Issue link: https://iconnect007.uberflip.com/i/750006
44 The PCB Design Magazine • November 2016 equal common and differential-mode propaga- tion velocity which helps reduce far end mode transformation. It can be seen in Figure 3, that the differen- tial impedance of this particular microstrip pair, levels-off at 100 ohms above 12mils trace clear- ance (blue curve). This is simulated quickly by multiple passes of the field solver . So, all other factors being equal, the differential impedance will always be 100 ohms regardless of increased spacing. This allows the pair to be split to tra - verse obstacles (vias or pins) without altering im- pedance. This curve provides a clear map of the design space and can increase your productivity by efficiently defining the stackup configuration for single ended and differential pairs. In this case, once the separation is greater than 12 mils, the two traces convey single ended signals. A few signaling standards have both differ- ential and common-mode impedances speci- fied, but many do not. This provides the free- dom for the user to set it according to their ap- plication. These two impedances are related to the coupling strength of the differential pair. As the traces get closer, both differential and com- mon-mode impedances are reduced. With loose coupling, there is always the case where the traces must be bought closer together to weave through constrictions such as connector fields, BGA balls, vias and other land patterns. As the traces are bought closer to- gether the impedance drops, so the traces must be made narrower to compensate. The major- ity of PCB fab shops limit the trace width to 3 mil minimum. And of course, the narrower the trace width; the more the expense. Also because of the narrower traces, the Figure 2: Differential pair symmetry. Figure 3: Differential impedance levels-off above 12 mils, simulated by the iCD Stackup Planner. UNCOMMON SENSE