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DECEMBER 2018 I DESIGN007 MAGAZINE 55 Consequently, although the amplitude of the harmonic frequency components decreases, as the frequency increases, the radiated frequency varies depending on the traces characteristics. For a synchronous bus—providing the receiver waits sufficiently long enough for the crosstalk to settle—before sampling the bus, the crosstalk on data and address signals within each group has little impact on the signal quality at the receiver. This can be ensured by always mak - ing the clock or strobe the longest signal of a matched length group. For a perfectly balanced differential signal, the radiation from one trace exactly cancels the radiation from the other as they are equal and opposite. However, a common-mode sig- nal represents an average of the two signals in a pair. The radiation is identical on both traces, and therefore, does not cancel but instead rein- forces. The transformation from differential to common-mode takes place on bends and non- symmetrical routing near via and pin obstruc- tions. This also impacts the impedance of the pair. To minimize radiation and crosstalk, one must think explicitly about the common-mode component of the differential signal; skew cre- ates this common-mode signal. Arguably, the principal source of imbalance is time-delay skew between the two traces. The easiest way to minimize this skew is to match the electrical lengths and to correct any shift immediately after it arises by adding length (hence the delay) to the shorter trace. VII. Analyze the Return Current Paths: All signal traces should be tightly coupled to a contigu- ous reference plane and have a clearly defined minimum loop inductance return current path. High-speed design is not as simple as sending a signal from the driver to the receiver over an interconnect. Rather, one should also consider the presence and interaction of the power dis- tribution network (PDN) and how and where the return current flows. Generally, PCB designers take great care to ensure that critical signals are routed exactly to length from the driver to the receiving device pins, but they take little care of the return current path of the signal. Current flow is a round trip, and the important issue is delay— not length. If it takes one signal longer for the return current to get back to the driver—around a gap in the plane for instance—then there will be skew between the critical timing signals. Return path discontinuities (RPDs) can cre- ate large loop areas that increase series induc- tance, degrading signal integrity and increasing crosstalk and electromagnetic radiation. Small discontinuities, such as vias and non- uniform return paths on a bus, are becoming an important factor for the signal integrity and timing of high-speed systems. RPDs produce impedance discontinuities due to the local return inductance and capacitive changes. Impedance discontinuities create reflected noise, contribute to differential channel-to- channel noise, and may promote mode con- version. In the case of differential pairs, the transformation from differential-mode to com- mon-mode typically takes place on bends, and asymmetrical routing near via and pin obstruc- tions, but can also be caused by small changes in impedance due to RPDs. Each signal layer should be adjacent to—and closely coupled to—a contiguous reference plane that creates a clear, uninterrupted return path and eliminates broadside crosstalk. As the layer count increases, this concept becomes easier to implement, but decisions regarding return current paths become more challenging. Although power planes can be used as refer- ence planes, ground is more effective as local stitching vias can be used for the return cur- rent transitions rather than stitching decou- pling capacitors, which add inductance. This keeps the loop area small and reduces radia- tion. As the stackup layer count increases, so does the number of possible combinations of the structure. But, if one sticks to the basic rules, then the best-performing configurations are obvious. Figure 4 shows the electric and magnetic fields emanating from a signal trace in both a microstrip and stripline configuration. Electric fields (blue) terminate when they come into contact with a solid plane while magnetic fields (red) are shielded by the planes, but the fringing fields still tend to radiate from the board edges .