Radiated emissions EMC test: pre-scan and final scan
Guide. EMC test method
Radiated emissions are the most-cited cause of schedule slip in CE marking and FCC certification campaigns. The measurement looks straightforward, an EUT, an antenna, a receiver, a graph, but it concentrates a rare number of methodological traps: antenna choice by band, measurement distance, detector, polarisation, cable arrangement, EUT operating mode. This guide describes the two-pass strategy, fast pre-scan in peak followed by slow final-scan in quasi-peak, which remains the reference practice of accredited labs. It covers the 30 MHz - 1 GHz band and the above-1 GHz band, with CISPR 32 and FCC Part 15 limits placed side by side.
Normative frame and boundaries between standards
Section titled “Normative frame and boundaries between standards”Radiated emissions denote the unintentional radio-frequency energy that an equipment under test (EUT) releases into its environment, as opposed to conducted emissions, which propagate through power and signal cables. Three families of standards bound the measurement.
CISPR 32 for multimedia equipment
Section titled “CISPR 32 for multimedia equipment”CISPR 32 (published as EN 55032 in the EU) covers information technology, audio, video and television-reception equipment. It replaced CISPR 22 in 2017. It defines limits by class (A for commercial / industrial, B for residential) and refers to CISPR 16 for measurement methods.
CISPR 11 for ISM equipment
Section titled “CISPR 11 for ISM equipment”CISPR 11 (published as EN 55011) covers industrial, scientific and medical equipment. It applies to RF heating, welding and plasma equipment, and to power converters, inverters and variable-speed drives. Its class structure (groups 1 and 2, classes A and B) overlaps with CISPR 32 but with group and band subtleties of its own.
Product standards
Section titled “Product standards”Above these two generic standards, product-specific standards may take precedence. Examples: EN 55014-1 for household appliances, EN 55015 for lighting, EN 55025 for automotive (a CISPR 25 derivative), EN 50121 for railway. A product standard takes precedence over the generic one where it exists.
47 CFR Part 15 on the FCC side
Section titled “47 CFR Part 15 on the FCC side”On the US side, FCC Part 15 Subpart B covers unintentional radiators (digital devices). The referenced method is ANSI C63.4, a functional counterpart of CISPR 16 with its own distance and detector conventions.
| Domain | EU generic | US generic |
|---|---|---|
| Multimedia / IT equipment | CISPR 32 (EN 55032) | 47 CFR 15 Subpart B |
| ISM equipment | CISPR 11 (EN 55011) | 47 CFR 18 |
| Measurement methodology | CISPR 16-2-3 | ANSI C63.4 |
| Instrumentation | CISPR 16-1-1 | ANSI C63.2 |
Frequency-band split and instrumentation
Section titled “Frequency-band split and instrumentation”The full test range typically spans 30 MHz to 6 GHz for most products, or up to 40 GHz for equipment with very fast clocks. No single antenna covers the whole range. The split into sub-bands is dictated by the physics of radiation and by the availability of calibrated antennas.
Antennas by band and measurement distance
Section titled “Antennas by band and measurement distance”| Frequency band | Typical antenna | Measurement distance | Site |
|---|---|---|---|
| 30 - 200 MHz | Biconical or hybrid biconilog | 3 m or 10 m | SAC or OATS |
| 200 MHz - 1 GHz | Log-periodic or hybrid biconilog | 3 m or 10 m | SAC or OATS |
| 1 - 6 GHz | Horn (double-ridge) | 3 m | SAC dedicated above 1 GHz |
| 6 - 18 GHz | Higher-frequency horn | 1 m (typical) | SAC or FAR |
| 18 - 40 GHz | Specific horn or open waveguide | 1 m | FAR (fully anechoic room) |
Quick legend:
- SAC = semi-anechoic chamber with a reflecting ground plane. The CISPR standard below 1 GHz.
- OATS = open area test site, an open-air range with a ground plane. The historical reference, still in service but sensitive to weather and ambient noise.
- FAR = fully anechoic room, with no ground plane, required by CISPR above 1 GHz to limit reflections.
Antenna height typically varies from 1 to 4 m below 1 GHz to sweep the worst-case radiation angle. Above 1 GHz, antenna height is fixed (typically 1.5 m), and the EUT is explored in azimuth and elevation instead.
Receivers and resolution bandwidth
Section titled “Receivers and resolution bandwidth”CISPR 16-1-1 specifies the EMI receiver resolution bandwidth (RBW), which is not a free parameter.
| Band | CISPR RBW | Primary detector |
|---|---|---|
| 30 MHz - 1 GHz | 120 kHz | Quasi-peak (CISPR) |
| 1 GHz - 18 GHz | 1 MHz | Peak + average |
| 18 - 40 GHz | 1 MHz | Peak + average |
The FCC, through ANSI C63.4, uses the same RBW values in practice. A measurement taken with a different RBW (for example 100 kHz on a generic spectrum analyser instead of 120 kHz) is non-compliant and cannot stand in a certification report.
Detectors
Section titled “Detectors”The three CISPR detectors are defined in CISPR 16-1-1 with precise time constants.
- Peak (Pk): instantaneous maximum in the resolution bandwidth. The fastest, the most pessimistic. Used in pre-scan and above 1 GHz.
- Quasi-peak (QP): time-weighted with the charge (1 ms) and discharge (550 ms) constants defined between 30 MHz and 1 GHz. The CISPR reference detector below 1 GHz. Slow.
