Issue link: https://iconnect007.uberflip.com/i/1348195
MARCH 2021 I DESIGN007 MAGAZINE 25 Key Points: • Parallel-plate waveguides (plane pairs) form a radial transmission line that propagates electromagnetic (EM) energy within a plane cavity. e parallel-plate waveguide also gets excited by currents flowing through the power/ground plane cavity. • Decoupling does not suppress PDN transients in a system where a radial transmission line exists. • EM energy can be dissipated or absorbed by terminating the edges of the PCB in its characteristic impedance. • A region under a large BGA densely populated with vias also appears as a discontinuity due to the large array of anti-pads eating a hole in the plane. eliminate the initial transient but does, how- ever, prevent it from being compounded. Edge effects can be particularly problematic since it is the board edges that are in such close proximity to the chassis and hence the radiat- ing fields can induce currents into the chassis frame. Termination elements inhibit standing waves from developing between the planes and protect peripheral signals and electronics from radiation. Where Do the Adjacent RC Terminations Need to be Placed? at highest effective frequency of concern is typically the fih harmonic of the fundamen- tal frequency, corresponding to the rise time. Assuming that the fastest rise time is one nano- second, then we are concerned with frequen- cies up to 5 GHz. e wavelength, in FR-4 at 5 GHz, is about 1.1 inches. So, in this case, AC- coupled terminations spaced one inch apart is a good approximation of a continuous termina- tion. At 2.5 GHz bandwidth (500 MHz clock), two-inch spacing would suffice. e resistor, capacitor and power/ground vias should be placed closely together with wide connecting traces to minimize inductance as in Figure 3. e added losses from the RC terminations are helpful in the overall response of the PDN as they dampen impedance peaks arising from connecting the devices to the board. e optimization of the PDN is a trial-and- error process that needs to be done in conjunc- tion with the stackup configuration, dielectric materials, decoupling and AC-coupled ter- minations to fully exploit all avenues. Adding the terminations to the periphery of the board will result in new anti-resonance peaks that need to be dampened. e iCD PDN Plan- ner shows the impact of RC terminations on the cavity resonance, allowing the designer to level out the peaks with a selection of decaps. Suppressing the plane resonance peaks to provide a low impedance profile at higher fre- quencies helps to minimize electromagnetic emissions. Figure 3: RC terminations (3R5 and 10nF) are spaced around the periphery of the PCB.