Electronics Guide

Standards and Regulatory Framework

Radiated emission measurement rests on two layers of documents. Basic standards define how to measure: what the instrument must do, what the site must look like, and how the numbers are computed. Product standards define what to measure and against which limits. Confusing the two is a common source of invalid test reports.

Basic Measurement Standards

• CISPR 16-1-1 specifies measuring receivers and detectors, including the CISPR measurement bandwidths and the peak, quasi-peak, average, and RMS-average detector responses.
• CISPR 16-1-4 specifies antennas and test sites for radiated disturbance measurements, together with the site validation procedures that qualify an open area test site or chamber.
• CISPR 16-2-3 specifies the radiated disturbance measurement methods themselves, including EUT arrangement, scanning, and reporting.
• CISPR 16-4-2 specifies measurement instrumentation uncertainty and how it is applied to compliance decisions.
• ANSI C63.4 is the corresponding American method standard, used for United States regulatory testing of unintentional radiators.
• IEC 61000-4-20 covers emission and immunity testing in transverse electromagnetic (TEM) waveguides such as GTEM cells, and IEC 61000-4-21 covers reverberation chamber test methods.

Product and Application Standards

• CISPR 32 sets emission requirements for multimedia equipment, replacing the earlier CISPR 22 for information technology equipment and CISPR 13 for broadcast receivers.
• CISPR 11 covers industrial, scientific, and medical radio-frequency equipment, and CISPR 14-1 covers household appliances and power tools.
• CISPR 25 addresses emissions from vehicle components, measured to protect receivers installed on the same platform rather than the general radio environment.
• MIL-STD-461 defines the military test methods, of which RE102 is the radiated electric field emission test.
• RTCA DO-160 Section 21 defines radiated emission requirements for airborne equipment.
• In the United States, 47 CFR Part 15 establishes the legal limits and the frequency range over which they apply.

The two regimes differ on how far up the spectrum a scan must reach. In both cases the upper measurement frequency is set by the highest frequency generated or used inside the product, but CISPR 32 caps the requirement at 6 GHz, whereas the FCC rules extend as far as 40 GHz for the fastest equipment. A product sold into both markets is therefore tested over different ranges under the same nominal design.

Limits are expressed as an electric field strength in dBµV/m at a stated distance, most often 3 m or 10 m. Where a standard permits measurement at a distance other than the one for which the limit is written, the limit is scaled by an inverse-distance relationship of 20 dB per decade, so a 10 m limit becomes roughly 10.5 dB higher at 3 m. This extrapolation assumes far-field behavior and becomes unreliable for large equipment at short distances, which is one reason standards restrict when it may be used.

Open Area Test Sites (OATS)

Open area test sites represent the traditional reference standard for radiated emission measurements and remain the benchmark against which alternative test methods are validated. An OATS provides a controlled outdoor environment that approximates free-space propagation conditions while incorporating a ground plane that creates predictable reflections.

OATS Construction and Requirements

A properly constructed OATS consists of a flat, unobstructed area with a conductive ground plane, typically built from bonded metal mesh or solid metal sheet laid over prepared earth. CISPR 16-1-4 requires the metallic ground plane to extend at least 1 m beyond the periphery of the equipment under test (EUT) and at least 1 m beyond the largest measuring antenna, covering the entire area between them. Larger ground planes are common because extending the conductive surface pushes edge diffraction farther from the measurement geometry and improves site attenuation at the lowest frequencies. Mesh openings must be small compared with the shortest wavelength of interest, and every seam must be electrically bonded; a corroded or intermittently bonded joint behaves as a slot radiator and shows up directly in site validation results.

The site must be free of reflecting objects within an elliptical zone with the EUT and the antenna at its foci. That ellipse has a major axis of twice the measurement distance and a minor axis of the measurement distance multiplied by the square root of three, so a 10 m site requires a clear area of roughly 20 m by 17.3 m. Buildings, fences, overhead wires, trees, and parked vehicles all create reflections that distort measurements. The ideal OATS location is remote from such structures and from sources of ambient electromagnetic interference.

Site attenuation measurements verify that an OATS meets performance requirements. The procedure measures transmission between two antennas positioned as they would be during actual testing and compares the result with the theoretical value for an ideal site. Deviations point to problems with ground plane conductivity, site geometry, or reflecting obstacles. ANSI C63.4 and CISPR 16-1-4 specify the validation procedures and tolerances: normalized site attenuation must agree with theory within plus or minus 4 dB from 30 MHz to 1000 MHz, and above 1 GHz the site voltage standing wave ratio criterion applies instead.

OATS Measurement Procedure

During OATS measurements, the EUT is placed on a non-conductive turntable at a specified height above the ground plane, typically 0.8 m for tabletop equipment, or directly on the ground plane, usually on a thin insulating pad, for floor-standing equipment. The receiving antenna is positioned at the specified measurement distance, commonly 3 m or 10 m, with 30 m used at some sites for low-frequency work. The EUT rotates through 360 degrees of azimuth while the antenna scans vertically from 1 m to 4 m to find the maximum emission level at each frequency.

This maximization procedure accounts for the complex interaction between direct radiation, ground reflection, and the EUT's directional emission pattern. At some frequencies, the direct and reflected signals add constructively; at others, they partially cancel. By varying EUT orientation and antenna height, the procedure finds the worst-case combination at each frequency.

