In a typical computer system found in a home or office, you normally see these "bumps" on themouse,keyboardandmonitorcables. You can also find them on power supply wires when a device (like aprinterorscanner) uses an external transformer.
These "bumps" are calledferrite beadsor sometimesferrite chokes. Their goal in life is to reduceEMI(electromagnetic interference) andRFI(radio-frequencyinterference). You can see these beads in the following photo:
Advertisement
A ferrite bead is simply a hollow bead or cylinder made offerrite, which is a semi-magnetic substance made from iron oxide (rust) alloyed with other metals. It slips over the cable when the cable is made, or it can be snapped around the cable in two pieces after the cable is made. The bead is encased in plastic -- if you cut the plastic, all that you would find inside is a black metal cylinder.
Computersare fairly noisy devices. Themotherboardinside the computer's case has anoscillatorthat is running at anywhere from 300 MHz to 1,000 MHz. The keyboard has its own processor and oscillator as well. Thevideo cardhas its own oscillators to drive the monitor. All of these oscillators have the potential to broadcastradio signalsat their given frequencies. Most of this interference can be eliminated by the cases around the motherboard and keyboard.
Another source of noise is the cables connecting the devices. These cables act as nice, long antennae for the signals they carry. They broadcast the signals quite efficiently. The signals they broadcast can interfere with radios andTVs. The cables can also receive signals and transmit them into the case, where they cause problems. A ferrite bead has the property ofeliminating the broadcast signals. Essentially, it "chokes" the RFI transmission at that point on the cable -- this is why you find the beads at the ends of the cables. Instead of traveling down the cable and transmitting, the RFI signals turn intoheatin the bead.
Advertisement
