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JULY 2021 I DESIGN007 MAGAZINE 89 the source rail's impedance in place, the finite source impedance associated with S21 and the infinite source impedance associated with the current source for Z21 will produce this large attenuation at low frequencies where the low impedance of the DC source dominates the impedance of the source rail. e other extreme case, when we leave out the source impedance altogether, would produce equally unrealistic results. We show this in Figures 10 and 11. S21 starts out at 0 dB at 100 Hz but then it drops unrealistically due to the 50-ohm source impedance, which would be very unexpected from a high-current rail impedance. Z21, on the other hand, starts with a large gain and then it settles at the ESR of C4. Figures 9 and 11 illustrate that both S21 and Z21 hugely depend on the impedance of the source rail, whereas the voltage transfer ratio, by its defi- nition, does not. We need to remember that details matter; the illustration and conclusions are valid for the stated category of cases when we con- nect a filter to a main rail, which represents a source-side impedance much lower than the load impedance of the filter output. If you are interested in further details, want to learn about filter measurements and are curi- ous about correlations between measurement and simulation, check out the further reading listed below. DESIGN007 Resources 1. "Frequency-Domain Characterization of Power Distribution Networks," Artech House, 2007, Sec- tion 5.4. 2. "Using Ferrites and Inductors in Power Distribu- tion Networks (PDN)," Samtec gEEk spEEk Webinar, December 3, 2020. 3. "Do You Really Need that Ferrite Bead in the PDN?" by Istvan Novak, Design007 Magazine, June 2020. Istvan Novak is the principal signal and power integrity engineer at Samtec with over 30 years of experience in high- speed digital, RF, and analog circuit and system design. He is a Life Fellow of the IEEE, author of two books on power integrity, and an instructor of signal and power integrity courses. He also provides a website that focuses on SI and PI techniques. To read past columns or contact Novak, click here. Figure 11: Transfer parameters for the circuit of Figure 10.

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