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Design007-Mar2021

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22 DESIGN007 MAGAZINE I MARCH 2021 Today's high-speed multilayer PCBs have multiple planes. e ground planes are used for shielding and to provide return current continuity. Whereas, closely coupled power/ ground plane pairs provide low inductance power to the ICs and reduce the AC impedance and plane resonance of the power distribution network (PDN). However, these parallel-plate waveguides (plane pairs) also form a radial transmission line. at is, they form a trans- mission line that propagates electromagnetic (EM) energy within a plane cavity emanating from a feed point, within the plane, outward in all directions. And, like all transmission lines, it will reflect if not terminated. e parallel-plate waveguide gets excited by currents flowing through the power/ground plane cavity which can lead to simultaneous switching noise. e current flowing through vias, connecting signal traces, can also cause a similar excitation at harmonic frequencies. In this month's column, I will look at how to dampen plane resonance, radiating from the fringing fields of the board, with RC termina- tion to match the plane's characteristic imped- ance. Dampening Plane Resonance with Termination e first line of defense against power supply transients is filtering, bypassing and decou- pling. However, in the critical 100 MHz-1 GHz band, the effectiveness of a typical decoupling capacitor is determined almost entirely by its series inductance. is is the frequency band now being used increasingly by digital logic. ese strategies lower the AC impedance but do not suppress PDN transients in a system where a radial transmission line exists between the planes. A termination that matches the transmission line's characteristic impedance reduces reflec- tions. But, in the case of parallel-plate reso- nance, termination also dampens the standing wave of propagating EM energy that can be built-up within the cavity and radiate from the edge fringing fields (Figure 1). EM energy can be dissipated or absorbed by terminating the edges of the PCB, in its characteristic imped- ance. As frequency and edge rates continue to increase, the impact of intrinsic electrical characteristics become more pronounced. AC switching currents in the power/ground planes can be very large. Under these circumstances, Beyond Design by Barry Olney, IN-CIRCUIT DESIGN PTY LTD / AUSTRALIA Figure 1: A standing wave built up by plane cavity resonance.

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