This glossary defines the terms used across the EMC standards and testing pages on this site. Each entry is self-contained. For how the terms fit together in practice, see EMC testing basics; for the standards that formally define them, see the EMC standards hub.

A - D

Ambient - the electromagnetic signals present at a test site that do not come from the equipment under test, such as broadcast stations and mobile networks. Ambients must be identified and separated from EUT emissions, which is a main reason shielded chambers replaced many open area test sites.

Anechoic chamber - a shielded room lined with RF-absorbing material so that reflections are suppressed and measurements approximate free space. A semi-anechoic chamber keeps a reflective ground plane to emulate an open area test site; a fully anechoic room has absorber on all six surfaces.

ANSI C63 - the US Accredited Standards Committee on Electromagnetic Compatibility, publisher of measurement standards including ANSI C63.4 (emission measurement methods used for FCC testing). Detailed on our ANSI C63 page.

Antenna factor - the ratio, usually expressed in dB/m, that converts the voltage an antenna delivers to the receiver into the field strength at the antenna. Calibrated per ANSI C63.5; adding the antenna factor and cable loss to the receiver reading yields the field strength in dBuV/m.

CISPR - the International Special Committee on Radio Interference, an IEC committee founded in 1934 that publishes the leading international emission standards (CISPR 11, 25, 32) and the CISPR 16 instrumentation series. See our CISPR page.

Common mode - a signal or noise current that flows in the same direction on all conductors of a cable, returning through ground or parasitic capacitance. Common-mode currents on cables are the dominant source of radiated emissions from most products.

Conducted emissions - RF disturbance a product couples onto its power or signal cables rather than radiating through the air. Measured with a LISN over 150 kHz to 30 MHz in most commercial standards and limited in dBuV.

dBuV - decibels relative to 1 microvolt; the logarithmic unit for conducted emission levels. 0 dBuV = 1 uV; 60 dBuV = 1 mV; 120 dBuV = 1 V.

dBuV/m - decibels relative to 1 microvolt per meter; the logarithmic unit for radiated field strength. The FCC Class B limit of 100 uV/m at 3 m (30-88 MHz) equals 40 dBuV/m.

Differential mode - a signal or noise current that flows out on one conductor and back on another, as intended circuit currents do. Differential-mode noise dominates low-frequency conducted emissions; filters treat it differently from common-mode noise.

Working with dBuV, uV and dBm all day? Use the converter:

E - H

EFT/Burst (electrical fast transient) - repetitive bursts of fast, low-energy transients caused by arcing at relay and switch contacts. The immunity test is defined in IEC 61000-4-4, coupling the bursts onto power and signal lines.

EMC (electromagnetic compatibility) - the ability of equipment to function satisfactorily in its electromagnetic environment without introducing intolerable disturbance to anything else in that environment. The discipline spans emissions, immunity, measurement and mitigation.

EMI (electromagnetic interference) - unwanted electromagnetic energy that degrades the performance of a device or radio service. EMI is the disturbance; EMC is the state of coexisting despite it.

EMS (electromagnetic susceptibility) - the degree to which equipment malfunctions in the presence of electromagnetic disturbance; the inverse framing of immunity. A highly susceptible product has low immunity.

ESD (electrostatic discharge) - the sudden transfer of charge between bodies at different potentials, such as a charged person touching a connector. ESD immunity testing per IEC 61000-4-2 applies contact discharges typically up to 8 kV and air discharges up to 15 kV.

EUT (equipment under test) - the product being evaluated in an EMC test, including its defined configuration, cables and operating mode. Standards also use DUT (device under test) interchangeably.

FCC Part 15 - the US regulation (47 CFR Part 15) governing unlicensed RF devices, setting conducted (Section 15.107) and radiated (Section 15.109) emission limits for Class A and Class B equipment. Explained on our FCC Part 15 page.

Flicker - perceptible variation in lamp brightness caused by voltage fluctuations that loads impose on the mains. Limited by IEC 61000-3-3 for equipment up to 16 A per phase.

GTEM cell (gigahertz transverse electromagnetic cell) - a tapered, absorber-terminated transmission-line enclosure that creates a defined test field in a compact volume, usable from DC to several GHz for pre-compliance emissions and immunity on small EUTs.

Harmonics - currents drawn by non-linear loads (rectifiers, switching supplies) at integer multiples of the 50/60 Hz mains frequency, distorting the supply voltage. Emission limits for harmonic currents are set by IEC 61000-3-2.

I - O

Immunity - the ability of equipment to perform without degradation in the presence of a defined electromagnetic disturbance. Immunity requirements and test levels come mainly from the IEC 61000-4 series; see our IEC 61000 page.

ISM equipment (industrial, scientific and medical) - equipment that intentionally generates RF energy for purposes other than communication, such as induction heaters, RF welders and diathermy machines. Emission requirements are set by CISPR 11.

LISN (line impedance stabilization network) - a network inserted between the mains and the EUT that presents a standardized 50 ohm / 50 uH impedance, blocks incoming grid noise, and provides a port for measuring the EUT’s conducted emissions. See what is a LISN.

Normalized site attenuation (NSA) - the site attenuation between two calibrated antennas with antenna factors removed, compared against theoretical values to validate a radiated emission test site. ANSI C63.4 requires measured NSA within +/-4 dB of theory.

OATS (open area test site) - an outdoor radiated emission test site with a metallic ground plane and an obstruction-free elliptical area around EUT and antenna. The historical reference environment that semi-anechoic chambers are built to replicate.

Q - T

Quasi-peak - a weighted detector, defined in CISPR 16-1-1, whose reading rises with the repetition rate of impulsive signals, approximating perceived annoyance to analog radio reception. Most emission limits below 1 GHz are specified as quasi-peak values.

Radiated emissions - electromagnetic fields a product releases through the air, measured with calibrated antennas at a standard distance (3 m or 10 m) and limited in dBuV/m by standards such as CISPR 32 and FCC Section 15.109.

RF (radio frequency) - electromagnetic energy in the range where radio communication operates, conventionally about 9 kHz to 300 GHz. EMC emission standards concentrate on 9 kHz to 40 GHz.

Shielding - the use of conductive enclosures or cable screens to attenuate electromagnetic fields passing between the interior and exterior. Shielding effectiveness is expressed in dB and depends heavily on apertures, seams and cable penetrations.

Surge - a high-energy transient overvoltage caused by lightning coupling or power-system switching. Immunity is tested per IEC 61000-4-5 with a 1.2/50 us open-circuit voltage waveform and 8/20 us short-circuit current waveform.

TEM cell (transverse electromagnetic cell) - a closed rectangular transmission-line structure that produces a calculable, uniform field between its septum and outer walls, used for small-EUT emissions and immunity measurements up to a cutoff frequency set by its size; the GTEM cell extends the concept into the GHz range.