Issue link: https://iconnect007.uberflip.com/i/1503357
JULY 2023 I DESIGN007 MAGAZINE 31 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 combined reflections from the open circuit at the edge of the board can cause a phenomenon known as voltage doubling, creating a standing wave. e standing wave appears to be vibrating ver- tically without traveling horizontally. ere are various approaches to reducing radiation edge effects from the PCB. In many cases, energy can be reflected, possibly creat- ing additional internal cavity resonance effects and coupling to internal vias, and resulting in increased radiation. When plane pairs reso- nate, their emissions come from the fring- ing fields at the board edges. With ground/ power plane pairs, edge-fired emissions can be reduced by reducing the plane separation and lowering the AC impedance. Alternatively, make the power planes slightly smaller (~200 mil) than the GND plane. is modifies the pattern of the fringing fields, pulling them back from the edge, and may help reduce emissions to some extent. Edge plating, as the name suggests, is the process of plating the edges around the PCB. is is an ele- gant (but expensive) solu- tion to prevent emissions from extremely high-speed SerDes signals on terabit routers, etc., but is an over- kill for a typical high-speed design. Another way to mitigate this problem is to create a via fence, stitched to ground around the perimeter of the PCB. If the spacing between the stitching vias is less than or equal to 1/12th of a wave- length, the via fencing will appear as a short circuit, causing the propagating wave to be reflected back to the source rather than being launched from the PCB edge. Unfortunately, most of the above techniques create reflections and possi- bly exasperate the issue. 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. e best solution to dampen the plane res- onance is to terminate the transmission line with an impedance-matching resistive element along the board edges. But in practice, this means approximating a continuous structure with resistors spaced around the perimeter. Obviously, multiple low-value resistors cannot be placed directly between the power supply and ground as it would needlessly dissipate a huge amount of DC power. To prevent this, they should be AC coupled with a ceramic capacitor of sufficient capacitance to allow the resulting impedance to appear predominately resistive at and above the lowest frequency of Figure 2: Standing wave is the superposition of two harmonic waves.