Both horizontal and vertical antenna polarizations must be tested because emissions may be predominantly polarized in either direction depending on their source structure within the EUT. The higher emission level at each frequency determines compliance.

OATS Advantages and Limitations

OATS measurements provide excellent correlation with real-world electromagnetic environments because they replicate the ground reflection geometry that exists in actual use. The outdoor location allows testing of large equipment without facility size constraints. OATS remains the reference method specified in many standards for type approval and certification testing.

However, OATS testing has significant practical limitations. Weather dependence restricts testing to acceptable conditions, and ambient electromagnetic noise from broadcast stations, cellular systems, and other sources can mask device emissions or create false readings. Site preparation and maintenance costs are substantial, and testing is time-consuming due to the mechanical maximization procedure. These factors have driven the development of alternative test methods that provide equivalent results in more controlled environments.

Semi-Anechoic Chambers

Semi-anechoic chambers have become the predominant facilities for radiated emission compliance testing, providing controlled indoor environments that simulate OATS conditions while eliminating weather dependence and ambient interference. These chambers represent a substantial investment but offer superior convenience, reproducibility, and throughput compared to outdoor testing.

Chamber Construction

A semi-anechoic chamber consists of a shielded enclosure lined with radio frequency absorbing material on the walls and ceiling, while the floor remains a conductive surface serving as the ground plane. The shielding, typically constructed from welded steel panels or multiple layers of sheet metal, prevents external signals from entering and internal signals from escaping. Shielding effectiveness of 80-100 dB is typical for compliance-grade chambers.

The absorbing material, usually pyramidal foam, ferrite tile, or a combination of the two, suppresses reflections from walls and ceiling that would otherwise create standing waves and measurement errors. Pyramidal absorbers are carbon-loaded polyurethane foam cut into cones or pyramids whose tapered geometry provides a graduated impedance transition from free space into the lossy material. Absorption depends on electrical depth, so the pyramid height sets the lowest effective frequency: useful performance requires a depth on the order of a quarter wavelength, which at 30 MHz means several meters of foam. Pure foam treatment for the bottom of the emission band is therefore impractical in most buildings.

Ferrite tile absorbers solve that problem. Sintered ferrite dissipates energy through magnetic losses rather than resistive losses, so a tile only a few millimeters thick performs well from roughly 30 MHz to several hundred megahertz, exactly where foam fails. Ferrite is heavier and considerably more expensive than foam, and its performance degrades above about 1 GHz. The dominant compliance-chamber construction is consequently a hybrid: ferrite tile bonded to the wall for the low band, with shorter foam pyramids in front of or bonded to the tile for the high band, achieving broadband performance in a fraction of the depth that either material would need alone.

Chamber Qualification and Validation

Before a semi-anechoic chamber can be used for compliance testing, it must be validated to demonstrate equivalence to an ideal OATS. The normalized site attenuation (NSA) procedure compares transmission between reference antennas in the chamber with the theoretical value for an ideal site, in both polarizations and over the full antenna height scan. CISPR 16-1-4 requires agreement within plus or minus 4 dB from 30 MHz to 1000 MHz. For a chamber the measurement is repeated with the transmitting antenna at several positions spanning the intended test volume, a volumetric NSA, because a chamber that qualifies only at the center of the turntable does not qualify for a large EUT.

Site voltage standing wave ratio (SVSWR) measurements validate the site from 1 GHz to 18 GHz, where NSA becomes impractical. A transmitting antenna is moved through a set of positions along each of several radial paths within the test volume, and the spread between the maximum and minimum received signal reveals the interference between the direct path and any residual reflection. CISPR 16-1-4 requires the SVSWR to be no greater than 2:1, equivalently 6.0 dB, at every tested position and frequency. Lower values indicate better absorber performance and a more uniform field. Because the criterion is sensitive to reflections from any hard surface, the ground plane between the EUT and the antenna is normally covered with absorber for measurements above 1 GHz, and turntable edges, cable trays, and antenna masts frequently prove to be the limiting reflectors.

Chamber validation must be repeated periodically and after any changes to the chamber structure or absorber configuration. Many facilities perform annual revalidation to ensure continued compliance with site qualification requirements.

Semi-Anechoic Chamber Measurement Procedures

Measurement procedures in semi-anechoic chambers closely parallel OATS procedures. The EUT is placed on a non-conductive turntable at the specified height, rotated through azimuth angles while the receiving antenna scans through heights. Both polarizations are tested, and the maximum emission at each frequency determines compliance.

Modern chambers often incorporate automated systems that control turntable rotation, antenna positioning, and measurement receiver operation. These systems can perform complete emission scans in a fraction of the time required for manual testing, improving throughput and reducing labor costs. Automated pre-scans quickly identify frequencies of interest, allowing detailed maximization at specific frequencies rather than time-consuming broadband maximization.

Chamber Size Considerations

Chamber size determines the maximum measurement distance and the largest EUT that can be accommodated. The internal dimensions must hold the measurement distance itself plus the turntable, the antenna and its mast, and the absorber depth and clearance on every surface, so a chamber is always substantially larger than its nominal test distance. Three-meter chambers are the workhorses of commercial compliance testing and suit most tabletop and small floor-standing products. Ten-meter chambers cost several times as much and are built when a standard specifies a 10 m measurement, when the product is large enough that a 3 m distance would violate far-field or EUT-size constraints, or when a laboratory wants to avoid the extrapolation error that comes with short-distance testing.

