A Ferrite bead is a dowel-like device which has a center holes and is composed of ferromagnetic material. When placed onto a current carrying conductor it acts as an RF choke. It offers a convenient, inexpensive, yet a very effective means of RF shielding, parasitic suppression and RF decoupling.
The most common noise generating suspects in high frequency circuits are power supply leads, ground leads and connections, and inter stage connections. Adjacent leads and unshielded conductors can also provide a convenient path for the transfer of energy from one circuit to another. A few ferrite beads of the appropriate material placed on these leads can greatly reduce or completely eliminate the problem. Best of all, they can be added to most any existing electronic circuit.
The amount of impedance is a function of both the material and the frequency, as well as the size of the bead. As the frequency increases, the permeability declines causing the losses to rise to a peak. With a rise in frequency the bead presents a series resistance with very little reactance. Since reactance is low there is little chance of resonance which could destroy the attenuation effect. Impedance is directly proportional to the length of the bead, therefor impedance is additive as each similar bead is slipped onto the conductor. Since the magnetic field is totally contained within, it does not matter if the beads are touching or separated. Ferrite beads do not have to be grounded and they cannot be detuned by external magnetic fields.