ferrite beads with today's ic

Hello everyone, microcontrollers working at high clock frequencies or even some ics, as ethernet controllers or switches, require the use on power supplies' pins, in addition to the usual bypass capacitors, of ferrite bead in order to reduce emi.

Let's say that I feed these chips with an output of a DC / DC converter which has already its emi filters.

My questions are:

first of all I would like to understand what is a ferrite bead and as at high frequencies it is able to suppress emi noise (I do not understand the question of impedance at different frequencies)

there could be problems (oscillations, and more) putting a ferrite bead in series to output of a DC / DC conveter (my opinion is that there should be no problems because usually the dcdc switching is around khz frequencies) .. .

Thanks let me know your opinions

Ferrite beads are lossy inductors, at frequencies below e.g. 10 MHz they behave purely inductive, at higher frequencies more like resistors.

LC filters in power supply always bring up a risk of oscillations with load transients, depending on the LC resonance frequency, characteristic impedance, load impedance and supply current waveforms. In case of doubt, you should calculate and/or simulate the expectable behaviour of the filters.

The point of placing ferrite beads near the EMI source is to keep the interferences from spreading allover the board, being radiated by power supply lines and planes. This can only work if low ESR/ESL capacitors are directly bypassing the switching currents at the chip.