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PCBD-Apr2017

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54 The PCB Design Magazine • April 2017 RETURN PATH DISCONTINUITIES configurations where one signal layer is sand- wiched between two planes. Which plane does the return current flow on? 4. Take Corrective Action to Mitigate the RPDs Unfortunately, RPDs can never be totally eliminated but we can take steps to minimize the effects significantly. As with PDN planning, it is all about inductance! If the return path loop area is increased, in any way by RPDs, then the inductance will also increase. There are the obvious rules to follow such as: • Never allow a high-speed signal to cross a gap or split in the plane. This creates a large return path loop area and tends to radiate. • Never route a high-speed signal near the edge of the reference plane. The fringing fields may wrap around the edge of the board and radiate. • Never place an IC over a split plane. The IC substrate is like a miniature multilayer PCB and may rely on a solid plane placed beneath the IC to provide a continuous return path. If there are sufficient planes in the substrate, or you have the freedom to add more, then the use of a number of central GND plane struc- tures, with signals on both sides, will mitigate the RPDs as the return path will be in the same plane–albeit on opposite sides. As mentioned, at high frequencies the skin effect forces the re- turn current into the surface of the plane, clos- est to the signal trace, as shown in Figure 4. So, as the signal transitions from one signal layer to the other about the common GND plane, the return current also needs to change planes sides. This is achieved through the outer surface of the via antipad, on the plane, creating only a small RPD due to the variation of impedance between the signal layer and via. Also, when a signal propagates from the driver to the receiver, it creates noise in the power/ground plane cavity. As a result, energy is being lost to the PDN, creating effects such as RPDs and increasing insertion loss. By reducing the size of the cavity with a thin, high dielec- tric constant (Dk) material between the planes, ringing at low frequencies is reduced and the cavity resonance moves to the upper band which is above the maximum bandwidth. Points to Remember • SSN is a major problem in high-speed systems. But, the underlying issue is really the management of return current paths. • Current flow is a 'round trip' and the im- portant issue is delay not length. • Ground impedance is at the root of vir- tually all signal and power integrity problems– low ground impedance is mandatory for both. • A ground plane serves well as a signal re- turn, provided the ground is continuous under the signal path. • RPDs have a huge impact on supply bounce of single-ended signals. They produce impedance discontinuities due to the local re- turn inductance and capacitive changes and cause timing push-outs. • RPDs typically manifest themselves as in- termittent operation and degrade the perfor- mance of the product which can be extremely difficult to debug. Figure 4: Central GND plane return path structure.

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