Issue link: https://iconnect007.uberflip.com/i/1071356
JANUARY 2019 I DESIGN007 MAGAZINE 43 it is best to compare the skew between, for example in Figure 3, the MDQ0 (data) and MDQS0 (strobe) at the receiver to ensure the flight times are correct. In this case, there is 10pS difference, so this is well within the timing specification. IX. Eliminate Crosstalk Scan the board for possible crosstalk. Crosstalk can be coupled trace-to-trace on the same layer or broadside coupled by traces on adjacent layers. Crosstalk is caused by the coupling of the electromagnetic fields. Electric fields cause signal voltages to capacitively couple into nearby traces. Capacitive coupling draws a surge of drive current which causes reflections on the transmission lines. Whereas, magnetic fields cause signal currents to be induced into nearby traces. Inductive coupling produces ground bounce and power supply noise. Crosstalk falls off rapidly with the square of the distance, and the degree of impact is related to the aggressor signal voltage, available board real estate and thus the proximity of signal traces. Crosstalk can be coupled trace-to-trace on the same layer (Figure 4) or can be broadside coupled by traces on adjacent layers (Figure 5). The coupling is three dimensional. Broadside coupling is difficult to spot as generally we look for trace clearances when evaluating crosstalk, but a simulator will pick this up. Traces routed in parallel and broadside cause greater amounts of crosstalk than those routed side-by-side due to the greater coupling area. Therefore, it is good practice to route adjacent signal layers in the stackup orthogonally to each other to minimize the coupling region. A better solution is to only have one signal layer between two planes to avoid broadside coupling altogether. When interactive routing, one tends to group signals for aesthetic reasons–this is the artistic side of the PCB Designer showing through. Figure 3: Flight times of data compared to strobe.