Glossary of Shielding Terms
AF - Audio Frequency - Components of a signal or noise having frequencies in the 15 Hz - 20 kHz range.
Air Filters, Shielded - Used to permit ventilating shielded rooms and cabinets while removing dust and other air suspended particles. The filter also provides a shield to RF energy, which would otherwise leak in or out of an enclosure. Air filters are often used where wave-guide-beyond-cut-off honeycomb shielding would otherwise be used.
Anechoic Chambers (rooms) - A lined metallic enclosure that isolates the electromagnetic ambient environment while providing low internal reflections. These chambers, sometimes called microwave dark rooms, are usually large rooms lined with absorbing materials to reduce R-F reflections.
Attenuation - The difference in dB [20 log10 (V-input/V-output)] of a device (e.g., a power line filter PLF) vs. frequency measured in a system of unknown (e.g., in-situ) or defined input and output impedances.
BeCu - Beryllium Copper - Among other applications, BeCu is used extensively for shielding finger stock gaskets because of its maximum spring properties of strength and fatigue resistance.
Biconical Antenna - A broadband dipole antenna used to measure and produce electric fields from approximately 30 MHz to 300 MHz.
Bilog Antenna - A combination of a biconical and a log periodic antenna with an automatic crossover network. It has a frequency range from about 26 MHz to 2 GHz.
Common Mode - Signals that are identical in amplitude and phase at both inputs; the potential or voltage that exists between neutral and ground. Most electronic equipment requires it to be as close to 0 V and not to exceed 1&Mac218;2 V.
Common-Mode Current - The component of the signal current that induces electric and magnetic fields that do not cancel each other. For example, in a circuit with one outgoing signal conductor and one ground conductor, the common-mode current is the component of the total signal current that flows in the same direction on both conductors. It is the primary source of EMI in many electronic systems.
Common-Mode Interference - Interference that appears between signal leads or the terminals of a measuring circuit and ground.
Common-Mode Rejection Ratio - The ratio of the common-mode interference voltage at the input of a circuit to the corresponding interference voltage at the output. A high ratio is desirable. The ratio expresses the capability of the device to reject the effect of a voltage that is applied simultaneously to both input terminals.
Conductive Elastomer - An elastomer containing metal powder or small flakes for bonding metal parts to achieve a defined shielding effectiveness.
Conducted Emission - The potential EMI generated inside equipment and carried through the I/O lines, the control leads, or power lines.
Conducted Susceptibility - The EMI that couples from the outside of equipment to the inside over the I/O cables, signal leads, or power lines.
Current Probe - An EMI measuring sensor that clamps onto a wire, wire pair, coaxial line, or cable harness. Snap-on current probes measure the normal-mode current in a wire pair, coax, or wire bundle. They help locate and quantify ground loops.
dB- Decibel - A unit of the logarithm of a ratio measurement = 10 log10 (P1/P2) = 20 log10 (V1/V2)-10 log10 (Z2/Z1).
Dipole Antenna - Antenna with the gain, pattern, and impedance defined at and near resonance of one-half wavelength. The antenna is split at its electrical center for connection to a transmission line. The radiation pattern is maximum at right angles to the axis of the antenna.
Double Shielded Enclosure - A type of shielded room or enclosure in which the inner wall is isolated from the outer wall except in the region of where power-line filters and coaxial connectors penetrate.
E3 = Electromagnetic Environmental Effects - A broad umbrella term used to cover EMC, RFI, EMI, EMP, ESD, RADHAZ, lightning and the like.
Electrical Gaskets - A gasket used between two mating metal members to secure a low-impedance path. Gaskets are used to fill and electrically join and seal leaky apertures between these mating members in order to maintain a minimum shielding effectiveness over a defined frequency spectrum.
Electromagnetic Compatibility - The capability of equipment to be operated in its intended operational environment at designed levels of efficiency without causing electromagnetic interference.
