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20 DESIGN007 MAGAZINE I OCTOBER 2020 noise from simultaneous switching devices can become significant. Unfortunately, as core voltages drop, noise margins become tighter. This voltage, emanating from the vicinity of the signal via, injects a propagating wave into the cavity, which can excite the cavity resonances or any other parallel structure (for instance, between copper pours over planes). Other sig- nal vias, also passing through this cavity, can pick up this transient voltage as crosstalk. And when the wave meets the PCB edge, the two reference planes form a slot antenna and will radiate noise with the potential to generate electromagnetic interference (EMI) to nearby equipment. The more switching signals that pass through the cavity, the more noise is induced into other signals; it affects vias all over the cavity, not just the ones in close proximity to the aggressor sig- nal vias. This cavity noise propagates as standing waves, spreading across the entire plane pair. This is the pri- mary mechanism by which high-fre- quency noise is injected into cavities by signals transitioning through cavi- ties, using each plane successively as the signal return path. Cavity resonance also affects the power/signal return layers at the edges of the PCB. Edge effects can be particularly problematic since it is the board edges that are in such close proximity to the chassis; hence, the radiation fields can induce currents into the chassis frame. When the cavity has open end boundary conditions, resonances arise when a multiple of half wavelengths can fit between the ends of the cavity. Figure 3 shows the cavity resonance of a plane pair with a resonant frequency of 1 GHz. If the signal clock frequency (or har- monics) are multiples of 1 GHz, then noise can be injected into the plane cavity. When the clock or data harmonics overlap with the cav- ity resonant frequencies, there is the potential for long-range coupling between any signals that run through the cavity. This is one reason Figure 2: Signals passing through a plane cavity intensify fringing fields. Figure 3: Amplitude at the far end of planes as input frequency is swept. (Source: Eric Bogatin)

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