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April 2014 • The PCB Design Magazine 19 should not be routed on the outer layers. Em- bedding the signals between the planes (on lay- ers 3 and 6) reduces the radiation by at least 10 dB. So fanout from the driver and drop to the internal signal layer routing up to the load through a short stub. In conclusion, apart from routing pin-to- pin, it is important to also consider how the return current propagates in the planes. When you plan your stackup, be aware of which planes—either power or ground—will be the re- turn path for your critical signals and make sure there is an unobstructed return path. Points to Remember • Current always flows in a loop. The out- bound pulse charges the local parasitic capaci- tance as it propagates down the transmission line and returns to the driver. • The ground plane is not a dumping ground for unwanted signals. • In a DC circuit, the return current takes the path of least resistance. • At high speeds, the return current takes the path of least inductance which just hap- pens to be the reference plane (either ground or power) directly above or below the trace. • Return current tends to couple to the sig- nal conductor, falling off in intensity, with the square of increased distance. • Because of skin affect, the high-frequency fields cannot penetrate the plane. • The current density will be the same re- gardless of the frequency. • Placing the trace closer to the reference plane will reduce crosstalk even if the trace spacing remains the same. • It is important to consider how the return current propagates in the planes. When you plan your stackup, be aware of which planes— either power or ground—will be the return path for your critical signals and make sure there is an unobstructed return path. • If there are multiple ground planes, then place a stitching via as close as possible to each layer transition (signal via). • If power planes are also used as the reference plane, then place decoupling capacitors as close as possible to each layer transition. PCBDESIGN References 1. Barry Olney Beyond Design columns – The Dumping Ground, Embedded Signal Routing 2. Howard Johnson—High-Speed Signal Propagation 3. Henry Ott—Electromagnetic Compatibil- ity Engineering 4. The ICD Stackup Planner and PDN Plan- ner can be downloaded from www.icd.com.au MyTHBuSTING: THERE ARE NO ONE-WAy TRIPS! continues feature Barry Olney is managing di- rector of in-Circuit Design Pty ltd (iCD), Australia. This PCB design service bureau special- izes in board-level simulation, and has developed the iCD stackup Planner and iCD PDn Planner software. To read past columns, or to contact Olney, click here. by Real Time with... NEPCON South China With just a single atom, light can be switched between two fibre optic cables at the vienna university of Technology. scientists are trying to build optical switches at the smallest possible scale in order to manip - ulate light. At the vienna university of Technol- ogy, this can now be done using a single atom. researchers capture light in so-called "bot- tle resonators." if such a resonator is brought into the vicinity of a glass fibre which is carry- ing light, the two systems couple and light can cross over from the glass fibre into the bottle resonator. The atom can be prepared in such a way that it occupies both switch states at once. Single-Atom Light Switch