Electromagnetic compatibility (EMC) measurement is a discipline of defined quantities. A result is meaningful only when the voltage, field strength, power, impedance, detector characteristic, and test environment have been specified with equal precision. The same physical disturbance can produce very different readings depending on whether it is measured as common-mode or differential-mode voltage, whether a quasi-peak or peak detector is used, and whether the measurement is made at a terminal, on a cable, or in a radiated field. Vocabulary therefore performs an engineering function: it fixes the measurement plane, the reference impedance, the detector weighting, and the interpretation of the result.
This reference organizes the core terms used in EMC measurement, equipment classification, and receiver and transmitter performance. It begins with disturbance quantities and the networks that make repeatable terminal measurements possible. It then moves through common-mode and differential-mode definitions, transfer impedance, effective radiated power, detector time constants, and the physical structures used for radiated testing. The equipment section distinguishes ISM equipment, RF heating apparatus, ISM frequency bands, and information technology equipment. The final section covers emissions, signal-to-disturbance and signal-to-noise ratios, protection ratio, spurious response, and parasitic oscillation.
Read together, these terms describe a chain: a source generates a disturbance; a coupling path converts it into a voltage, current, or field; a transducer or network presents that quantity to a receiver; a detector weights it in the time domain; and an indicating instrument displays it. Classification and receiver terminology then connect the measurement to the equipment's intended function and to its ability to reject unwanted responses.
Highlights:
- EMC results are defined by the measurement plane and network: disturbance voltage requires a specified reference impedance, and an artificial mains network both stabilizes impedance and isolates the equipment under test from the power source.
- Common-mode and differential-mode voltages are separate quantities. Delta networks resolve them in single-phase circuits, while V-networks measure conductor-to-ground voltages.
- Detector choice changes the indicated value. Quasi-peak, peak, RMS, and average detectors respond differently to pulse repetition rate, and their time constants are part of the measurement definition.
Measurement
Measurement terms establish what is being measured, where it is measured, and under which electrical conditions. The definitions below separate disturbance quantities from the networks and instruments used to capture them. This separation is essential because a voltage, a field strength, and a power value are not interchangeable descriptions of the same emission. Each has its own measurement plane and its own reference conditions.
Disturbance Quantities and Reference Impedance
Disturbance voltage. Disturbance voltage is the voltage caused by an electromagnetic disturbance between two separate conductors, measured under specified conditions. The phrase "two separate conductors" is important because it fixes the measurement points. Without those points and the specified conditions, the voltage has no comparable PCB EMC meaning.
Disturbance field strength. Disturbance field strength is the field strength produced by an electromagnetic disturbance at a given location, measured under specified conditions. It is a radiated quantity, so the measurement location and the specified conditions form part of the definition rather than being incidental details.
Disturbance power. Disturbance power is the electromagnetic disturbance power measured under specified conditions. It applies when the relevant emission is best characterized as power rather than as a terminal voltage or as a field strength.
Reference impedance. A reference impedance is an impedance of specified value used to calculate or measure the electromagnetic disturbance generated by equipment. It provides the electrical reference against which a disturbance quantity is defined, allowing measurements made on different setups to be compared.
Artificial Mains Networks and Terminal Voltages
Artificial mains network. An artificial mains network is a network inserted in series with the power supply line of the equipment under test (EUT). Within a given frequency range, it provides a specified load impedance for disturbance voltage measurement and isolates the EUT from the power supply. It is also called a line impedance stabilization network (LISN).
Delta network. A delta network is an artificial mains network that can separately measure common-mode and differential-mode voltages in a single-phase circuit. Its role is to resolve the two voltage components at the measurement port rather than presenting only their combined effect.
V-network. A V-network is an artificial mains network that can separately measure the voltage of each conductor to ground. It may be designed for any number of conductors.
Differential-mode voltage. Differential-mode voltage is the voltage between any two of a specified group of live conductors. It is also called symmetrical voltage.
Common-mode voltage. Common-mode voltage is the average of the phase voltages between each conductor and a specified reference point, usually ground or the enclosure.
