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April 2017 • The PCB Design Magazine 53 2. Understand the Importance of Referencing Each signal layer should be adjacent to, and closely coupled to, a reference plane, which 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 cur- rent 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 current transitions, rather than stitching decoupling ca- pacitors which add inductance. This keeps the loop area small and reduces radiation. As the stackup layer count increases, so does the num- ber of possible combinations of the structure. But, if one sticks to the basic rules then the best performing configurations are obvious. Figure 2 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. 3. Identify the Location of the RPDs The return current of a high-speed, fast rise time digital signal will always follow the path of least inductance which is directly beneath the signal path. However, RPDs tend to divert the return current increasing the loop area, in- ductance and delay–which is not desirable. The best way to identify the RPDs is to follow the signal path and imagine the return path closely coupled on the nearest plane. A via that provides the connection between signal traces, referenced to different planes, cre- ates RPDs. In other words, the return current has to jump between the planes to close the current loop, which in turn increases the induc- tance of the current loop, thus affecting signal integrity. This return current also excites the parallel plate mode, causing significant EMI. If the reference planes are at the same DC po- tential, then they can be connected by stitch- ing vias near the signal via transition to provide shorter paths for return currents. However, if the planes are at different DC potentials, then decoupling capacitors must be connected across the planes at these points. In addition, some of the return current flows through the interplane capacitance to close the loop. Figure 3 illustrates the spreading of return current density across the planes above and be- low the signal path. As the frequency approach- es a couple of hundred megahertz, the skin ef- fect forces the return current to the surface clos- est to the signal trace. It is important to have a clearly defined return current path and to know exactly where the return current will flow. This is particularly critical with asymmetric stripline RETURN PATH DISCONTINUITIES Figure 2: Electric (blue) and magnetic (red) fields (source: HyperLynx). Figure 3: Return path current density for asym- metric stripline (source: iCD Design Integrity).