The quiet zone, the region in which absorber reflections are suppressed enough for accurate measurement, depends on chamber geometry and absorber performance. Larger chambers provide larger quiet zones. The EUT and its cabling must sit entirely within the validated volume, and this, rather than the physical room size, is the true constraint on what a chamber can test. A product that overhangs the qualified volume produces results that no site validation supports.

Fully Anechoic Rooms (FAR)

Fully anechoic rooms extend the absorber treatment to cover all surfaces including the floor, eliminating ground reflections entirely. This configuration simulates true free-space conditions, making FARs particularly valuable for antenna measurements, radar cross-section testing, and emission measurements where ground reflection effects are undesirable.

FAR Construction and Applications

FAR construction follows similar principles to semi-anechoic chambers but requires floor absorber treatment that can support the EUT weight while maintaining electromagnetic performance. Solutions include raised floors with grating that allows absorber placement beneath, or specialized walkway materials with minimal electromagnetic impact. Some facilities use removable floor sections that allow conversion between FAR and semi-anechoic configurations.

Emission measurements in a FAR produce different results from OATS or semi-anechoic measurements because the ground reflection is absent. A ground-plane site can deliver a constructive addition of the direct and reflected rays worth up to roughly 6 dB at favorable geometries, so the same source generally reads lower in a FAR. The difference is not a single correction factor that can be applied after the fact: it varies with frequency, antenna height, and the vertical distribution of the source. For that reason, a FAR is not a drop-in substitute for a ground-plane site. Where a product standard recognizes the fully anechoic method, it does so with its own limit set and its own site validation requirement, and where a standard specifies only OATS or semi-anechoic testing, a FAR result is not acceptable for compliance.

Above 1 GHz the distinction largely disappears in practice. CISPR procedures call for absorber on the floor between the EUT and the antenna at these frequencies, so the measurement is effectively performed in a free-space environment even in a nominally semi-anechoic chamber, and site validation switches from NSA to SVSWR. The simplified propagation geometry improves repeatability, which is one reason the height scan is reduced or eliminated in many above-1 GHz procedures.

FAR versus Semi-Anechoic Selection

The choice between FAR and semi-anechoic configurations follows the intended measurements. For emission compliance testing to standards written around OATS correlation, the semi-anechoic chamber is the required configuration below 1 GHz. For antenna pattern work, radiated power and receiver sensitivity testing of wireless products, radar cross-section measurement, and any application in which a ground reflection corrupts the quantity of interest, the FAR is superior. Because the two roles overlap in most laboratories, many facilities either maintain both or build a semi-anechoic chamber with removable floor absorber that converts between the configurations.

Reverberation Chambers

Reverberation chambers offer a fundamentally different approach to radiated emission measurement, using mechanical stirring to create a statistically uniform electromagnetic environment rather than absorbing reflections. This technique provides several unique advantages including high field strength capability, reduced measurement time for certain applications, and potentially lower facility costs.

Operating Principles

A reverberation chamber is a shielded enclosure with highly reflective walls, creating a cavity that supports numerous electromagnetic modes at any given frequency. A rotating metal stirrer, typically a large paddle or tuner, continuously changes the boundary conditions, causing the mode structure to shift. Over time, or averaged over many stirrer positions, the electromagnetic field becomes statistically uniform throughout the chamber volume.

Unlike anechoic chambers that attempt to create free-space conditions with controlled wave propagation, reverberation chambers deliberately exploit reflections to create an isotropic, unpolarized field environment. Every orientation and polarization of emission from the EUT contributes equally to the received signal, eliminating the need for turntable rotation and antenna height variation.

Measurement Procedures

Emission measurements in reverberation chambers involve measuring the received power as the stirrer rotates through a complete revolution or over a statistically adequate number of positions. The maximum received power, or the statistical average depending on the standard, characterizes the emission level. Calibration factors convert received power to equivalent field strength for comparison with regulatory limits.

The statistical nature of reverberation chamber measurements requires careful attention to stirrer effectiveness and to the number of independent stirrer positions sampled. At lower frequencies, where the cavity supports only a handful of modes, statistical uniformity cannot be achieved and the method breaks down. The lowest usable frequency (LUF) is conventionally taken as roughly three times the chamber's fundamental resonance, and it is confirmed empirically through the field uniformity validation defined in IEC 61000-4-21. A large chamber of several hundred cubic meters may reach an LUF below 100 MHz, while a modest laboratory chamber may not be usable until several hundred megahertz.

Advantages and Applications

Reverberation chambers offer significant time savings for radiated emission measurements because no mechanical maximization through rotation and antenna height variation is needed. The isotropic environment automatically samples all emission directions and polarizations simultaneously. For EUTs with unknown or complex emission patterns, this approach ensures that maximum emissions are captured without elaborate scanning procedures.

The high field strength capability of reverberation chambers makes them valuable for immunity testing, where they can generate field strengths of hundreds of volts per meter with modest input power. This same capability can be leveraged for emission testing of low-level radiators that might be difficult to measure in anechoic environments.

The reference document for the technique is IEC 61000-4-21, which defines reverberation chamber test methods for radiated immunity, for intentional and unintentional radiated emissions, and for shielding effectiveness measurement. The method reports total radiated power rather than a field strength in a particular direction, so it answers a different question from a ground-plane site measurement. Correlation studies have demonstrated reasonable agreement with OATS and semi-anechoic results when appropriate procedures and conversion factors are applied, but mainstream commercial type approval under the CISPR product standards still relies on ground-plane sites. Reverberation chambers are most established in aerospace, automotive, and defense programs and in shielding effectiveness work.