Common-mode conversion. Common-mode conversion is the process by which a common-mode voltage produces a differential-mode voltage.
Symmetrical terminal voltage. Symmetrical terminal voltage is the differential-mode voltage at specified terminals measured with a delta network.
Asymmetrical terminal voltage. Asymmetrical terminal voltage is the common-mode voltage at specified terminals measured with a delta network.
V-terminal voltage. V-terminal voltage is the terminal voltage between the power line and ground measured with a V-network.
Transfer Impedance and Radiated Power
Transfer impedance of a screened circuit. Transfer impedance of a screened circuit is the ratio of the voltage between two specified points in the screened circuit to the current on a specified cross section of the screen. It describes how current on the screen couples to voltage inside the screened circuit.
Surface transfer impedance of a coaxial line. Surface transfer impedance of a coaxial line is the ratio of the voltage induced in unit length L of the inner conductor to the current on the outer surface of the coaxial line. It is a measure of the coupling through the outer conductor of the coaxial structure.
Effective radiated power of any device in a given direction. Effective radiated power is the power that must be applied to the input of a lossless reference antenna to produce, at any specified distance in a given direction, the same radiated power flux density as the device. Unless otherwise stated, the lossless reference antenna is a half-wave dipole.
Detectors and Indicating Instruments
Electrical charge time constant of a detector. The electrical charge time constant is the time required for the detector output voltage to reach (1 - 1/e) of its steady-state value after a sinusoidal voltage at the design frequency is suddenly applied to the detector input.
Electrical discharge time constant of a detector. The electrical discharge time constant is the time from the sudden removal of the sinusoidal input voltage until the detector output voltage falls to 1/e of its initial value.
Mechanical time constant of an indicating instrument. The mechanical time constant is the free oscillation period of the instrument indicator divided by 2π. The free oscillation is characterized by undamped motion.
Overload factor of a receiver. The overload factor is the ratio of the maximum sinusoidal input signal amplitude to the input amplitude at full-scale deflection of the indicating instrument. At this maximum input, the amplitude-to-amplitude characteristic of the receiver circuit before the detector must depart from linearity by no more than 1 dB.
Quasi-peak detector. A quasi-peak detector is a detector with specified electrical time constants. When a regular train of equal-amplitude pulses is applied, its output voltage is a fraction of the pulse peak, and that fraction approaches 1 as the pulse repetition rate increases.
Quasi-peak voltmeter. A quasi-peak voltmeter is the combination of a quasi-peak detector and an indicating instrument with a specified mechanical time constant.
Pulse response characteristic of a quasi-peak voltmeter. The pulse response characteristic is the relationship between the voltmeter indication and the repetition rate of a regular train of equal-amplitude pulses.
Peak detector. A peak detector produces an output voltage equal to the peak value of the applied signal.
Root-mean-square detector. A root-mean-square detector produces an output voltage equal to the RMS value of the applied signal.
Average detector. An average detector produces an output voltage equal to the average value of the applied signal envelope. The average must be taken over a specified time interval.
Test Facilities and Measurement Accessories
Artificial hand. An artificial hand is an electrical network that simulates the impedance of the human body between a hand-held appliance and ground under normal operating conditions.
Radiation test site. A radiation test site is a site that satisfies specified conditions for correct measurement of the electromagnetic emissions of the EUT.
Stop (quarter-wave) filter. A stop filter is a movable coaxial tunable mechanism arranged around a conductor. It limits the radiating length of the conductor at a given frequency.
Absorbing clamp. An absorbing clamp is a measuring device that can be moved along the power line of equipment or a similar device to obtain the maximum radiated radio-frequency power of the equipment or device.
Stripline. A stripline is a transmission line formed by two parallel plates with matched terminations. Electromagnetic waves propagate between the plates in the transverse electromagnetic (TEM) mode, producing an electromagnetic field for test use.
TEM cell. A TEM cell is a closed system, usually a rectangular coaxial line, in which electromagnetic waves propagate in the TEM mode, producing a specified electromagnetic field for test use.
