias, it is very hard to predict the precise impact.

They will increase loss, crosstalk, and emissions—but how much? Assigning an overall equivalent impact, such as claiming an overall usable distance reduction of an inch per, helps clarify the need to minimize such features in a high-speed pair. In some designs, this might be overstating the fact. In others it might not.