Chokes and Ferrite Beads

Ferrite Beads and Ferrite Chokes both havecomputer cables. Usually you will see a bump or long
impedance properties that are used as filters andcylinder on cables going to monitors. These are simple
shields to suppress unwanted interference in electronicsnap-on ferrite beads that suppress the potential RF
circuits. Stray high frequency signals, commonlyinterference from the cables.
referred to as "noise", can interfere with properAnother common use or proposed use is on cell
operation of other surrounding circuits. Unwanted noisephones. There is great debate on whether the
on top of the main signal is similar to when you areradiation from cell phones will cause brain damage and
talking 1 on 1 with someone at a party but it is difficultthere are several products on the market that are
to hear because there is so much noise from thesimilar to ferrite chokes or shields that are designed to
other people (circuits) that surround you. In circuits,suppress the radiation and the interference. You can
through electromagnetic coupling, one circuit with veryaudibly hear this interference if your cell phone is
sharp pulses can introduce extra signals on top of theanywhere near your computer speakers. The cables
main signal in an adjacent circuit and thus becomes aon those speakers pick up the energy radiated from
source of noise and makes it difficult for the receivingthe cell phone and you typically hear an annoying buzz.
circuit to understand the main signal.In addition to adding beads to your phone, you can also
So what are they? A ferrite bead is a passiveadd the snap on beads to your speakers to quiet the
electronic component and is made of ferromagneticinterference down (unless you like the 1 second
compounds containing iron and some amount of nickel,warning that your phone is about to ring).
zinc, or manganese oxides. The impedance propertiesWith faster and faster circuits these days, along with
of the ferrite choke allow it to act with a highmore and more packed onto a circuit board, noise
resistance to high frequency signals and lowsuppression with ferrite beads and other techniques is
resistance to lower frequency signals; thus dampingparamount. Digital designers only think ones and zeros
out the high frequency noise and the absorbed energysignaling until discovering the nightmares of the analog
is converted to a very small amount of heat. The typeworld with noisy circuit behavior or interference from
of ferrite material used in the bead will determine thepower supply or grounding issues. In addition to
range of frequency suppression, and the physicalprotecting yourself from your own circuits, there are
dimensions and shape of the ferrite bead determinemany so called harden applications where one must
the amount of suppression possible. The beadsprotect the circuits and suppress any noise introduced
themselves can be a simple dowel looking device withby outside sources such as a strong transmitter either
a hole in the middle or what looks like a metal blob thatby accident (such as a cell phone on an airplane) or on
is a multi-layer bead used in surface mount applications.purpose (attempted jamming in a military application).
Electromagnetic Interference (EMI) suppression beads,Ferrite bead and chokes are simple devices and can
while similar to inductors, are designed to rise inbe the first level of defense for RF suppression and
impedance with frequency until a resonance point. Butother noise filtering applications. A bead manufacturer
unlike a regular inductor whose impedance would startcan provide you with curves of impedance versus
to drop as frequency continued to increase, the ferritefrequency for the various bead types they produce
bead inductance flattens out and remains a goodand they typically have application engineers to help
suppressor for a wide frequency band.with the selection of the proper type of bead.
One very simple application of ferrite chokes is on