8.6 Summary

Electric and magnetic coupling between conductors causes the impedance and time of flight to vary as neighboring signals switch, leading to data-dependent jitter. This effect is readily modeled in SPICE-type simulators, and it's not necessary to include all of the conductors of a wide bus to obtain accurate results. Coupling falls off with distance, especially for low-impedance lines, so that only a few signals on either side of a victim need be included.

The switching activity can be categorized as either odd- or even-mode behavior. The way mutual capacitance and inductance is summed depends if the neighboring traces are switching in phase (even mode) or out of phase (odd mode).

Coupling also causes voltages to be induced from one or more culprit lines to one or more unswitching (passive) victim lines. The crosstalk voltages appearing at the line's near end are called NEXT; the far end voltages are called FEXT.

Simple equations for predicting NEXT and FEXT are useful for lossless lines but are somewhat more prone to error as losses increase, especially for FEXT. Multiconductor transmission line modeling using SPICE-type simulators that includes skin effect resistance provide accurate results, especially if the loads present at the far and near ends are well modeled.

In stripline k _C = k _L, which in lossless lines means that FEXT will be zero. It's nonzero in lossy lines, and the polarity depends on the relative magnitudes of the induced capacitive and inductive currents.