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When common sense fails, tap into your uncommon sense. While common sense is considered conventional wisdom, uncommon sense is a re-examination of that conventional wisdom. Basically, common sense teaches us that the way it has always been done is the right way, and that’s just how things are. Following common sense is usually the safe way to go. But the people who are really making a difference in the world are usually the people who try something new. Tapping into our uncommon sense allows us to take a deeper look at things we often take for granted.
It is remarkable that with all of today’s high-performance systems, in which very complicated electromagnetic effects play a dominant role, many of us still hold misconceptions about the fundamental nature of how signals interact with interconnects. In this month’s column, I will look at the contemporary ways of addressing an old issue (déjà view, as I call it) and go beyond the design of PCBs.
There is always a debate regarding how a differential pair should be routed. Conventional wisdom tells us that since the two halves of the pair carry equal and opposite signals, a good ground connection is not required as the return current flows in the opposite signal. And tight coupling between the signals is better than loose coupling as it reduces undesirable coupling/crosstalk from aggressor signals. And let’s face it, it is easier to route a pair together so that we logical manage the planes, aggressor signals and matched delay simultaneously particularly in complex designs.
Some argue that beyond the fact that differential pairs transfer equal and opposite signals, there are no special requirements that need to be considered when using differential pairs. They should be treated as two single ended signals. The signals of a differential pair don’t need to be routed together, should not be tightly coupled and are not required to be routed to the differential impedance.
Basically, a differential pair is two complementary transmission lines that transfer equal and opposite signals down their length. We assume that tightly coupled differential pairs have no current in the adjacent planes because the return current of one line is carried by the other. That is not correct. On a PCB, the return current path, of each trace of the pair, flows directly below each trace in the reference plane as seen in Figure 1.
If the differential pair is well balanced, then tight coupling will achieve an effective degree of field cancellation. However, if they are not perfectly balanced, then the degree of cancellation is not determined by the spacing, but rather by the common-mode balance of the differential pair. Most digital drivers have poor common-mode balance and therefore differential pairs often radiate far more power in the common-mode than in the differential-mode. In such a case, one gains no radiation benefit from coupling the differential traces more closely together.
According to the FCC Class B compliancy standard, the differential-mode radiation from a microstrip pair, with 20mil separation, should theoretically yield a 40dB radiation improvement at 1 GHz over the radiation one would measure from the same signal routed as a single ended trace. It is the common-mode signal that dominates the radiation and decreasing the pair spacing will not improve this situation.
To read this entire article, which appeared in the November 2016 issue of The PCB Design Magazine, click here.