Dummy lamp. A dummy lamp is a device that simulates the radio-frequency impedance of a fluorescent lamp. It can replace the fluorescent lamp in lighting equipment so that the insertion loss of the lighting equipment can be measured.
Balun. A balun is a device used to convert between unbalanced and balanced voltages.
Current probe. A current probe is a device for measuring the current in a conductor without disconnecting the conductor and without introducing significant impedance into the corresponding circuit.
Ground (reference) plane. A ground reference plane is a conductive plane whose potential is used as a common reference potential.
Equipment Classification
Equipment classification provides the category framework within which EMC terms are applied. The definitions below distinguish equipment by its intended function, by the way it uses radio-frequency energy, and by the frequency allocations associated with its operation. Classification matters because a term such as "ISM equipment" or "information technology equipment" carries specific scope conditions that affect how the equipment is described and assessed.
ISM Equipment and Frequency Bands
ISM (qualifier). ISM denotes equipment or apparatus designed for industrial, scientific, medical, household, or similar purposes, to generate and use radio-frequency energy locally. It excludes equipment used in telecommunications. ISM is the abbreviation for industrial, scientific, and medical. For some organizations, the category does not include information technology equipment.
Radio-frequency heating apparatus. Radio-frequency heating apparatus is ISM equipment that uses radio-frequency energy to produce heating effects.
ISM frequency band. An ISM frequency band is a frequency band allocated to ISM equipment.
Information Technology Equipment
Information technology equipment (ITE). Information technology equipment is equipment used for the following purposes: (1) receiving data from an external source, for example through a keyboard or a data line; (2) processing the received data, for example by computation, data conversion, recording, filing and classification, storage, and transfer; and (3) providing data output, either to another device or as a reproduction of data and images. The definition includes units or systems that mainly generate various periodic binary electrical or electronic pulse waveforms and perform data processing functions. Examples include word processing, electronic computation, data conversion, recording, filing, classification, storage, recovery, and transfer, as well as the reproduction of data by images.
Receiver and Transmitter
Receiver and transmitter terms describe emissions that lie outside the intended signal path and the ability of a receiver to reject unwanted responses. They also define the ratio quantities used to express signal quality and protection. These definitions separate emissions caused by the modulation process from emissions that can be reduced without affecting information transfer, and they distinguish disturbance from noise in ratio calculations.
Transmitter Emissions
Spurious emission of a transmitting station. Spurious emission is emission at one or more frequencies outside the necessary bandwidth. Its level can be reduced without affecting the corresponding information transmission. Spurious emission includes harmonic emission, parasitic emission, intermodulation products, and frequency-conversion products. Out-of-band emission is excluded.
Out-of-band emission. Out-of-band emission is emission at one or more frequencies immediately outside the necessary bandwidth, caused by the modulation process. Spurious emission is excluded.
Receiver Response and Protection Terms
Signal-to-disturbance ratio. Signal-to-disturbance ratio is the ratio of the useful signal level to the electromagnetic disturbance level, both measured under specified conditions. The term signal-to-interference ratio should not be used to express this concept.
Signal-to-noise ratio. Signal-to-noise ratio is the ratio of the useful signal level to the electromagnetic noise level, both measured under specified conditions.
Protection ratio. Protection ratio is the minimum signal-to-disturbance ratio required for a device or equipment to achieve its specified performance.
Spurious response frequency. A spurious response frequency is an electromagnetic disturbance frequency that produces an unwanted response in a given piece of equipment. For a receiver tuned to f0, many spurious response frequencies can exist. The associated variables are fs, the spurious response frequency; fl, the local oscillator frequency; fi, the intermediate frequency; and m, n, and h, which are integers.
Spurious response rejection ratio. Spurious response rejection ratio is the ratio of the level of a signal at a spurious response frequency that produces a specified output power in a device to the level of the useful signal that produces the same output.
Parasitic oscillation. Parasitic oscillation is an unwanted oscillation generated by equipment. Its frequency is unrelated to the operating frequency and also unrelated to the frequencies associated with producing the desired oscillation.