Limitations

The lower usable frequency limitation restricts reverberation chamber application for measurements below approximately 100-200 MHz. The statistical nature of measurements means that specific emission directions or polarizations cannot be isolated, which may be important for troubleshooting. Additionally, the complex electromagnetic environment can stress EUT circuitry differently than the more orderly fields in anechoic environments, potentially affecting EUT behavior during testing.

GTEM Cells

Gigahertz transverse electromagnetic (GTEM) cells provide compact, economical facilities for radiated emission measurement from sub-megahertz frequencies to several gigahertz. These tapered transmission line structures create a region of uniform electromagnetic field, enabling measurements without the facility size and cost of full-scale chambers.

GTEM Cell Structure

A GTEM cell consists of a tapered, asymmetric coaxial transmission line with a rectangular cross-section. The outer conductor forms the cell walls, while an internal septum (center conductor) creates the transverse electromagnetic field region. The cell tapers from a small coaxial connector at one end to absorber termination at the large end, providing a smooth impedance transition across a wide frequency range.

The EUT is placed in the test volume between the septum and the cell floor, where the electromagnetic field is relatively uniform. The cell dimensions determine the maximum EUT size and the upper frequency limit, with larger cells accommodating larger equipment but with reduced high-frequency performance. Cell sizes range from desktop units for small devices to walk-in configurations for complete equipment assemblies.

Measurement Principles

When the EUT radiates electromagnetic energy, this energy couples to the cell's transmission line mode and propagates to the measurement port. The received power relates to the radiated emission level through cell factors that account for geometry and propagation characteristics. Multiple orientations of the EUT within the cell are typically measured to capture emissions in different directions.

Unlike far-field measurements in anechoic chambers, GTEM cell measurements sample the coupling of the EUT to a guided TEM mode, which requires a different interpretation of the results. The established approach measures the EUT in three mutually orthogonal orientations, resolves the equivalent radiating dipole moments from those three readings, and computes the field the same source would produce at a standard measurement distance over a ground plane. IEC 61000-4-20, the basic standard for emission and immunity testing in TEM waveguides, defines the procedure, the validation of the usable test volume, and the reporting requirements. The algorithms have been refined through extensive correlation studies and agree well with OATS measurements for many equipment types.

GTEM Cell Applications

GTEM cells excel as pre-compliance and design verification tools, providing quick feedback during product development at a fraction of the cost of full compliance testing facilities. Their compact size allows installation in engineering laboratories, enabling designers to evaluate emissions iteratively as development progresses.

For troubleshooting, GTEM cells offer rapid measurements that help identify emission sources and verify mitigation effectiveness. The immediate feedback accelerates the design cycle and reduces the risk of failures during formal compliance testing.

Although IEC 61000-4-20 establishes GTEM measurement as a recognized method, correlation with ground-plane sites depends heavily on EUT characteristics, and individual product standards decide whether the method may be cited for compliance. Equipment with external cables presents particular difficulty because the cell constrains cable routing and because a cable in a bounded TEM structure radiates differently from the same cable in free space. The three-orientation model also assumes the EUT behaves as a compact set of dipoles, an assumption that weakens as the EUT approaches the dimensions of the usable test volume. For final compliance testing, semi-anechoic chambers or an OATS generally remain preferred, with GTEM cells filling the valuable role of pre-compliance screening.

Antenna Types and Selection

Receiving antennas are critical measurement system components that capture radiated emissions for analysis. Different antenna types offer varying characteristics across frequency ranges, and appropriate selection is essential for accurate, standards-compliant measurements.

Biconical Antennas

Biconical antennas consist of two conical or cage elements arranged point to point, forming a broadband dipole structure usable from approximately 30 MHz to 300 MHz. In practice a biconical covers the lower part of the 30 MHz to 1000 MHz emission range and a log-periodic array takes over above roughly 200 MHz to 300 MHz; the CISPR 16-1-4 site validation procedure follows the same division, using a biconical pair over the lower range and log-periodic pair above it.

Biconical antenna factors vary with frequency and must be applied to convert received voltage to field strength. Manufacturers provide calibrated antenna factor data traceable to national standards. The relatively large size of a biconical, typically 1 m to 1.5 m across, demands adequate clearance from chamber walls and absorber tips, and its low gain and rising antenna factor at the bottom of the band make 30 MHz to 50 MHz the most sensitivity-limited part of a typical emission scan.

Log-Periodic Dipole Arrays

Log-periodic dipole array (LPDA) antennas provide directional reception over frequency ranges typically spanning 200 MHz to 1 GHz or beyond. The antenna consists of multiple dipole elements of progressively varying length and spacing, creating a frequency-independent design with consistent characteristics across its bandwidth.

LPDA antennas offer higher gain than biconical antennas, improving sensitivity for detecting low-level emissions. Their directional pattern focuses sensitivity in the forward direction, which can be advantageous for isolating emissions from specific sources but requires proper alignment with the EUT.

Horn Antennas

Horn antennas provide excellent performance at frequencies above 1 GHz, offering high gain, well-defined patterns, and stable characteristics. The horn structure guides electromagnetic waves from a waveguide feed to free-space radiation through a flared aperture. Different horn sizes cover different frequency bands, with smaller horns serving higher frequencies.