Electromagnetic interference (EMI) - Any conducted radiated or magnetically induced voltage which degrades, obstructs, or repeatedly interrupts the desired performance of electronic equipment.
Electromagnetic Spectrum - The entire range of wavelengths or frequencies of electromagnetic radiation extending from gamma rays to the longest radio waves and including visible light.
EMC = Electromagnetic Compatibility - Operations of equipment and systems in their installed environments which cause no unfavorable response to or by other equipments and systems in the same environment.
EMI = Electromagnetic Interference - When an electrical disturbance from a natural phenomena (e.g., lightning or ESD) or an electrical/electronic device or system causes an undesired conducted or radiated response in a victim. EMI is the opposite of EMC.
EMI Filter - A circuit or device containing series-inductive and parallel-capacitive components that provide a low impedance path for high-frequency noise around a protected circuit.
Emission, EMI - The unintentional or undesired exiting of potentially interfering electromagnetic energy from electrical / electronic sources.
Faraday Shield - A conductive material used to contain or control an electric field. It is placed between the primary and secondary windings of a transformer to reduce coupling capacitance and common-mode noise. The shield provides electrostatic shielding while passing electromagnetic waves. Ground is not sneeded.
Ferrites, EMI - Powered magnetic (permeable) material in the form of beads, rods, and blocks used to absorb conducted interference on wires, cables and harnesses.
Ferrite Material - Made by calcining a combination of metal oxides sintered into tiles. Material only a few millimeters thick absorbs low frequencies. Tiles may be used with dielectric materials or as a hybrid combination with dielectric pyramids.
Filter - A device for blocking the flow of EMI current while passing the desired 50/60/400-Hz current. In communications circuits, it suppresses unwanted frequencies, noise, or separates channels.
Filters, EMI/RFI - Filters designed for powerline and/ or signal line applications to pass a defined band and reject emissions above the cutoff frequency.
Finger Stock - A beryllium copper, electrical gasket used to bond metal panel members on doors, sills or covers designed to accommodate many openings and closings with limited aging effects.
Gaskets, Electrical - Gaskets are used to fill and electronically bond and seal leaky apertures between mating panel member parts in order to maintain a minimum shielding effectiveness over a defined frequency spectrum.
Honeycomb, EMC Airflow - A hexagonal cell configuration (honeycomb) to permit smooth airflow into and out of shielded enclosures while also blocking electromagnetic radiation leakage.
Horn Antenna - A microwave antenna made by flaring out the end of a circular or rectangular waveguide into the shape of a horn; for radiating radio waves into space.
Insertion Loss - The ratio between the power received at a specified load before and after the insertion of a filter at a given frequency. It is an indication of the attenuation provided by a filter.
IEEE/EMC - The EMC society within the IEEE (Institute of Electrical Engineers.
Log Periodic Antenna - A broadband antenna. The electrical lengths and element spacings are chosen so the bi-directional radiation pattern, impedance, and other antenna properties are repeated for several frequencies. The bandwidth is approximately the ratio of the longest dipole element to the shortest.
Loop Antenna - An antenna consisting of one or more complete turns of a conductor; usually tuned to resonance by a variable capacitor connected to the terminals of the loop. It measures magnetic-field strengths at frequencies <30 kHz.
Method of Moments - Equations for numerically computing electromagnetic fields.
MHz - Megahertz - 1000 kHz = 10-6 Hz
Ohm's Law - The mathematical relationship between current (I), voltage (V), and resistance ( R) where V=I x R. If any of the two variables are known, the third can be calculated.
Parasitic Capacitance - The capacitive leakage across a component such as a resistor, inductor, filter, isolation transformer, or optical isolator that adversely affects high-frequency performance.
Permeability - The extent to which a material can be magnetized; often expressed as the parameter relating the magnetic-flux density induced by an applied magnetic-field intensity.
RADHAZ Meter = Radiation Hazard Meter - A hand-held, battery operated, broad-band receiver designed to measure the electric-power density (electric-field strength) from typically 10 kHz to about 18 GHz in two or three bands.
