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Beyond Design: Plane Cavity Resonance

Plane pairs in multilayer PCBs are essentially unterminated transmission lines—just not the usual traces or cables we may be accustomed to. They also provide a very low-impedance path, which means that they can present logic devices with a stable reference voltage at high frequencies. But as with signal traces, if the transmission line is mismatched or unterminated, there will be standing waves: ringing. The bigger the mismatch, the bigger the standing waves and the more the impedance will be location dependent.

Following on from my previous columns, Return Path Discontinuities and The Dark Side—Return of the Signal, in this month’s column, I will cover plane cavity resonance and look at how it impacts electromagnetic radiation.

When return current flows through the impedance of a cavity, between two planes, it generates voltage. Although quite small (typically in the order of 5mV) the accumulated noise from simultaneous switching devices can become significant. And unfortunately, as core voltages drop, noise margins become tighter. Figure 1 illustrates the electromagnetic fields resonating in a cavity. 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 signal vias also passing through this cavity can pick up this transient voltage as crosstalk.

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 signal vias. This cavity noise propagates as standing waves spreading across the entire plane pair. This 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.

To read this entire column, which appeared in the September 2017 issue of The PCB Design Magazine, click here.