Bandwidth (of a Device)
Bandwidth of a device is the width of the frequency band over which a specified characteristic of the device or transmission channel remains within a stated value or ratio of its reference value. The specified characteristic may be an amplitude/frequency response, a phase/frequency response, or a delay/frequency response. In each case, the bandwidth defines the range over which the device or channel meets the chosen deviation limit.
Bandwidth (of an Emission or Signal)
Bandwidth of an emission or signal is the width of a frequency band such that the level of any spectral component outside the band does not exceed a specified percentage of the reference level. This definition characterizes the spectral extent of an emission or signal relative to a defined reference level.
Broadband Emission
Broadband emission is an emission whose bandwidth is greater than the bandwidth of a specified measuring equipment or receiver. The comparison is made against the bandwidth of the measuring or receiving system used for the assessment.
Broadband Device
A broadband device is a device whose bandwidth is sufficient to accept and process all spectral components of a specified emission. It can therefore pass the complete emission without rejecting any of its spectral content.
Narrowband Emission
Narrowband emission is an emission whose bandwidth is less than the bandwidth of a specified measuring equipment or receiver. Its spectral extent is smaller than the measurement or reception bandwidth used for the comparison.
Narrowband Device
A narrowband device is a device whose bandwidth is only sufficient to accept and process part of the spectral components of a specified emission. Consequently, part of the emission spectrum falls outside the device passband.
Selectivity
Selectivity is the ability of a receiver to distinguish a given wanted signal from an unwanted signal, or the measure of that ability. It describes how well the receiver rejects unwanted signals while receiving the wanted signal.
Effective Selectivity
Effective selectivity is selectivity under specified special conditions, for example when the receiver input circuit is overloaded. The overload condition can alter the receiver response and therefore its effective ability to reject unwanted signals.
Adjacent Channel Selectivity
Adjacent channel selectivity is selectivity measured with a signal separation equal to the channel spacing. It characterizes the receiver response to signals in adjacent channels at the defined spacing.
Desensitization
Desensitization is a reduction in the wanted output of a receiver caused by an unwanted signal. The unwanted signal reduces the receiver useful output even when the wanted signal conditions are otherwise unchanged.
Crossmodulation
Crossmodulation is the modulation of a wanted signal by an unwanted signal generated through interaction of signals in a nonlinear device, electrical network, or propagation medium. The unwanted signal is produced by the signal interaction and then modulates the wanted signal.
Intermodulation
Intermodulation is a process that occurs in a nonlinear device or propagation medium. Spectral components of one or more input signals interact to produce new components whose frequencies are linear combinations of integral multiples of the input component frequencies. Intermodulation may be caused by a single non-sinusoidal input signal or by multiple sinusoidal or non-sinusoidal signals applied to the same or to different input terminals.
Intermediate Frequency Rejection Ratio
Intermediate frequency rejection ratio is the ratio of a specified signal level at any intermediate frequency used in a receiver to the level of a wanted signal that produces the same output power. It indicates the receiver rejection of signals at its intermediate frequencies relative to a wanted signal producing equal output.
Image Rejection Ratio
Image rejection ratio is the ratio of a specified signal level at the image frequency of a receiver to the level of the tuned wanted signal that produces the same output power. It expresses the receiver ability to reject signals at the image frequency relative to the wanted signal at the tuned frequency.
Single-Signal Method
The single-signal method is a method of measuring a receiver response to an unwanted signal in the absence of a wanted signal. Only the unwanted signal is applied, and the receiver response is measured under that condition.
Two-Signal Method
The two-signal method is a method of determining a receiver response to an unwanted signal in the presence of a wanted signal. Both the wanted signal and the unwanted signal are applied. For this method, detailed test procedures and the standards to be used must be specified for each type of receiver under test.
Related Reading:
EMC Engineer Skills and Common Components: Chokes, Ferrite Beads, Filter Capacitors
EMC Design Principles and EMI/EMC Questions for PCB Engineers
PCB EMC Design Q&A: Differential Pairs, Grounding and Filtering
FCC and EMC Compliance Requirements Explained for PCB Design
Ferrite Beads vs. Inductors for High-Frequency Filtering on PCBs
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EMC Fundamentals: Disturbance and Interference Control Terms
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