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A ferrite bead (also known as a ferrite block, ferrite core, ferrite ring, EMI filter, or ferrite choke [1] [2] ) is a type of choke that suppresses high-frequency electronic noise in electronic circuits.
Ferrite beads employ high-frequency current dissipation in a ferrite ceramic to build high-frequency noise suppression devices.
Ferrite beads prevent electromagnetic interference (EMI) in two directions: from a device or to a device. [1] 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. [1]
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.
Ferrite beads are one of the simplest and least expensive interference filters to install on preexisting electronic cabling. For a simple ferrite ring, the wire is wrapped around the core through the center, typically five or seven times.[ citation needed ] Clamp-on cores are also available, which attach without wrapping the wire: this type of ferrite core is usually designed so that the wire passes only once through it. If the fit is not snug enough, the core can be secured with cable ties, or if the center is large enough, the cabling can loop through one or more times. (However, although each loop increases the impedance to high frequencies, it also shifts the frequency of the highest impedance to a lower frequency.) Small ferrite beads can be slipped over component leads to suppress parasitic oscillation. [3]
Surface-mount ferrite beads are available. Like any other surface-mount inductor, these are soldered into a gap in the printed circuit board trace. Inside the bead component, a coil of wire runs between layers of ferrite to form a multi-turn inductor around the high-permeability core. [4]
Ferrite beads are used as a passive low-pass filter by dissipating radio frequency (RF) energy as heat by design.
Ideal inductors, on the other hand, have no resistance and hence do not dissipate energy as heat. Ideal inductors only have inductive reactance, which reduces the flow of high-frequency signals by returning some of their energy back towards the signal source (possibly reducing the amount of power drawn) rather than dissipating that energy as heat (as done by the resistance in ferrite beads). While an inductor's reactance may commonly be referred to simply as impedance, impedance generally can be any combination of resistance and reactance. [ citation needed ]
The geometry and electromagnetic properties of coiled wire over the ferrite bead result in an impedance for high-frequency signals, attenuating high-frequency EMI/RFI electronic noise. The energy is either reflected back up the cable or dissipated as low-level heat. Only in extreme cases is the heat noticeable.
A ferrite bead can be added to an inductor to improve, in two ways, its ability to block unwanted high frequency noise. First, the ferrite concentrates the magnetic field, increasing inductance and, therefore, reactance, which filters out the noise. Second, if the ferrite is so designed, it can produce an additional loss in the form of resistance in the ferrite itself. The ferrite creates an inductor with a very low Q factor. [3] This loss heats the ferrite, generally by a negligible amount. While the signal level is large enough to cause interference or undesirable effects in sensitive circuits, the energy blocked is typically relatively small. Depending on the application, the resistive loss characteristic of the ferrite may or may not be desired.
A design that uses a ferrite bead to improve noise filtering must consider specific circuit characteristics and the frequency range to block. Different ferrite materials have different properties concerning frequency, and the manufacturer's literature helps select the most effective material for the frequency range. [3] [5]
An electromagnetic coil is an electrical conductor such as a wire in the shape of a coil. Electromagnetic coils are used in electrical engineering, in applications where electric currents interact with magnetic fields, in devices such as electric motors, generators, inductors, electromagnets, transformers, sensor coils such as in medical FMRi imaging devices with coils going upto 3-7 and even higher Tesla. Either an electric current is passed through the wire of the coil to generate a magnetic field, or conversely, an external time-varying magnetic field through the interior of the coil generates an EMF (voltage) in the conductor.
An inductor, also called a coil, choke, or reactor, is a passive two-terminal electrical component that stores energy in a magnetic field when electric current flows through it. An inductor typically consists of an insulated wire wound into a coil.
In telecommunications and professional audio, a balanced line or balanced signal pair is an electrical circuit consisting of two conductors of the same type, both of which have equal impedances along their lengths, to ground, and to other circuits. The primary advantage of the balanced line format is good rejection of common-mode noise and interference when fed to a differential device such as a transformer or differential amplifier.
Electromagnetic compatibility (EMC) is the ability of electrical equipment and systems to function acceptably in their electromagnetic environment, by limiting the unintentional generation, propagation and reception of electromagnetic energy which may cause unwanted effects such as electromagnetic interference (EMI) or even physical damage to operational equipment. The goal of EMC is the correct operation of different equipment in a common electromagnetic environment. It is also the name given to the associated branch of electrical engineering.
Balanced audio is a method of interconnecting audio equipment using balanced interfaces. This type of connection is very important in sound recording and production because it allows the use of long cables while reducing susceptibility to external noise caused by electromagnetic interference. The balanced interface guarantees that induced noise appears as common-mode voltages at the receiver which can be rejected by a differential device.
A balun is an electrical device that allows balanced and unbalanced lines to be interfaced without disturbing the impedance arrangement of either line. A balun can take many forms and may include devices that also transform impedances but need not do so. Sometimes, in the case of transformer baluns, they use magnetic coupling but need not do so. Common-mode chokes are also used as baluns and work by eliminating, rather than rejecting, common mode signals.
An antenna tuner is a passive electronic device inserted between a radio transmitter and its antenna. Its purpose is to optimize power transfer by matching the impedance of the radio to the signal impedance on the feedline to the antenna.
