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EMC, EMI, RF and Electrical Shielding

Absorber: A material which absorbs electromagnetic energy by converting the wave energy into heat.

Absorption Loss: The attenuation of an electromagnetic wave as it passes through a shield. This loss is primarily due to induced currents and the associated heat loss.

Ambient Level: Those levels of radiated and conducted energy existing at a specified location and time when a test sample is deenergized. Atmospheric noise signals, both desired and undesired, from other sources and the internal noise level of the measuring instruments all contribute to the "ambient level".



Antenna: A device employed as a means for radiating or receiving electromagnetic energy.

Aperture: An opening in a shield through which electromagnetic energy passes.

Attenuation: A general term used to denote a decrease in magnitude of power or field strength in transmission from one point to another caused by such factors as absorption, reflection, scattering, and dispersion. It may be expressed as a power ratio or by decibels.

Bond: The electrical connection between two metallic surfaces established to provide a low-resistance path between them.

Bonding: The process of establishing the required degree of electrical continuity between the conductive surfaces to be joined.

Common mode current: The component of signal current that induces electric and magnetic fields that don't tend to cancel one another. For example, in a circuit with one outgoing signal conductor and one return (“ground”) conductor, the common mode current is the component of the total signal current that flows in the same direction on both conductors. Common mode current is the primary source of EMI in many electronic systems.

Common mode voltage: The voltage that drives directed common mode (noise) currents.

Conducted immunity: A product's relative ability to withstand electromagnetic energy that penetrates it through external cables, power cords, I/O interconnects, or chassis. EMI may also couple to a chassis, if interconnects are improperly implemented.

Conductive Interference: Undesired signals that enter or leave an equipment along a conductive (wire or metallic) path.

Containment: A process whereby RF energy is prevented from exiting an enclosure, generally by shielding a product within a metal enclosure (Faraday cage or Gaussian structure) or by using a plastic housing with an RF conductive coating. Reciprocally, we can also speak of containment as preventing RF energy from entering the enclosure.

Coupling: Energy transfer between circuits, equipments, or systems.

Coupling, Free-Space: Energy transfer via electromagnetic fields not in a conductor.

Curie Temperature T C (°C): The temperature above which a ferromagnetic material (such as ferrite) loses its unique magnetic properties. Above the Curie temperature, the relative permeability of a ferrite drops to a value of 1.0.

Cutoff Frequency: The frequency below which electromagnetic energy will not propagate readily in a waveguide.

dB: Decibel, a unit of voltage or power ratio. Defined as follows:
dB = 10 log P2/PI for power or
dB = 20 log V2/V1 for voltage.
"dB" is commonly used to specify shielding effectiveness since very large differences in the input/output fields are generally required by the shielding specification. Specifications on the order of 60-100 dB are typical. This means that if one watt of power impinges on the shield, then only one millionth to one ten trillionth of a watt exits on the other side.

Degradation: A decrease in the quality of a desired signal (i.e., decrease in the signal-to-noise ratio or an increase in distortion), or an undesired change in the operational performance of equipment as the result of interference.

Differential mode voltage: The voltage that drives equal and oppositely directed currents to achieve an intended circuit function; the source of differential mode currents.

Differential mode current: The intended signal currents that are equal and oppositely directed on pairs of signal and return (“ground”) conductors.

Earth Electrode System: A network of electrically interconnected rods, plates, mats, or grids installed for the purpose of establishing a low-resistance contact with earth. The design objective for resistance to earth of this subsystem should not exceed
10 ft.

Electric Field: A vector field about a charged body. Its strength at any point is the force which would be exerted on a unit positive charge at that point.

Electromagnetic Compatibility (EMC): The capability of equipment or systems to be operated in their intended operational environment at designed levels of efficiency without causing or receiving degradation owing to unintentional electromagnetic interference. Electromagnetic compatibility is the result of an engineering planning process applied during the life cycle of the equipment. The process involves careful considerations of frequency allocation, design, procurement, production, site selection, installation, operation, and maintenance.

Electromagnetic Interference (EMI): Any conducted, radiated, or induced voltage which degrades, obstructs, or repeatedly interrupts the desired performance of electronic equipment. The lack of EMC, since the essence of interference is the lack of compatibility. EMI is the process by which disruptive electromagnetic energy is transmitted from one electronic device to another via radiated or conducted paths (or both). In common usage, the term refers particularly to RF signals. EMI can occur in the frequency range commonly identified
as "anything greater than DC to daylight."

