Electronics Guide

Measurement Fundamentals

Radiated emission measurements determine the electric field strength at a specified distance from the equipment under test. Standards typically specify measurement distances of 3 meters, 10 meters, or 30 meters, with 10 meters being the most common reference distance for commercial products. The field strength is expressed in decibels referenced to one microvolt per meter (dBuV/m), providing a logarithmic scale convenient for the wide dynamic range of emission levels encountered in practice.

The measurement captures the maximum emission at each frequency across all spatial orientations of the equipment and receiving antenna. The equipment under test is typically rotated through 360 degrees while the antenna height is varied to find the maximum field strength, accounting for reflections from the ground plane that create angle-dependent maxima. This procedure ensures that the worst-case emission orientation is identified regardless of how the equipment might be positioned in actual use.

Frequency scanning across the specified measurement range, typically 30 MHz to 1 GHz or higher for modern standards, generates an emission spectrum that can be compared against regulatory limits. Different detector functions, primarily peak, quasi-peak, and average, weight the measurements according to signal characteristics. The quasi-peak detector particularly reflects the subjective annoyance potential of emissions to broadcast radio reception.

Test Facilities

The Open Area Test Site (OATS) represents the reference environment for radiated emission measurements. An OATS consists of a flat, conductive ground plane in a location free from ambient electromagnetic noise and reflecting structures. The ground plane, typically at least three times the measurement distance in each direction, provides a well-defined electromagnetic environment with a single ground reflection. OATS measurements established the baseline for EMC testing, and alternative test facilities are validated against OATS performance.

Semi-anechoic chambers provide controlled measurement environments by lining walls and ceiling with RF absorber material while maintaining a conductive floor. The absorbers eliminate reflections from the chamber boundaries, while the conductive floor provides the ground plane reflection present in OATS measurements. Semi-anechoic chambers offer protection from external ambient signals and consistent conditions regardless of weather, making them the most common choice for commercial EMC testing.

Fully anechoic chambers line all surfaces including the floor with absorber material, eliminating all reflections. These chambers approximate free-space conditions and are used primarily for antenna calibration and measurements where ground reflections would complicate interpretation. For standard radiated emission testing, semi-anechoic chambers with ground planes are preferred as they match the OATS reference conditions.

Chamber performance must be validated through site attenuation or normalized site attenuation measurements that verify the facility produces results equivalent to an OATS. This validation process compares the transmission loss between calibrated antennas in the chamber against theoretical OATS values. Regular revalidation ensures that absorber degradation or facility changes have not compromised measurement accuracy.

Antennas for Emission Measurement

Radiated emission measurements require antennas with known characteristics across the frequency range of interest. Different antenna types are used for different frequency bands based on their practical size, sensitivity, and pattern characteristics. Proper antenna selection and calibration are essential for accurate measurements.

Biconical antennas cover the lower frequency range, typically from 30 MHz to 300 MHz. These horizontally polarized antennas present a broad, predictable pattern and reasonable sensitivity despite their modest gain at the lowest frequencies. Biconical antennas are mechanically robust and maintain consistent performance over extended use.

Log-periodic dipole array (LPDA) antennas extend coverage from approximately 200 MHz to 1 GHz or higher. The overlapping frequency coverage with biconical antennas allows correlation checks between antenna types. LPDAs provide higher gain than biconicals at the upper frequencies, improving sensitivity and signal-to-noise ratio. Modern broadband LPDAs can cover the entire 30 MHz to 1 GHz range, simplifying test setups.

Horn antennas provide high gain and directional patterns at frequencies above 1 GHz, where smaller aperture sizes become practical. Double-ridged horn antennas can cover very wide bandwidths, making them suitable for broadband emission measurements. Their directional patterns help discriminate against ambient interference and chamber imperfections.

Antenna calibration provides the antenna factor needed to convert received voltage to field strength. The antenna factor varies with frequency and must be determined through calibration against reference antennas or reference field strengths. Calibration uncertainty contributes directly to measurement uncertainty, making regular recalibration essential for maintaining measurement accuracy.

Receivers and Spectrum Analyzers

EMI receivers are specialized instruments designed for emission measurements according to regulatory standards. They include the specific detector functions required by standards, particularly the quasi-peak detector with its defined charge and discharge time constants. EMI receivers provide preselector filtering to reject out-of-band signals and pulse limiters to protect against transient overloads. Their frequency accuracy, amplitude accuracy, and noise floor are specified to meet measurement requirements.

Spectrum analyzers can also perform radiated emission measurements, though care must be taken to understand the differences between spectrum analyzer and EMI receiver behavior. Spectrum analyzers typically include peak, sample, and average detection modes, but may not include quasi-peak detection or may implement it differently than required by standards. Modern EMI receivers often incorporate spectrum analyzer functions, providing flexible measurement capabilities.