In an electrical system, a ground loop or earth loop occurs when two points of a circuit are intended to have the same ground reference potential but instead have a different potential between them. This is typically caused when enough current is flowing in the connection between the two ground points to produce a voltage drop and cause two points to be at different potentials. Current may be produced in a circular ground connection by electromagnetic induction.
Electromagnetic interference (EMI), also called radio-frequency interference (RFI) when in the radio frequency spectrum, is a disturbance generated by an external source that affects an electrical circuit by electromagnetic induction, electrostatic coupling, or conduction. The disturbance may degrade the performance of the circuit or even stop it from functioning. In the case of a data path, these effects can range from an increase in error rate to a total loss of the data. Both human-made and natural sources generate changing electrical currents and voltages that can cause EMI: ignition systems, cellular network of mobile phones, lightning, solar flares, and auroras. EMI frequently affects AM radios. It can also affect mobile phones, FM radios, and televisions, as well as observations for radio astronomy and atmospheric science.
A magnetic core is a piece of magnetic material with a high magnetic permeability used to confine and guide magnetic fields in electrical, electromechanical and magnetic devices such as electromagnets, transformers, electric motors, generators, inductors, loudspeakers, magnetic recording heads, and magnetic assemblies. It is made of ferromagnetic metal such as iron, or ferrimagnetic compounds such as ferrites. The high permeability, relative to the surrounding air, causes the magnetic field lines to be concentrated in the core material. The magnetic field is often created by a current-carrying coil of wire around the core.
In electronics, a choke is an inductor used to block higher-frequency alternating currents (AC) while passing direct current (DC) and lower-frequency ACs in a circuit. A choke usually consists of a coil of insulated wire often wound on a magnetic core, although some consist of a doughnut-shaped ferrite bead strung on a wire. The choke's impedance increases with frequency. Its low electrical resistance passes both AC and DC with little power loss, but its reactance limits the amount of AC passed.
A braid-breaker is a filter that prevents television interference (TVI). In many cases, TVI is caused by a high field strength of a nearby high frequency (HF) transmitter, the aerial down lead plugged into the back of the TV acts as a longwire antenna or as a simple vertical element. The radio frequency (RF) current flowing through the tuner of the TV tends to generate harmonics which then spoil the viewing.
A radio transmitter or receiver is connected to an antenna which emits or receives the radio waves. The antenna feed system or antenna feed is the cable or conductor, and other associated equipment, which connects the transmitter or receiver with the antenna and makes the two devices compatible. In a radio transmitter, the transmitter generates an alternating current of radio frequency, and the feed system feeds the current to the antenna, which converts the power in the current to radio waves. In a radio receiver, the incoming radio waves excite tiny alternating currents in the antenna, and the feed system delivers this current to the receiver, which processes the signal.
In electronics, a ferrite core is a type of magnetic core made of ferrite on which the windings of electric transformers and other wound components such as inductors are formed. It is used for its properties of high magnetic permeability coupled with low electrical conductivity. Moreover, because of their comparatively low losses at high frequencies, they are extensively used in the cores of RF transformers and inductors in applications such as switched-mode power supplies, and ferrite loopstick antennas for AM radio receivers.
A variety of types of electrical transformer are made for different purposes. Despite their design differences, the various types employ the same basic principle as discovered in 1831 by Michael Faraday, and share several key functional parts.
In the field of EMC, active EMI reduction refers to techniques aimed to reduce or to filter electromagnetic noise (EMI) making use of active electronic components. Active EMI reduction contrasts with passive filtering techniques, such as RC filters, LC filters RLC filters, which includes only passive electrical components. Hybrid solutions including both active and passive elements exist. Standards concerning conducted and radiated emissions published by IEC and FCC set the maximum noise level allowed for different classes of electrical devices. The frequency range of interest spans from 150 kHz to 30 MHz for conducted emissions and from 30 MHz to 40 GHz for radiated emissions. Meeting these requirements and guaranteeing the functionality of an electrical apparatus subject to electromagnetic interference are the main reason to include an EMI filter. In an electrical system, power converters, i.e. DC/DC converters, inverters and rectifiers, are the major sources of conducted EMI, due to their high-frequency switching ratio which gives rise to unwanted fast current and voltage transients. Since power electronics is nowadays spread in many fields, from power industrial application to automotive industry, EMI filtering has become necessary. In other fields, such as the telecommunication industry where the major focus is on radiated emissions, other techniques have been developed for EMI reduction, such as spread spectrum clocking which makes use of digital electronics, or electromagnetic shielding.
In electrical engineering, a common-mode signal is the identical component of voltage present at both input terminals of an electrical device. In telecommunication, the common-mode signal on a transmission line is also known as longitudinal voltage.
Sheath current filters are electronic components that can prevent noise signals travelling in the sheath of sheathed cables, which can cause interference. Using sheath current filters, ground loops causing mains hum and high frequency common-mode signals can be prevented.
A sheath current is a form of charge transfer in wires. Sheath currents can run along the outer sheath of a coaxial cable. This can be caused by a geographically proximate or remote ground potential.
This glossary of electrical and electronics engineering is a list of definitions of terms and concepts related specifically to electrical engineering and electronics engineering. For terms related to engineering in general, see Glossary of engineering.
