Issue link: https://iconnect007.uberflip.com/i/1503357
32 DESIGN007 MAGAZINE I JULY 2023 interest. A 10nF, X7R ceramic capacitor is a typical value. e addition of loss to dampen modal resonances is more important than the exact termination value and distribution. Notice how the 20-series RC terminations of 3R5Ω and 10nF (blue) dampen the plane reso- nance, pushing the effective PDN impedance (Zpdn) down below the resonance, above 200MHz (Figure 3b). A standing wave can be generated at switching locations within the interior of the PCB. ere- fore, there may be hot spots within that cavity that would benefit from the placement of an additional AC-coupled load at or near the source. Reflections occur in a transmission line only when there is a discontinuity. If the edge of the board is terminated in the characteristic impedance of the radial transmission line, then the EM energy will be absorbed and there will be no reflections. is will not eliminate the initial transient but does, however, prevent it from being compounded. Termination ele- ments inhibit standing waves from developing between the planes and protect peripheral sig- nals and electronics from radiation. Key Points • Plane cavities in multilayer PCBs are essentially unterminated radial transmis- sion lines that will reflect if not terminated. • e energy that radiates from the edges of the plane pair is called a fringing field. • When displacement current flows through the impedance of a cavity between two planes it generates voltage. • Signal vias passing through a plane cavity can pick up the transient voltage as crosstalk. • A standing wave is produced by the super- position of two harmonic waves of equal amplitude, frequency, and wavelength moving in opposite directions. • Standing waves impact vias all over the cavity, not just the ones in proximity to the aggressor signal vias. • 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 possibility of exciting the cavity resonance. • A region under a large BGA densely popu- lated with vias also appears as a disconti- nuity due to the large array of anti-pads eating a hole in the plane. • e edges of the board cause the greatest amount of reflection since an edge is a totally abrupt open circuit surrounding the board. • e standing wave appears to be vibrating vertically without traveling horizontally. Figure 3: a) Zpdn with 4x100uF; b) Zpdn with added RC damping. (Source: iCD PDN Planner)