Radiated Emission (RE) - The potential EMI, which radiates from escape-coupling paths such as cables, leaky apertures, or inadequately shielded housings.
Radiated Susceptibility - Undesirable EMI radiated into equipment from outside electromagnetic sources.
Radiation - The outward flow of energy from any source in the form of electromagnetic energy.
Radio Frequency - A frequency at which coherent electromagnetic radiation of energy is useful for communications. Radio frequencies are designated as very low: <30 kHz, low: 30 to 300 kHz, medium: 300 to 3,000 kHz, high: 3 to 30 MHz, very high: 30 to 300 MHz, ultrahigh: 300 to 3,000 MHz, superhigh: 3 to 30 GHz, and extremely high: 30 to 300 GHz.
Radio Frequency Interference (RFI) - A high frequency, cyclic series of spikes or noise injected onto an electrical line by means of radio wave energy or by a piece of equipment connected to the line. Exists when either the transmitter or receiver is carrier operated (has an antenna), causing undesired responses to or from other electronic equipment or systems.
RFI/EMI Filters - Low-pass filters designed for power lines and/ or control and signal leads.
Ripple - The AC component of the output of a DC signal. The term typically refers to the residual line-frequency-related AC part in the output of a DC power supply that arises as a result of incomplete or inadequate filtering. The amount of filtering depends on the ripple frequency and the load resistance. As load resistance decreases, more filtering is required.
Shielded Cables - To protect EMI from entering or exiting a cable, shield(s) may be added. Braids and foils are the most popular shields.
Shielding Effectiveness - The relative capability of a shield to screen out undesirable electric and magnetic fields and plane waves. The measurement is the ratio of the signal received without the shield to the signal received inside the shield.
Shielding Gasket - A material that maintains shielding effectiveness across a seam or gap in an electronic enclosure. It is made from a variety of materials including fabric-wrapped foam, wire mesh, stamped metal, and elastomer.
Shielded Room - A room made free from EMI by applying shielding to the floor, walls, and ceiling, and by suppressing interference entering through the power lines. Typical construction shields from 70 dB to 140 dB from 10 kHz to 10 GHz.
Shielded Windows - Shielding accomplished by using a thin conducted film on the glass, or a fine-wire mesh or metalized open-mesh textile.
Shielding (Electrical Shielding) - A process of preventing radiation from coupling into or out of defined areas or regions. Shielding materials are always metals, metalized plastics (conductive coatings) or conductive composites.
Shielding Fabrics - Fabrics made of (1) metal threads or yarn, or (2) conductive-coated yarn, woven to form a shielded fabric.
Shielding Foils and Sheets - Thin sheets of metallic foil are used for both shielding and grounding. Foil sheets are usually adhesive backed to line nonconductive boxes, cabinets, and walls. They also make low-impedance ground plates.
Shielding Gaskets - Electrical gaskets used to bond to pieces of metal or to fill voids between mating metal members to block aperture leaks.
Shielding Vents - Used for HVAC or simple ventilation of shielded products, cabinets, enclosures, and rooms. Some provide high shielding, such as honeycomb, while others provide air filtering, as well.
Shielding Windows - Metal mesh screen or thin films deposited on a substrate used to cover displays to block RF radiation while permitting optical viewing.
TEM Cells - Transverse electromagnetic cell; a chamber that maintains its characteristic impedance throughout its volume. Cable, connector assemblies, and electronic devices are placed inside the cell. The cell also can be used as a detector to measure radiation emitted by devices inside the cell.
Voltage Standing Wave Ratio - A measure of the degree to which a load is impedance matched to its transmission line. A perfect match has a VSWR of 1.0 while an imperfect match has a greater standing wave ratio value.
Waveguide - A dielectric or metallic medium that confines, supports, and guides the energy of propagating wave, such as an electromagnetic or acoustic wave, along a prescribed and controllable path.