Double-ridged horn antennas extend the usable bandwidth by loading the horn with ridged waveguide, enabling single-antenna coverage from below 1 GHz to beyond 18 GHz. These versatile antennas have become standard equipment for broadband emission measurements at higher frequencies.

Combined and Broadband Antennas

Some antenna designs combine multiple elements to cover extended frequency ranges with a single antenna. Biconical-log-periodic combination antennas (BiLog or BiConiLog) integrate biconical and log-periodic elements, providing coverage from 30 MHz to 1 GHz or higher without antenna changes. These antennas reduce measurement time and eliminate uncertainty associated with antenna substitution.

Antenna Factors and Calibration

The antenna factor quantifies the relationship between the electric field at the antenna location and the voltage delivered to the measurement receiver. It varies with frequency and must be applied to convert the received voltage into the field strength required by standards, normally dBµV/m at the specified measurement distance. In logarithmic form the complete measurement equation is straightforward:

field strength (dBµV/m) = receiver reading (dBµV) + antenna factor (dB/m) + cable loss (dB) − preamplifier gain (dB)

Every term on the right carries its own calibration and its own uncertainty, which is why the accuracy of a radiated emission result depends as much on the accessory chain as on the receiver itself. A cable loss error of 1 dB or a stale preamplifier gain figure shifts every reported number by that amount.

Antenna calibration is typically performed at accredited calibration laboratories using reference antenna methods or standard site methods. Calibration data provides antenna factors at discrete frequency points, with interpolation used for intermediate frequencies. Calibration should be traceable to national standards (such as NIST in the United States) and repeated periodically to account for any degradation or damage.

Antenna factor uncertainty contributes to overall measurement uncertainty and must be considered when evaluating compliance margins. Well-calibrated antennas from reputable manufacturers typically provide antenna factor uncertainties of plus or minus 1-2 dB.

Measurement Receiver Specifications

The measurement receiver, typically an EMI receiver or spectrum analyzer, detects and quantifies the signals captured by the receiving antenna. EMC standards specify receiver characteristics to ensure consistent, comparable measurements across different laboratories and equipment.

EMI Receivers versus Spectrum Analyzers

EMI receivers are specialized instruments designed specifically for EMC measurements, incorporating features required by standards including precisely specified bandwidth, detector types, and preselection. Traditional EMI receivers use analog designs with stepped tuning, measuring one frequency at a time across the emission spectrum.

Modern instruments combine the speed of swept or FFT-based analysis with standards-compliant measurement functions. The significant advance is the FFT-based time-domain scan, in which the receiver digitizes a wide segment of spectrum at once and computes hundreds or thousands of CISPR-bandwidth channels in parallel, applying the standard detector weighting to each. A stepped scan that once occupied hours can be completed in minutes, and because the whole segment is observed continuously rather than one channel at a time, intermittent emissions that a stepped receiver would miss are captured reliably. CISPR 16-1-1 now recognizes FFT-based instruments, subject to requirements on segment overlap and on the observation time per channel. The practical workflow is a rapid pre-scan to find frequencies of interest, followed by compliant final measurement at those frequencies.

Bandwidth Requirements

CISPR 16-1-1 divides the spectrum into bands and fixes a measurement bandwidth for each, so that results are consistent regardless of the receiver used. The bandwidths are specified at the 6 dB points rather than the 3 dB points familiar from ordinary spectrum analysis, which is why a general-purpose analyzer set to a nominally identical resolution bandwidth does not produce a CISPR-equivalent reading:

• Band A, 9 kHz to 150 kHz: 200 Hz
• Band B, 150 kHz to 30 MHz: 9 kHz
• Band C, 30 MHz to 300 MHz, and Band D, 300 MHz to 1000 MHz: 120 kHz
• Band E, 1 GHz to 18 GHz: 1 MHz

Radiated emission work therefore uses 120 kHz below 1 GHz and 1 MHz above it. These values are a compromise between frequency resolution and the ability to capture broadband impulsive noise. A narrower bandwidth resolves closely spaced spectral lines but understates broadband emissions and lengthens the scan; a wider bandwidth does the reverse. Fixing the bandwidth by standard is what makes results from different laboratories comparable, and using a non-standard bandwidth invalidates a compliance measurement even when the instrument is otherwise accurate.

Detector Types

EMI receivers incorporate several detector types that respond differently to the same signal. CISPR 16-1-1 defines their charge, discharge, and meter time constants for each band, and the product standard states which detector applies to which limit:

• Peak detection captures the maximum instantaneous amplitude within the measurement interval. It is the fastest and most sensitive detector and never reads below the others, which makes it ideal for pre-scans: a frequency that passes on peak needs no further examination.
• Quasi-peak detection weights a signal by its repetition rate through a defined charge and discharge network, so a pulse train repeating a few times per second reads well below a continuous carrier of the same peak amplitude. The weighting approximates the subjective annoyance of interference to analog audio and video reception, and it remains the primary detector for radiated emission limits from 30 MHz to 1000 MHz.
• Average detection measures the mean of the envelope, reading far below quasi-peak for intermittent or pulsed signals and equal to it for a steady unmodulated carrier. Several standards impose an average limit alongside the quasi-peak limit.
• RMS-average detection applies RMS weighting above a defined corner frequency and average weighting below it, giving a pulse-repetition-rate dependence better matched to the impairment of digital radio services than the classical quasi-peak curve. It appears in newer standards for that reason.

