EMI/RFI Suppression

Electromagnetic interference (EMI) or radio-frequency interference (RFI) is an undesirable property of electrical systems. If severe enough, the interference can cause nearby electronic devices to malfunction.
In the US, the FCC regulates the total radiated power a device is allowed to emit. Exceeding that limitation can cause the product to be withheld from the market and, therefore, trigger costly product redesign and re-qualification. When dealing with EMI/RFI, an ounce of prevention is worth a pound of cure. Proper consideration of all radiation sources (cables, long PCB trace routes, and insufficient decoupling capacitors) throughout the design process can reduce or eliminate the need EMI/RFI suppression when the product is nearing release.
Because electromagnetic radiation cannot propagate through a conductor, the simplest and most common method of preventing EMI is to place the electronic system inside a metal chassis or enclosure that acts as a shield. Strictly speaking, this does not eliminate the source of radiation, but it does prevent the radiation from 'leaking' to the outside world.
A ferrite bead (or choke) is an electrical filtering component that is widely used for preventing EMI/RFI. Usually, the choke is installed on a cable connected to the system to prevent it from acting as an unintentional radiator. The choke not only prevents unwanted radiation, but also provides immunity from other sources of unwanted electromagnetic interference.