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56 The PCB Design Magazine • April 2017 • Each signal layer should be adjacent to, and closely coupled to, an uninterrupted refer- ence plane, which creates a clear, uninterrupted return path. • Although power planes can be used as ref- erence 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. • The return current of a high-speed, fast rise time digital signal will always follow the path of least inductance. • RPDs tend to divert the return current in- creasing the loop area, inductance and delay– which is not desirable. • A via that provides the connection between signal traces, referenced to different planes, cre- ates RPDs. • The skin effect forces the return current to the surface closest 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. • RPDs can never be totally eliminated but we can take steps to minimize them significantly. • It is all about inductance! If the return path loop area is increased, in any way by RPDs, then the inductance will also increase. • The use of a number of central GND planes, with signals on either side, will mitigate the RPDs. • By reducing the size of the cavity with a thin, high dielectric constant (Dk) material, ringing at low frequencies is reduced and the cavity resonance moves in to the upper band. PCBDESIGN References 1. Barry Olney Beyond Design columns: Stackup Planning, Part 3, The Dumping Ground. 2. Power Integrity Modeling and Design for Semiconductors and Systems, by Madhavan Swaminathan. 3. Qualifying the Impact of Non-Ideal Re- turn Path, by Byers and Hall. 4. High-Speed Digital Design, by Howard Johnson. Barry Olney is managing director of In-Circuit Design Pty Ltd (ICD), Australia. This PCB design service bureau specializes in board-level simulation, and has developed the ICD Stackup Planner and ICD PDN Planner software. The software can be download- ed from To read past columns, or to contact Olney, click here. RETURN PATH DISCONTINUITIES Harnessing wave energy by localizing it and suppressing its propagation through a medium is a powerful technique. Now, Al- agappin Gandhi and Png Ching Eng Jason from the A*STAR Institute of High Performance Computing have calculated a design that localizes light in tiny loops, within a two-dimensional structure created by merging two lattices of slightly differing periodicities. The new technique is not limited to light, and may enable the design of systems that can precisely control wave energy in any realm and at any scale. The ability to create resonators in which light is localized on the surface of a device also has applica- tions in quantum computing components based on light, such as defects in diamond. Gandhi and Png designed the structures by superimposing lattices of small circular dielectric materials with periods in a simple ratio R:R-1 — for example one lattice is merged with another whose spacing is 4/3 as big, or 5/4, 6/5 etc. "It creates a two-dimensional effect similar to beats between two waves of very close frequency," Gandhi said. "Where there are antinodes the light is localized in the form of a closed path." Gandhi and Png ran numerical simulations of the propagation of light in a range of wavelengths slightly below that of the lattice spacing, and calcu - lated the energy band structure. The Perfect Pattern to Trap Light

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