Electromagnetic Pulse (EMP): A large impulsive-type electromagnetic wave generated by nuclear or chemical explosions.

Electrostatic discharge (ESD): A transfer of electric charge between bodies of different electrostatic potential in proximity to each other or through direct contact. This definition is observed as a high-voltage pulse that may cause damage or loss of functionality to susceptible devices. Although lightning differs in magnitude as a high-voltage pulse, the term ESD is generally applied to events of lesser amperage and more specifically to events triggered by human beings.

Facility: A building or other structure, either fixed or transportable in nature, with its utilities, ground networks, and electrical supporting structures.

Far Field: The region of the field of an antenna where the radiation field predominates, and where the angular field distribution is essentially independent of the distance from the antenna. A variety of guidelines is used; for some shielding calculations, 1/6th of a wavelength has been found useful.

Fault: An unintentional short circuit or partial short circuit (usually of a power circuit) between energized conductors or between an energized conductor and ground.

Field Strength: A general term that means the magnitude of the electric field vector (in volts per meter) or the magnitude of the magnetic field vector (in ampere-turns per meter). As used in the field of EMC/EMI, the term "field strength" shall be applied only to measurements made in the far field and shall be abbreviated as FS. For measurements made in the near field, the term "electric field strength" (EFS) or
"magnetic field strength" (MFS) shall be used, according to whether the resultant electric or magnetic field, respectively, is measured.

Filter: A device for use on power or signal lines, specifically designed to pass only selected frequencies and to attenuate substantially all other frequencies.

Ground: The electrical connection to earth through an earth electrode subsystem. This connection is extended throughout the facility via the facility ground system, consisting of the signal reference subsystem, the fault protection subsystem, and the lightning protection subsystem.

Immunity: A relative measure of a device or a system's ability to withstand EMI exposure while maintaining a predefined performance level.

Impedance Z (ohm): The impedance of a ferrite may be expressed in terms of its complex permeability:

Incremental Permeability (μD): The permeability of a magnetic material about a specified operating point and applied H (especially under DC bias). The incremental permeability is expressed as the slope of the B-H characteristic about the given operating point:

Initial Permeability (μi): The measured permeability of a magnetic material (with zero initial magnetization) at small flux densities under 10 gauss (1 milliTesla).

Leakage Flux and Inductance: Leakage flux is the small fraction of the total magnetic flux in a transformer or common mode choke that does not contribute to the magnetic coupling of the windings of the device. In a transformer with a single set of primary and secondary windings, the leakage flux is that portion of flux that's produced by the primary that does not link the secondary.

The presence of leakage flux in a transformer or common mode choke is modeled as a small “leakage” inductance in series with each winding. In a multi-winding choke or trans-former, leakage inductance is the inductance measured at one winding with all other windings short circuited.

Line Conducted Interference (LCI): Refers to RF energy in a power cord or AC mains input cable. Conducted signals don't propagate as fields but may propagate as conducted waves.

Loss Tangent: The measure of the loss of a magnetic material at high operating frequencies due to the oscillation of microscopic magnetic regions within the material. The loss tangent is expressed as the ratio of the imaginary permeability component μ” to the real permeability μ’ of the material.

Magnetic Field: A vector field produced by a continuous flow of charge.

Magnetic Field Intensity or Magnetizing Force (H) (measured in Oersteds or Amperes/m): The mmf per unit length. H can be considered to be a measure of the strength or effort that the magnetomotive force applies to a magnetic circuit to establish a magnetic field. H may be expressed as H = NI/l where l = the mean length of the magnetic circuit in meters. Common industry practice uses a magnetizing force of 0.01 Oersted (79.6 amp-turns/m) to find the initial permeability of a magnetic material.

Magnetic Flux Density (B) Gauss: The number of magnetic flux lines per area (measured in Gauss or Webers/m2) that's induced by an applied magnetic field intensity H. The B results from an applied H is given by B = µH, where µ is the permeability (sometimes referred to as the absolute permeability) of the magnetic material in which the flux is contained.

Magnetomotive Force (mmf): The electromagnetic force that causes a magnetic flux to flow in a magnetic circuit. It is analogous to the electromotive force (emf) that gives rise to current in an electrical circuit. The mmf applied to a multi-turn inductor is expressed as F = mmf = NI ampere turns, where N is the number of conductor turns and I is the series current through the inductor.

Multipoint Ground: More than one path to ground.

National Electrical Code®: (NEC®), A standard governing the use of electrical wire, cable, and fixtures installed in buildings.