Above 1 GHz the picture changes: quasi-peak weighting is not defined in Band E, and product standards specify peak and average limits instead, typically with a 20 dB separation between them. Because the quasi-peak measurement is slow, the usual workflow is a fast peak pre-scan across the whole range followed by quasi-peak or average measurement only at the frequencies that come within a few decibels of the limit.

Dynamic Range and Sensitivity

Measurement receivers must provide adequate sensitivity to detect emissions at levels below regulatory limits while handling strong signals without overload or distortion. Dynamic range, the ratio between the strongest signal that can be measured accurately and the noise floor, determines the range of signal levels that can be characterized in a single measurement.

Preamplifiers increase system sensitivity for detecting weak emissions but reduce dynamic range and may introduce overload susceptibility. Attenuators reduce signal levels to prevent overload from strong emissions but raise the effective noise floor. Proper configuration of preamplifiers and attenuators optimizes the measurement system for the expected emission levels.

Preselection and Image Rejection

Preselector filters in EMI receivers reject out-of-band signals that might otherwise create spurious responses through receiver nonlinearities. This capability is essential when measuring in environments with strong ambient signals or when EUT emissions include strong narrowband components at some frequencies.

Modern receivers use multiple conversion stages with careful frequency planning to reject image responses and other spurious signals. Specifications for image rejection and spurious response levels ensure that detected signals genuinely represent EUT emissions rather than receiver artifacts.

Pre-Compliance Testing Setups

Pre-compliance testing enables engineers to evaluate radiated emissions during product development, identifying problems early when corrections are least expensive. Effective pre-compliance setups balance measurement capability against cost and convenience, providing actionable results without the expense of full compliance facilities.

Essential Pre-Compliance Equipment

A functional pre-compliance setup for radiated emissions includes:

• Spectrum analyzer or EMI receiver. A receiver with CISPR bandwidths and detectors gives the most transferable results, but a general-purpose analyzer is adequate for identifying emission frequencies and relative levels. The useful minimum is coverage from 30 MHz to at least 1 GHz, a displayed average noise level low enough to see emissions well below the limit, a 120 kHz resolution bandwidth setting, and a stable, calibrated amplitude response.
• Broadband antenna. A combination biconical and log-periodic antenna covering 30 MHz to 1 GHz or beyond spans the primary regulated range without antenna changes. Precision calibration is valuable, but even approximate antenna factors support useful relative comparisons.
• Low-noise preamplifier. At a short measurement distance in an untreated room the limiting factor is usually the analyzer noise floor rather than the ambient, and a preamplifier of 20 dB to 30 dB gain often makes the difference between seeing a harmonic and guessing at it.
• Near-field probes. Small loop and monopole probes localize emission sources on boards, cables, and enclosure seams. They give no absolute field strength, but they answer the question that matters during debugging, which is where the energy is coming from.

Test Environment Considerations

Full shielding and anechoic treatment are usually impractical for pre-compliance work. Practical alternatives include:

• Shielded room without absorber. Ambient interference disappears, but internal reflections produce standing waves that can push readings several decibels either side of the true level. Results identify emission frequencies and relative changes reliably; absolute levels do not transfer.
• GTEM cell. Compact, shielded, and broadband, with the best correlation to far-field results of any low-cost option for small equipment.
• Open environment. Measuring in an ordinary laboratory or office at a reduced distance, often 1 m or 3 m, exposes major emission sources and gross failures at no cost beyond the instruments. Ambient broadcast, cellular, and wireless local area network signals limit sensitivity, so a baseline sweep with the EUT powered down is essential for distinguishing product emissions from the environment.

Interpreting Pre-Compliance Results

Pre-compliance measurements typically underestimate formal compliance test results due to differences in measurement environment, equipment calibration, and procedure. A common approach applies margin, perhaps 6-10 dB, to pre-compliance results when estimating formal test outcomes. Emissions approaching limits during pre-compliance testing warrant design modifications before formal testing.

Relative measurements during pre-compliance testing provide valuable design guidance even when absolute accuracy is limited. Comparing emissions before and after design changes quantifies improvement regardless of absolute calibration uncertainty. This approach accelerates design optimization and builds confidence that formal testing will succeed.

Correlation Between Test Methods

Different test methods each have unique characteristics that affect measurement results. Understanding correlation between methods enables appropriate interpretation of results and supports regulatory acceptance of alternative test approaches.

OATS and Semi-Anechoic Chamber Correlation

Semi-anechoic chambers are designed to replicate OATS conditions, and properly constructed and validated chambers provide excellent correlation. The ground plane and one-reflection geometry are common to both methods, ensuring similar propagation characteristics. Site validation procedures, particularly normalized site attenuation measurements, verify that chamber performance matches theoretical OATS behavior within specified tolerances.

Minor differences may arise from chamber size constraints, absorber imperfections, or differences in ground plane characteristics. These effects are typically small for properly validated chambers and are accounted for within measurement uncertainty budgets.

Reverberation Chamber Correlation

Reverberation chamber measurements correlate with OATS results through statistical relationships and conversion factors derived from chamber theory and empirical validation. The isotropic, unpolarized field environment in a reverberation chamber effectively averages all emission directions, requiring different interpretation than the directional measurements of OATS or semi-anechoic methods.

Correlation factors convert reverberation chamber maximum or average received power to an equivalent field strength for comparison with limits expressed in traditional units. These factors depend on chamber characteristics, on the chamber calibration performed with a reference antenna, and on procedure details. IEC 61000-4-21 defines the calibration and validation steps on which any such conversion depends.

