Like professionals in other disciplines, EMC engineers have their own specialized “language.” To help you better understand their terminology, here’s a selected list of definitions and acronyms.
Glossary
Anechoic Chamber: A room lined with absorbing material to reduce reflections of sound or electro-magnetic waves. A full anechoic room simulates a free-space environment. It has absorber materials on all surfaces, including the floor. A semi-anechoic chamber has absorbing materials on the walls and ceiling. The floor has no absorbing material, and is reflective to create a ground-plane effect.
Attenuation: A reduction in energy. The amount of attenuation generally is expressed in decibels.
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 ½ 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.
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.
Conductive Elastomer: A rubber-like material containing metal powder or small flakes for conducting electricity to achieve a specified shielding effectiveness.
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.
Decibel: Logarithm of a ratio measurement. It is a unit for measuring the power of a sound or the strength of a signal. Increases or reductions of 3 dB double or halve the power in a circuit. The corresponding figure for doubling or halving voltage is 6 dB.
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.
Electromagnetic Compatibility: The capability of equipment or systems to be used in their intended environment within designed efficiency levels without causing or receiving degradation due to unintentional EMI.
Electromagnetic Interference: Any natural or man-made electrical or electromagnetic event conducted or radiated, and resulting in unintentional and undesirable responses,
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.
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. No ground is needed.
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.
Finger Stock: A beryllium copper electrical gasket used to bond metal panel members on doors, sills, or covers.
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.
Log Periodic Antenna: A broadband antenna. The electrical lengths and element spacings are chosen so the bidirectional 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.
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.
Radiated Emission: The potential EMI that emits from paths including cables, leaky apertures, or inadequately shielded housings.
Radiated Susceptibility: Undesirable EMI radiated into equipment from outside electromagnetic sources.
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.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.
SAE J551-2: The Society of Automotive Engineers standard for measuring RFI in cars, boats, and other vehicles.
SAE J1113: The Society of Automotive Engineers standard for determining EMC for vehicles. It includes Part 27 for measuring immunity to radiated EM fields.
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.
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.
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.
Acronyms
ACIL American Council of Independent Laboratories
ANSI American National Standards Institute
CENELEC European Committee for Electrotechnical Standardization
CISPR International Special Committee on Radio Interference
dB decibel
dBm decibels above or below 1 milliwatt
EIA Electronics Industries Association
ELF extremely low frequency
EMC electromagnetic compatibility
EMF electromagnetic fields or electromagnetic force
EMI electromagnetic interference
EMP electromagnetic pulse
EN European Norm
FCC Federal Communications Commission
HIRF high intensity radiated fields
IEC International Electrotechnical Commission
IEEE Institute of Electrical and Electronic Engineers
ISO International Organization for Standardization
LISN line impedance stabilization network
NIST National Institute of Standards and Technology
NMRR normal-mode rejection ratio
NVLAP National Voluntary Laboratory Accreditation Program
PARD periodic and random deviation
RF radio frequency
RFI radio frequency interference
SAE Society of Automotive Engineers
SE shielding effectiveness
SIXPAC system for inertial experiment priority and attitude control
SNR signal-to-noise ratio
TEM transverse electromagnetic
THD total harmonic distortion
VLF very-low frequency
Copyright 1997 Nelson Publishing Inc.
July 1997