text: Use Ferrite beads prevent electromagnetic interference (EMI) in two directions: from a device or to a device. A conductive cable acts as an antenna – if the device produces radio-frequency energy, this can be transmitted through the cable, which acts as an unintentional radiator. In this case, the bead is required for regulatory compliance to reduce EMI. Conversely, if there are other sources of EMI, such as household appliances, the bead prevents the cable from acting as an antenna and receiving interference from these other devices. This is particularly common on data cables and medical equipment.
  Large ferrite beads are commonly seen on external cabling. In addition, various smaller ferrite beads are used internally in circuits—on conductors or around the pins of small circuit-board components, such as transistors, connectors, and integrated circuits.
  Beads can block low-level unintended radio frequency energy on wires intended to be DC conductors by acting as a low-pass filter. For example, on unbalanced coax transmission lines (such as video cables), the cable is designed to contain the signal, and beads can be used to block stray common mode current from using the cable as an antenna while not interfering with the signal carried inside the cable. In this use, the bead is a simple form of a balun.