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30 DESIGN007 MAGAZINE I JULY 2023 tric field lines associated with this effect are called fringing fields (Figure 1). When displacement current flows through the impedance of a cavity between two planes, it generates voltage. Although quite small (typ- ically in the order of 5 mV), the accumulated noise from simultaneous switching devices can become significant. is voltage, emanat- ing from the vicinity of the signal via, injects a propagating wave into the cavity, which can excite the cavity resonances or any other par- allel structure (for instance, between copper pours and planes). Other signal vias also pass- ing through this cavity can pick up this transient voltage as crosstalk. When the wave meets the PCB edge, the two reference planes form a slot antenna that will radiate noise with the poten- tial to generate electromagnetic interference (EMI) to nearby equipment. e more switching signals that pass through the cavity, the more noise is induced into other signals. It impacts vias all over the cavity, not just the ones in proximity to the aggressor sig- nal vias. is cavity noise propagates as stand- ing waves spreading across the entire plane pair. is is the primary mechanism by which high-frequency noise is injected into cavities: by signals transitioning through cavities, 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 proximity to the chassis and, hence, the radi- ation fields can induce currents into the chas- sis 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. If the clock or data harmonics overlap with the cavity resonant frequencies, there is the potential for long-range coupling between any signals that run through the cav- ity. is is one reason why all return planes should be GND layers, so that stitching vias between GND planes can be placed adjacent to each signal via transition to minimize the pos- sibility of exciting the cavity resonance. Fig- ure 2 shows a standing wave produced by the superposition of two harmonic waves of equal amplitude, frequency, and wavelength moving in opposite directions. A region under a large BGA densely popu- lated with vias also appears as a discontinuity due to the large array of anti-pads eating a hole in the plane. A discontinuity reflects propagat- ing energy because it represents a mismatch with the characteristic impedance of the trans- mission line. Figure 1: Signals passing through a plane cavity intensify fringing fields.