The receiver bandwidth affects measurement results, with specified bandwidths for different frequency ranges defined in the standards. Typical bandwidths are 9 kHz for frequencies below 150 kHz, 9 kHz for 150 kHz to 30 MHz, and 120 kHz for 30 MHz to 1 GHz. Using incorrect bandwidth settings will produce inaccurate measurements that may not correlate with compliance test results.

Preamplifiers may be needed to improve system sensitivity, particularly for measurements at greater distances or with lower gain antennas. The preamplifier gain must be accounted for in the measurement calculation, and its noise figure affects the overall system noise floor. Care must be taken to avoid overload from strong ambient signals or high emission levels.

Measurement Procedure

A complete radiated emission measurement follows a systematic procedure to ensure that maximum emissions are captured at each frequency. The equipment under test is positioned on a turntable at a specified height above the ground plane, typically 0.8 meters for tabletop equipment or floor standing for large equipment. Power and signal cables are arranged in a standardized manner to ensure reproducibility.

The measurement antenna is positioned at the reference distance and scanned in height over a specified range, typically 1 to 4 meters. For each equipment rotation angle and antenna height, the receiver measures the emission level. The combination of turntable rotation, antenna height scan, and polarization switching finds the maximum emission at each frequency regardless of the spatial orientation.

Initial scans typically use peak detection to quickly identify frequencies where emissions are present. Once emission frequencies are identified, measurements at those frequencies using quasi-peak and average detectors determine the final emission levels for comparison against limits. This approach balances measurement speed with the detailed characterization needed for compliance determination.

The equipment under test should operate in its most emissive condition during measurement. This may require testing multiple operating modes, configurations, or loads to identify the worst case. Any external cables, peripherals, or accessories that would normally be connected in use should be present during testing, as these often act as antennas for emissions.

Pre-Compliance Testing

Pre-compliance testing during product development identifies potential emission problems before formal compliance testing. Early identification allows design changes when they are least costly, avoiding expensive iterations after the design is finalized. Pre-compliance testing need not achieve full compliance measurement accuracy but must be sufficiently indicative to guide design decisions.

Near-field probes allow localization of emission sources on the equipment under test. These small probes, including current probes for cable common-mode currents and H-field probes for magnetic fields near circuit boards, can identify which specific circuits or cables are responsible for observed emissions. This diagnostic information is invaluable for targeted mitigation efforts.

Simplified far-field measurements using spectrum analyzers and basic antennas can estimate emission levels without the full complexity of compliance facilities. While such measurements may not correlate exactly with formal compliance results, they indicate relative emission levels and track the effect of design changes. Understanding the correlation between pre-compliance and compliance results for a particular setup improves interpretation.

TEM cells and GTEM cells provide alternative pre-compliance measurement environments that can fit in typical laboratory spaces. These devices create uniform fields for immunity testing and can also measure emissions, though with different field distributions than open-air measurements. Results from TEM/GTEM measurements must be interpreted with understanding of their limitations and correlation to far-field results.

Measurement Uncertainty

All measurements have associated uncertainties that affect the confidence with which compliance can be determined. For radiated emission measurements, uncertainty sources include antenna calibration, cable losses, receiver accuracy, site imperfections, and positioning errors. Understanding and quantifying these uncertainties is essential for meaningful compliance assessment.

Antenna factor uncertainty typically contributes several decibels to overall measurement uncertainty. Cables connecting the antenna to the receiver introduce losses that must be measured and accounted for, with their own associated uncertainties. Receiver amplitude accuracy, bandwidth accuracy, and detector conformance all add uncertainty components.

Site imperfections cause variations between facilities that should theoretically produce identical results. Site validation measurements quantify facility performance, but residual variations remain. The distance from equipment under test to the antenna must be accurately measured and maintained, as field strength varies with distance.

Standards specify measurement uncertainty requirements and may define how uncertainty should be accounted for in compliance determination. Some regulatory schemes require that uncertainty be considered such that compliance is demonstrated at the limit minus the measurement uncertainty, while others accept measurements at the limit without explicit uncertainty subtraction. Understanding the applicable rules is essential for correct compliance assessment.

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.

Summary

Radiated emission measurement is a fundamental EMC engineering discipline that enables characterization and compliance verification of electronic products. The measurement process involves controlled environments, calibrated equipment, and standardized procedures that ensure consistent results across test facilities. Understanding measurement fundamentals, uncertainty sources, and result interpretation enables effective use of measurement data for design improvement and compliance demonstration.

From pre-compliance testing during development through formal compliance verification, radiated emission measurements guide the design process toward products that coexist successfully in the shared electromagnetic environment. The combination of far-field measurements for compliance determination and near-field techniques for source identification provides the complete picture needed for efficient EMC engineering.