My answer to What purpose does the small cylindrical module near the end of my laptop charger cable serve?
Answer by Allen He:
This video discusses the basics of ferrite beads, and their uses for basic filtering applications. It discusses and demonstrates how ferrites act as resistors at high frequencies, and how this is used for EMI/RFI Suppression, improved power supply filtering, parasitic oscillation suppression, and others. The affect of the frequency dependent resistive characteristic of the ferrite beads is demonstrated by showing its effect on a swept RF signal, its effect on the rise/fall time of a digital signal, and its effectiveness of eliminating parasitic oscillation. Here are some links to the other videos mentioned in this video: