Issue link: https://iconnect007.uberflip.com/i/1391285
88 DESIGN007 MAGAZINE I JULY 2021 First, we can notice that the S21 and Z21 curves run in parallel. e vertical difference between them is 25x, which comes from the parallel equivalent of the two 50-ohm termina- tions we need to calculate S21. We also notice that both curves start at very low values at 100 Hz; this may seem to be good news, but in real- ity, it is misleading. In this test we assume that the input side of the filter is driven by a high- current low-impedance supply rail. We get a low S21 or Z21 value simply because the low source-side impedance shunts out our test sig- nal. In reality, whatever noise appears across the source rail will be forced across the input of the filter, which represents a much higher impedance and as such it has very little influ- ence on the input noise. When we calculate the noise transfer with Figure 9: Z21, S21 and voltage transfer ratio. Figure 10: Simulation deck for filter transfer function without including the impedance of the input-side supply rail.