Issue link: https://iconnect007.uberflip.com/i/241303
beyond design pdn planning and capacitor selection, part 2 continues Figure 5: The optimized value approach with 21 capacitors. the 60mΩ impedance of the V shape of the target frequency approach—some as high as 800mΩ below 1GHz which is 13 times higher than the target impedance. If an odd harmonic was to fall on that particular frequency, then emissions would also be very high at that frequency. From extensive simulations, I have noticed that there is a direct correlation between AC impedance peaks and electromagnetic radiation. In fact, if a board fails electromagnetic compliancy, emissions can be dampened by changing the capacitors to ones that have a self-resonant frequency (SRF) close to the radiating frequency. The optimized value approach, illustrated in Figure 5, has 21 capacitors of different values and numbers to optimize the overall AC impedance. In this case, 21 capacitors from 10uF to 4.7nF are used. This approach gives a response below the target impedance from 2MHz to 158MHz. The low end is of little consequence, as the operating frequency of concern is much higher. And you will also note that there are no antiresonant peaks, in this case, because the values are so close together the overlaps dampen the 36 The PCB Design Magazine • January 2014 peaks. When the SRFs are spread, the parallel resonant impedance sets the limits to the PDN performance. There are a few ways to reduce the anti-resonant peaks: 1. Adjust the capacitor values so that the SRFs are closer to the anti-resonate peaks. 2. Add a capacitor with a SRF at the antiresonant peak. 3. Add more ESR by increasing the number of capacitors. 4. Increase the capacitance of the planes by using thinner plane to plane dielectrics. The resonant and anti-resonant peaks of the bypass and decoupling capacitors have now been taken into account but we also need to deal with the plane resonance. Ideally the planes, a perfect lumped element capacitor of this size, should provide a very low impedance between power and ground at very high frequencies (several hundred MHz and higher). But planes, left open at the edges, behave like wide un-terminated traces, from a signal integrity point of view,