GTEM Cell Correlation

GTEM cell correlation with far-field methods is more complex because GTEM measurements occur in the near field with different field distributions than free-space propagation. Correlation algorithms use multiple EUT orientations within the cell to estimate far-field emission levels, with accuracy depending on EUT radiation characteristics.

For small, electrically simple EUTs, GTEM correlation can be quite good. Larger equipment or devices with significant cable emissions may show poorer correlation due to cable behavior differences between GTEM and free-space environments. Correlation studies for specific product types help establish confidence levels for GTEM pre-compliance results.

Factors Affecting Correlation

Several factors influence correlation between test methods:

• EUT radiation characteristics. Equipment with a directional pattern reads differently depending on how thoroughly each method samples direction. Compact, nearly omnidirectional radiators correlate best; a large system with a strongly lobed pattern correlates worst.
• Frequency range. Correlation generally improves with frequency as environment effects become more consistent. The low end of the range is the hardest, where chamber modes, absorber limitations, and antenna size all work against an ideal field.
• Cable configuration. External cables are among the strongest emission sources, and their behavior differs sharply between environments. Standardized layouts and termination impedances improve reproducibility but do not fully represent real installations, which is why a product can pass in the laboratory and still cause interference in the field.
• EUT operating mode. Emissions depend on what the product is doing. Comparing methods is meaningful only when the operating mode, data patterns, and display or communication activity are held constant.

Measurement Uncertainty

All measurements include uncertainty that must be quantified and considered when evaluating compliance. Radiated emission measurements are particularly subject to uncertainty due to the complex interaction of multiple factors affecting results.

Sources of Uncertainty

Major uncertainty contributors in radiated emission measurements include:

• Antenna factor calibration. Uncertainty in the antenna factor propagates directly into the calculated field strength. Typical calibration uncertainties range from 1 dB to 3 dB depending on antenna type, frequency, and calibration method.
• Antenna directivity and phase center. A directional antenna does not sample a large or distributed EUT uniformly, and the effective phase center of a log-periodic array moves along the boom with frequency, changing the true measurement distance.
• Receiver accuracy. Amplitude accuracy, frequency response, linearity, and detector implementation all contribute. Well-calibrated receivers hold amplitude accuracy within plus or minus 1 dB to 2 dB.
• Site imperfections. Residual deviation from ideal site behavior varies between facilities. Validation bounds these effects but does not remove them, and the permitted plus or minus 4 dB NSA window is itself a significant allowance.
• Mismatch. Impedance mismatch between antenna, cables, and receiver causes reflections that shift the measured level. The effect varies with frequency and commonly contributes 1 dB to 2 dB.
• Cable loss and attenuator calibration. Attenuation between antenna and receiver must be characterized, and cable loss drifts with temperature, bending, and connector wear.
• EUT and setup repeatability. Cable dressing, turntable position, and the reproducibility of the maximization search frequently dominate the budget in practice, particularly for products with cable-driven common-mode emissions.

Uncertainty Budgets

Laboratories develop measurement uncertainty budgets that quantify and combine the individual contributors according to established procedures such as those in ISO/IEC Guide 98-3, the Guide to the Expression of Uncertainty in Measurement (GUM). The combined uncertainty is expressed as an expanded uncertainty with a stated coverage factor, conventionally k = 2, corresponding to approximately 95 percent confidence.

CISPR 16-4-2 publishes reference values for this expanded uncertainty, denoted U_cispr. For radiated disturbance measured as electric field strength on an open area test site or an equivalent alternative site from 30 MHz to 1000 MHz, U_cispr is 5.2 dB. Comparable figures apply to the above-1 GHz and fully anechoic room methods. A laboratory whose own budget, U_lab, comes out at or below the reference value compares its measured result directly with the limit. A laboratory whose budget exceeds it must add the excess to the measured value before comparison, so that poorer measurement capability translates into a stricter effective limit rather than a hidden risk to the market.

Uncertainty and Compliance Decisions

When measured emissions approach regulatory limits, uncertainty becomes decisive. Three decision rules are in common use:

• Simple acceptance. A result below the limit passes, whatever the uncertainty. This is the CISPR approach once the laboratory's uncertainty is bounded by U_cispr, and it is deliberately permissive: a product measuring just under the limit may genuinely exceed it.
• Shared risk. The result is compared with the limit without adjustment, both parties accepting that some compliant products will fail and some non-compliant products will pass. It is the same arithmetic as simple acceptance, framed as an explicit agreement about who bears the consequences.
• Guard band. The limit is reduced by the measurement uncertainty, so a passing result carries high confidence of true compliance. This conservative rule places the burden on the manufacturer and is common in internal release criteria even where the regulator does not require it.

Regulatory regimes and accreditation bodies differ in which rule they apply, and the accredited scope of a laboratory states its declared uncertainty. Engineers should know which rule governs their testing before interpreting a 1 dB margin as a pass. A prudent internal target is a margin of 6 dB or more below the applicable limit, which absorbs both measurement uncertainty and the unit-to-unit variation that a single test sample cannot reveal.

Practical Measurement Considerations

Successful radiated emission measurements require attention to numerous practical details beyond equipment selection and facility design. These considerations affect measurement quality, reproducibility, and correlation with regulatory test results.

