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18Sep/090

Chokes and Ferrite Beads

Ferrite Beads and Ferrite Chokes both have impedance properties that are used as filters and shields to suppress unwanted interference in electronic circuits. Stray high frequency signals, commonly referred to as "noise", can interfere with proper operation of other surrounding circuits. Unwanted noise on top of the main signal is similar to when you are talking 1 on 1 with someone at a party but it is difficult to hear because there is so much noise from the other people (circuits) that surround you. In circuits, through electromagnetic coupling, one circuit with very sharp pulses can introduce extra signals on top of the main signal in an adjacent circuit and thus becomes a source of noise and makes it difficult for the receiving circuit to understand the main signal.

So what are they? A ferrite bead is a passive electronic component and is made of ferromagnetic compounds containing iron and some amount of nickel, zinc, or manganese oxides. The impedance properties of the ferrite choke allow it to act with a high resistance to high frequency signals and low resistance to lower frequency signals; thus damping out the high frequency noise and the absorbed energy is converted to a very small amount of heat. The type of ferrite material used in the bead will determine the range of frequency suppression, and the physical dimensions and shape of the ferrite bead determine the amount of suppression possible. The beads themselves can be a simple dowel looking device with a hole in the middle or what looks like a metal blob that is a multi-layer bead used in surface mount applications. Electromagnetic Interference (EMI) suppression beads, while similar to inductors, are designed to rise in impedance with frequency until a resonance point. But unlike a regular inductor whose impedance would start to drop as frequency continued to increase, the ferrite bead inductance flattens out and remains a good suppressor for a wide frequency band.

One very simple application of ferrite chokes is on computer cables. Usually you will see a bump or long cylinder on cables going to monitors. These are simple snap-on ferrite beads that suppress the potential RF interference from the cables.

Another common use or proposed use is on cell phones. There is great debate on whether the radiation from cell phones will cause brain damage and there are several products on the market that are similar to ferrite chokes or shields that are designed to suppress the radiation and the interference. You can audibly hear this interference if your cell phone is anywhere near your computer speakers. The cables on those speakers pick up the energy radiated from the cell phone and you typically hear an annoying buzz. In addition to adding beads to your phone, you can also add the snap on beads to your speakers to quiet the interference down (unless you like the 1 second warning that your phone is about to ring).

With faster and faster circuits these days, along with more and more packed onto a circuit board, noise suppression with ferrite beads and other techniques is paramount. Digital designers only think ones and zeros signaling until discovering the nightmares of the analog world with noisy circuit behavior or interference from power supply or grounding issues. In addition to protecting yourself from your own circuits, there are many so called harden applications where one must protect the circuits and suppress any noise introduced by outside sources such as a strong transmitter either by accident (such as a cell phone on an airplane) or on purpose (attempted jamming in a military application).

Ferrite bead and chokes are simple devices and can be the first level of defense for RF suppression and other noise filtering applications. A bead manufacturer can provide you with curves of impedance versus frequency for the various bead types they produce and they typically have application engineers to help with the selection of the proper type of bead.

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