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34 The PCB Design Magazine • January 2016 The worst-case noise response of a PDN is formed when a long, slow oscillation is followed by a short, fast oscillation. This phenomenon is referred to as a 'rough wave' and in extreme cas- es can cause total system failure. The long and slow oscillation is the clock and its odd harmon- ics, while the short and fast oscillation is due to the high-frequency plane resonance peaks. This is similar to an oceanographic 'rogue wave' phe- nomenon that is formed when a sudden quick wave hits a long, slow wave. The ICD PDN Planner displays this plane resonance effect in Figure 2 and the projected EMI in Figure 3 (the red line represents the FCC Class B limit). Although the current EMC lim- its are only defined to 1GHz, one could assume that these will one day be increased to cover the entire bandwidth. The EMI plot represents the projected maximum radiated noise if a high- speed signal excites the plane resonance at a particular frequency. If the plane size is increased, then the plane resonance is typically reduced. A combination of modifications to dielectric thickness and dielectric constant of the material in the ICD Stackup Planner, together with an adjustment of plane size, can usually establish the mini- mum resonance for the configuration. One should also ensure that the resonance peaks do not occur at the odd harmonics (red dotted ver- tical line), which tend to further radiate. In conclusion, multiple planes are essen- tial for high-speed design. But, one needs to select the right configuration to manage all of the diverse technologies, return current paths, broadside coupling and multiple power sup- plies requirements in order to achieve a high- performance, reliable product. Ensuring planes do not resonate, with the clock period, and that slow and fast frequency resonances do not com- bine will help avoid that dreaded 'rough wave' phenomenon. Power integrity issues generally manifest themselves as intermittent problems, which are otherwise difficult to nail. beyond design PLANE CRAzY, PART 2 Figure 3: Projected PDn eMI.