EUT Configuration and Operating Modes

The EUT must be configured and operated in a manner representative of typical use and consistent with standards requirements. Exercise all functions that might generate emissions, including communication interfaces, display modes, and processing activities. Standards often specify that measurements be performed in worst-case operating modes identified through preliminary testing.

Cable configurations significantly affect emissions. Standards specify cable layouts including lengths, routing, and termination conditions. Cables should be arranged to represent typical installation while following standard layouts that enable reproducible measurements. Excess cable length is typically arranged in a specified bundle configuration.

Support Equipment

Equipment needed to operate the EUT, such as monitors, keyboards, or communication partners, must be positioned and configured to minimize their contribution to measured emissions. Use equipment known to have low emissions, maintain specified distances from the EUT, and orient cables to minimize coupling. Some standards specify testing with support equipment removed if the EUT can operate independently.

Measurement Procedure Optimization

Complete broadband measurements covering all frequencies, rotations, antenna heights, and polarizations can be time-consuming. Efficient approaches begin with fast pre-scans using peak detection to identify frequencies of interest, followed by detailed maximization measurements at specific frequencies using appropriate detectors.

Automated measurement systems control turntables, antenna masts, and receivers to execute measurement procedures efficiently. Software algorithms optimize scanning patterns and focus measurement effort on frequencies approaching limits. These systems can reduce measurement time from days to hours while improving reproducibility.

Documentation and Reporting

Thorough documentation supports measurement validity and enables troubleshooting if questions arise. Essential documentation includes EUT identification and serial numbers, software versions, operating modes, cable configurations, support equipment, environmental conditions, equipment calibration status, and any deviations from standard procedures.

Test reports should present results clearly, typically in tabular and graphical formats showing emission levels versus frequency compared to applicable limits. Margin to limits, measurement uncertainty, and any identified non-compliances should be clearly indicated.

Interpreting Measurement Results

Emission spectra reveal information about the sources and characteristics of radiated emissions. Narrowband emissions at regular frequency intervals typically indicate clock harmonics, with the fundamental frequency identified from the harmonic spacing. Broadband noise floors may result from random digital activity or switching power supply noise. The spectral shape provides clues about the emission mechanisms and appropriate mitigation strategies.

Comparing emissions measured in different antenna polarizations indicates the dominant radiation mechanism. Horizontal polarization often results from cable common-mode currents, while vertical polarization may indicate emissions from vertical structures or slots in enclosures. Understanding the polarization characteristics helps identify which structures are acting as radiating antennas.

Correlation between emission frequencies and known internal frequencies helps identify emission sources. Clock frequencies, their harmonics, and intermodulation products can be mapped to specific circuits. Power supply switching frequencies and their harmonics indicate converter-related emissions. This frequency correlation guides targeted investigation and mitigation.

Changes in emissions with equipment configuration or operating mode provide diagnostic information. If emissions change with processor load, digital circuits are likely responsible. If emissions correlate with power converter operation, the power supply is the source. If emissions change with cable position or length, cable common-mode currents are involved. Systematic investigation using these relationships efficiently identifies emission sources.

Advanced Measurement Techniques

Time-domain measurements capture emission waveforms rather than just spectral amplitudes, providing additional information about emission sources and characteristics. The temporal signature of emissions can distinguish between different source types and may reveal intermittent emissions that could be missed in continuous frequency sweeps. Time-domain techniques are particularly valuable for pulsed or modulated emissions.

Emissions from digitally modulated equipment may be difficult to characterize with traditional swept measurements. Modern EMI receivers include features for capturing peak, average, and statistical distributions of time-varying emissions. These capabilities ensure that modulated emissions are properly characterized against applicable limits.

Near-field scanning creates spatial maps of electromagnetic fields near the equipment under test. By scanning a probe across the equipment surface and recording field magnitude and phase at each point, the near-field distribution can be visualized. This information helps locate emission sources and can even be used to predict far-field patterns through mathematical transformation.

Reverberation chamber measurements provide an alternative to anechoic chamber testing, particularly for large or complex equipment. The reverberant environment statistically samples emissions from all angles simultaneously, potentially reducing measurement time. Correlation with traditional anechoic measurements requires understanding of the statistical nature of reverberation chamber results.

Conclusion

Radiated emission measurement is a sophisticated discipline requiring understanding of electromagnetic theory, specialized facilities, precision equipment, and standardized procedures. The evolution from open area test sites to modern semi-anechoic chambers, reverberation chambers, and GTEM cells provides engineers with multiple approaches suited to different applications, from research and development through formal compliance certification.

Success in radiated emission measurement depends on proper facility design and validation, appropriate equipment selection and calibration, rigorous adherence to measurement procedures, and thorough understanding of uncertainty and correlation factors. Pre-compliance testing integrated into the development process identifies problems early and builds confidence that formal compliance testing will succeed.

The discipline continues to move. Measurement ranges have pushed upward as product clock rates and radio bands have risen, carrying with them the shift from NSA to SVSWR site validation and from quasi-peak to peak and average weighting. FFT-based time-domain receivers have compressed scan times by orders of magnitude and made intermittent emissions visible. Densely occupied spectrum has made ambient management, rather than instrument sensitivity, the practical limit at many sites. Engineers who understand why each requirement exists, and not merely what it says, are best placed to absorb these changes and to keep their products compliant in every market they enter.

Related Topics

Radiated emission measurement is one facet of a broader electromagnetic compatibility discipline. The following topics address where emissions originate, the complementary forms of EMC, and the design practices that keep measured levels within regulatory limits.