Cable Ferrites prevent electromagnetic interference (EMI) in two directions: from a device or to a device. A lead wire acts like an antenna: if the device produces RF radio frequency energy, it can be transmitted through the wire, which acts as an unintended radiator. In this case, it is necessary for regulatory compliance, to reduce EMI. On the contrary, if there are other sources of EMI, such as appliances, the ferrite filter prevents the cable from acting as an antenna and receiving interference from these other devices. This is particularly common in data cables and medical equipment, those that carry the video signal on TV monitors and computers, etc.
Large ferrite beads or filters are commonly seen in external wiring. Several smaller ferrite beads are used internally in circuits, in conductors, or around the pins of small components on the circuit board, such as transistors, connectors, and integrated circuits.
In cables intended to be DC conductors, the beads can block unwanted low-level RF energy by acting as a low-pass filter. In unbalanced coaxial transmission lines (for example, USB cables and video cables), the cable is designed to contain the signal, and the beads can be used to prevent the parasitic common mode current from using the cable as an antenna without interfering with the signal carried inside. the wire. In this use, the pearl or bead is a simple form of ball.