I am reading AN 583: Designing Power Isolation Filters with Ferrite Beads for Intel® FPGAs and in page 2, figure 2, there is a SPICE circuit to emulate the ferrite beads frequency response.
Why there is an AC current source in the circuit? Why not just "plug" the bead to the AC voltage source and ground and run the simulation?
With a 1 ampere AC current source in series with the ferrite bead model, the voltage across the ferrite bead at its terminals (including all of the R's, L', and C's) is numerically equal to its impedance. Thus plotting that voltage against frequency gives the desired result of ferrite bead impedance vs. frequency which are the plots given in the application note.
