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CISPR 16-2-3: Radiated Disturbance Measurements
Standard methods for measuring radiated emissions from equipment and systems
1. Scope and Measurement Fundamentals
CISPR 16-2-3 specifies the methods for measuring radiated radio disturbance from all types of equipment in the frequency range of 9 kHz to 18 GHz. This standard provides the definitive procedures for obtaining reproducible radiated emission measurements on open-area test sites (OATS), semi-anechoic chambers (SAC), and fully-anechoic chambers (FAR). The measurement fundamentally determines the electric field strength radiated by the EUT at a defined measurement distance.
The standard defines two types of measurement: (1) the traditional method, where the measurement antenna is scanned in height (1–4 m) to capture the maximum field strength resulting from constructive interference between the direct and ground-reflected waves, and (2) the fully-anechoic chamber method, where absorbers on all surfaces eliminate reflections and the antenna is positioned at a fixed height corresponding to the far-field condition.
The choice between OATS/SAC and FAR methods significantly affects both the measured values and the test duration. For equipment with dominant vertical polarization emissions (common for devices with vertical PCB orientation), the FAR method often yields higher measured values because there is no ground reflection to provide cancellation at certain antenna heights. Understanding this difference is crucial for correlating results between different test facilities.
2. Test Setup and Antenna Positioning
The standard specifies the EUT placement, cable routing, and antenna positioning in detail. For tabletop equipment, the EUT is placed on a turntable 0.8 m above the reference ground plane. The turntable enables 360° azimuth scanning to find the direction of maximum emission. For floor-standing equipment, the EUT is placed directly on the ground plane.
| Frequency Band | Antenna Type | Polarization | Antenna Height Scan | Measurement Distance |
|---|---|---|---|---|
| 9 – 150 kHz | Rod or loop | Vertical or H-field | Fixed at 1 m | 3 m or 10 m |
| 150 kHz – 30 MHz | Rod (1 m) | Vertical | Fixed at 1 m | 3 m or 10 m |
| 30 – 200 MHz | Biconical | Horizontal and vertical | 1 – 4 m | 3 m or 10 m |
| 200 – 1000 MHz | Log-periodic | Horizontal and vertical | 1 – 4 m | 3 m or 10 m |
| 1 – 18 GHz | Horn | Horizontal and vertical | Fixed at far-field height | 1 m or 3 m |
For the traditional OATS/SAC method, the receiving antenna is scanned from 1 m to 4 m height while the EUT is rotated on the turntable. This two-dimensional search (height + azimuth) ensures that the maximum field strength is captured regardless of the EUT’s emission pattern and the interference geometry. The measurement is performed separately for horizontal and vertical polarizations.
One of the most challenging aspects of radiated emission measurement is the handling of cable bundles. The standard specifies that all cables should be routed to the front edge of the table and then downward to the ground plane. Excess cable length should be bundled in a non-inductive figure-8 pattern. Improper cable routing can change the measured emission by 6–10 dB, particularly at frequencies above 200 MHz where cable resonances become significant.
3. Measurement Procedures for Different Equipment Types
CISPR 16-2-3 provides specific procedures for different equipment categories. For large equipment (exceeding the measurement antenna’s illumination area), the EUT is measured in sections with the antenna positioned at multiple locations around the EUT. For modular systems, the measurement is performed with the system in its typical configuration, and additional measurements may be required for each module in its standalone configuration.
For equipment with wireless transmission capabilities, the measurement must differentiate between the intentional radiator emissions (the communication signal) and the unintentional emissions (noise from the device electronics). The standard specifies that the wireless transmitter should be operated in a test mode with continuous carrier transmission at maximum power, with the measurement performed at the spurious emission frequencies (outside the allocated band) and, where required, at the band edges.
The standard also addresses measurements in the presence of ambient signals. If ambient signals (broadcast stations, other transmitters) interfere with the measurement at specific frequencies, the standard allows the use of a “CISPR notch filter” to reject the ambient signal, provided the filter’s insertion loss at the measurement frequency is accounted for in the calibration. Alternatively, the measurement may be performed in a shielded environment (SAC or FAR) where ambient signals are attenuated.
A significant advancement in modern EMC testing is the use of multi-channel FFT-based receivers that simultaneously acquire data from multiple antennas (horizontal and vertical polarization) in real time. This approach reduces test time by 40–60% compared to traditional single-channel scanning. For 3 m SAC measurements, dual-polarized antennas combined with a two-channel receiver can capture both polarizations in a single turntable rotation, cutting the test time from 30 minutes to under 10 minutes per EUT configuration.
4. Frequently Asked Questions
Q: When should I use a 3 m vs. a 10 m measurement distance?
A: A 3 m distance is typically used for equipment with dimensions less than 1 m and is the standard distance for FAR measurements. A 10 m distance provides better far-field approximation and is required for formal compliance in many regulatory frameworks, but requires larger test facilities.
A: A 3 m distance is typically used for equipment with dimensions less than 1 m and is the standard distance for FAR measurements. A 10 m distance provides better far-field approximation and is required for formal compliance in many regulatory frameworks, but requires larger test facilities.
Q: Why is the antenna height scanned from 1 m to 4 m?
A: On an OATS or SAC with a reflective ground plane, the direct and reflected waves interfere constructively or destructively depending on the antenna height. Scanning from 1 m to 4 m ensures that the maximum of this interference pattern is captured. The 1–4 m range covers the first interference maximum for frequencies above 30 MHz at 3 m and 10 m distances.
A: On an OATS or SAC with a reflective ground plane, the direct and reflected waves interfere constructively or destructively depending on the antenna height. Scanning from 1 m to 4 m ensures that the maximum of this interference pattern is captured. The 1–4 m range covers the first interference maximum for frequencies above 30 MHz at 3 m and 10 m distances.
Q: How do I determine the worst-case EUT operating mode?
A: The worst-case mode is determined by measuring the emission spectrum of all available modes and selecting the mode that produces the highest emission levels. For equipment with adjustable parameters (clock speed, output power, data rate), the configuration producing the highest emissions should be tested.
A: The worst-case mode is determined by measuring the emission spectrum of all available modes and selecting the mode that produces the highest emission levels. For equipment with adjustable parameters (clock speed, output power, data rate), the configuration producing the highest emissions should be tested.
Q: What is the measurement uncertainty of a typical radiated emission test?
A: For a well-characterized 3 m SAC, the expanded measurement uncertainty (k=2) is typically 4.5–5.5 dB below 1 GHz and 5.0–6.0 dB above 1 GHz. Factors contributing to uncertainty include antenna factor calibration (±1 dB), cable loss variation (±0.5 dB), EUT positioning (±0.5 dB), and site imperfections (±1 dB).
A: For a well-characterized 3 m SAC, the expanded measurement uncertainty (k=2) is typically 4.5–5.5 dB below 1 GHz and 5.0–6.0 dB above 1 GHz. Factors contributing to uncertainty include antenna factor calibration (±1 dB), cable loss variation (±0.5 dB), EUT positioning (±0.5 dB), and site imperfections (±1 dB).