Near Field: The region of the field immediately surrounding an antenna where the inductive and capacitive fields predominate. In this region, the angular distribution of the field varies with distance from the antenna.

Neutral: The ac power system conductor which is intentionally grounded on the supply side of the service disconnecting means. The neutral provides a current return path for ac power currents, whereas the ground (or green) should not, except during fault
conditions.

PCB: Printed circuit board

Penetration: The passage through a partition or wall of an equipment or enclosure by a wire, cable, pipe, or other conductive object.

Permeability (μ): The extend to or ease with which a material can be magnetized, often expressed as the parameter relating the magnetic flux density B induced by an applied magnetic field intensity H, as B = μH. The “absolute” permeability of a given material is expressed as the product of its relative permeability μr (a dimensionless constant) and the free space constant μo.

Plane Wave: An electromagnetic wave which predominates in the far-field region of an antenna, and with a wavefront which is essentially a flat plane. In free space, the characteristic impedance of a plane wave is 377 Ω.

Radiated immunity: A product's relative ability to withstand electromagnetic energy that arrives via free-space propagation.

Radiation: The emission and propagation of electromagnetic energy through space.

Radiation Resistance: The resistance which, if inserted in place of an antenna, would consume the same amount of power that's radiated by the antenna.

Radio Frequency Interference (RFI): Often synonymous with electromagnetic interference, RFI is a frequency range containing coherent electromagnetic radiation of energy useful for communication purposes—roughly the range from 10 kHz to 100 GHz. This energy may be transmitted as a byproduct of an electronic device's operation. RF is transmitted through two basic modes:

  • Radiated emissions: The component of RF energy that's transmitted through a medium as an electromagnetic field. RF energy is usually transmitted through free space; however, other modes of field transmission may occur.
  • Conducted emissions: The component of RF energy that's transmitted through a medium as a propagating wave, generally through a wire or interconnect cables.

RF-Tight: Offering a high degree of electromagnetic shielding effectiveness.

Reflection Loss: The portion of the transition loss, expressed in decibels, that's due to the reflection of power at a barrier or shield. Reflection loss is determined by the magnitude of the wave impedance inside the barrier relative to the wave impedance in the propagation medium outside the barrier.

Resistivity (r): The intrinsic property measured in ohm-centimeters that quantifies a material’s oppositon to free electron motion. Resistivity is the reciprocal property to conductivity. The resistance of a homogeneous material of uniform cross section A and length l can be found by:

Saturation: The point at which the flux density B in a magnetic material does not increase with further applications of greater magnetization force H. At saturation, the slope of a material’s B-H characteristic curve becomes extremely small, with the instantaneous permeability approaching that of free space (relative permeability = 1.0)

Saturation Flux Density: The flux density B at which saturation of a magnetic material occurs.

Shield: A housing, screen, or cover which substantially reduces the coupling of electric and magnetic fields into or out of circuits or prevents the accidental contact of objects or persons with parts or components operating at hazardous voltage levels.

Shielding Effectiveness: A measure of the reduction or attenuation in the electromagnetic field strength at a point in space caused by the insertion of a shield between the source and that point.

Shielded Enclosure: An area (box, room, or building) specifically designed to attenuate electromagnetic radiation, or electromagnetic radiation and acoustical emanations, originating either inside or outside the area. Necessary openings in shielded enclosures, such as doors, air vents, and electrical feedthroughs, are specially designed to maintain this attenuation.

Signal Reference Subsystem: This subsystem provides the reference points for all signal grounding to control static charges, noise, and interference. It may consist of any one or a combination of the lower frequency network, higher frequency network, or
hybrid signal reference network.

Suppression: The process of reducing or eliminating RF energy that exists without relying on a secondary method, such as a metal housing or chassis. Suppression may include shielding and filtering as well.

Susceptibility: A relative measure of a device, or a system's propensity to be disrupted or damaged by EMI exposure to an incident field or signal. It's the lack of immunity.

TEMPEST: A code word (not an acronym) which encompasses the government/industrial program for controlling the emissions from systems processing classified data. Individual equipment may be 'TEMPESTED" or commercial equipment may be placed in shielded enclosures.

Wave Impedance: The ratio of the electric field strength to the magnetic field strength at the point of observation.

Wavelength: The ratio of C, the speed of light, to F, the frequency. Wavelength (ft) = 984/F (MHz).

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Updated: Tuesday, 2008-08-19 17:15 PST