Issue link: https://iconnect007.uberflip.com/i/1348195
24 DESIGN007 MAGAZINE I MARCH 2021 a plane pair acts more like a radial transmission line rather than a planar capacitor. 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. e edges of the board cause the greatest amount of reflection since an edge is a totally abrupt (open circuit) surrounding the board. Reflected energy is accompanied by phase reversals in its components, and com- bined reflections from the open circuit at the edge of the board can cause a phenomenon known as voltage doubling creating a standing wave. Parallel planes in multilayer PCBs exhibit multiple resonances, which increase the impedance and the EM radiation. A typical FR-4 laminate, of 4-mil thickness, produces a characteristic impedance of about 3 to 5Ω for adjacent planes. e larger the plane area, the lower the impedance. is is a good reason to make planes as large as possible. How Do We Dampen the Plane Resonance? Resistive termination along the board edges reduces the resonance peaks. But in practice, this means approximating a continuous struc- ture with resistors spaced around the perim- eter. Obviously, multiple low-value resistors cannot be placed directly between the power supply and ground as it would needlessly dis- sipate a huge amount of DC power. To pre- vent this, they should be AC coupled with a ceramic capacitor of sufficient capacitance to allow the resulting impedance to appear pre- dominately resistive at and above the lowest frequency of interest. A 10nF, X7R ceramic capacitor is a typical value. e addition of loss to dampen modal resonances is more impor- tant than the exact termination value and dis- tribution. Notice how the 20 RC terminations (blue) dampen the plane resonance pushing the effective Zpdn down below the resonance, above 300MHz in Figure 2 (right). A standing wave can be generated at switch- ing locations within the interior of the PCB. erefore, 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 characteris- tic impedance of the radial transmission line, then there will be no reflections. is will not Figure 2: Zpdn with 4x100uF (left), Zpdn with added RC damping (right). (Source: iCD PDN Planner)