A Practical Guide to Lossy Differential Lines

In recent years differential signaling [1], [2], [1] has made large inroads in high-speed transmission schemes. Estimates are that in a few years almost 100% of all PCB’s will have at least some differential signal paths on them. There are several reasons for this: On one hand having a symmetrical pair of lines carrying opposite signals close to each other greatly reduces electromagnetic emissions because the electromagnetic far-fields of the two lines largely cancel. Second, since in such a transmission scheme the total signal current over the two lines of a differential pair is constant (at least as long as no differential skew is present), the current spikes drawn by the drivers are reduced by an order of magnitude, reducing power supply noise and ground bounce (see the section below about power decoupling). Third, differential transmission is much less sensitive to residual ground bounce or external influences than single ended signaling because the influences on the two lines of a pair are of similar size (as long as the two lines are close together) and so largely cancel out since the receiver is only sensitive to their difference. In contrast to wide-spread belief, there is nothing really special required per se for two lines to be “differential” the only distinctive feature is that the signals these two lines carry are not independent, but are always complementary. Things like “coupling” and “differential impedance” are the result of specific design techniques associated with differential signaling rather than prerequisites for it. This means that differential lines share a lot of issues – for example losses with single ended lines. This paper intends to show how differential lines can be better understood and modeled with a minimum of effort and with easily available tools. The reader will gain insight into the risks associated with differential signals like delay mismatches (skew), crosstalk, mode conversion, and signal losses. With a good understanding of those effects differential lines lose a large part of their mystery and the risks of using them can be well controlled.