- Average (AV): integration over time. Used above 1 GHz and for some conducted emissions.
Fundamental property: for any given signal, Peak >= QP >= AV. This inequality is what makes the pre-scan / final-scan strategy described below possible.
CISPR 32 emission limits
Section titled “CISPR 32 emission limits”CISPR 32 Class A and Class B limits are defined in dB(uV/m) at the reference measurement distance. Class A applies to a commercial or industrial environment, Class B to a residential environment.
30 MHz - 1 GHz band
Section titled “30 MHz - 1 GHz band”| Band | Class A (10 m) | Class B (10 m) | Detector |
|---|---|---|---|
| 30 - 230 MHz | 40 dB(uV/m) | 30 dB(uV/m) | Quasi-peak |
| 230 - 1000 MHz | 47 dB(uV/m) | 37 dB(uV/m) | Quasi-peak |
Class B limits are 10 dB tighter than Class A across both sub-bands. The discontinuity at 230 MHz reflects a physical reality: above 230 MHz, the internal structures of an EUT (cables, traces, apertures) become more efficient antennas, and the standard accepts a slightly higher emission level.
Above 1 GHz
Section titled “Above 1 GHz”| Band | Class A (3 m) | Class B (3 m) | Detector |
|---|---|---|---|
| 1 - 3 GHz | 56 dB(uV/m) peak / 76 dB(uV/m) peak | 50 dB(uV/m) peak / 70 dB(uV/m) peak | Peak / Average |
| 3 - 6 GHz | 60 dB(uV/m) peak / 80 dB(uV/m) peak | 54 dB(uV/m) peak / 74 dB(uV/m) peak | Peak / Average |
The CISPR 32 reference distance above 1 GHz is 3 m, against 10 m below. The change in distance and detector requires distinct instrumentation: broadband horn, tracking generator if analysis is performed in a fully anechoic chamber.
FCC Part 15 emission limits
Section titled “FCC Part 15 emission limits”On the US side, limits are set in 47 CFR 15.109 (Class A and Class B unintentional radiators). The reference distance is 3 m for Class B and 10 m for Class A, which complicates a direct comparison with CISPR.
30 MHz - 1 GHz band
Section titled “30 MHz - 1 GHz band”| Band | FCC Class A at 10 m | FCC Class B at 3 m | Detector |
|---|---|---|---|
| 30 - 88 MHz | 39 dB(uV/m) | 40 dB(uV/m) | Quasi-peak |
| 88 - 216 MHz | 43.5 dB(uV/m) | 43.5 dB(uV/m) | Quasi-peak |
| 216 - 960 MHz | 46.4 dB(uV/m) | 46 dB(uV/m) | Quasi-peak |
| Above 960 MHz | 49.5 dB(uV/m) | 54 dB(uV/m) | Quasi-peak |
Useful conversion between 3 m and 10 m: in the far field, 20 log(10/3) = 10.5 dB. An FCC Class B limit at 3 m of 40 dB(uV/m) maps, scaled to 10 m, to about 29.5 dB(uV/m). That is close to the CISPR Class B limit at 10 m (30 dB(uV/m)) at the low end, but the gap widens above 230 MHz where CISPR becomes the tighter regime.
Above 1 GHz
Section titled “Above 1 GHz”Above 1 GHz, the FCC historically referenced limits in 47 CFR 15.109(g), with peak and average detectors at 3 m. The values are not strictly identical to CISPR 32 but are close, to within around 6 dB on average. A product sized for CISPR 32 generally clears the FCC limits above 1 GHz without modification.
Comparative reading
Section titled “Comparative reading”| Band | Tighter side | Approximate gap |
|---|---|---|
| 30 - 88 MHz | FCC Class B (scaled to 10 m) | +0.5 dB FCC vs CISPR Class B |
| 88 - 230 MHz | FCC Class B (scaled to 10 m) | +3 dB FCC vs CISPR Class B |
| 230 - 960 MHz | CISPR Class B | -1.5 dB CISPR vs FCC |
| Above 960 MHz | CISPR Class B | -5 dB CISPR vs FCC |
Design implication for dual-market: targeting CISPR Class B above 230 MHz covers FCC by margin. Between 30 and 230 MHz, FCC sets the bar. See CE vs FCC, EMC comparison for the detailed reasoning.
Pre-scan and final-scan strategy
Section titled “Pre-scan and final-scan strategy”This is where lab time is won or lost. A full quasi-peak sweep over 30 MHz - 1 GHz, with a 50 kHz step and a dwell time sufficient for the QP discharge constant, takes hours per polarisation per antenna height. Multiplied by EUT operating modes, that is days of chamber time. The two-pass strategy brings that down to hours.
Pass 1: pre-scan in peak
Section titled “Pass 1: pre-scan in peak”The objective is to identify candidate frequencies for overshoot, not to produce a compliance measurement. The sweep is run with:
- Peak detector (the fastest),
- CISPR-compliant RBW (120 kHz below 1 GHz),
- Short dwell time per point (typically 1 to 10 ms),
- Full coverage of both polarisations and the antenna height range,
- EUT configured in the most-emissive mode identified by prior analysis.
The output is a peak vs frequency plot. Any frequency whose peak level is below the standard's QP limit minus 6 dB is considered passed, since QP <= Peak by construction. Only frequencies above that threshold are retained as candidates.
Pass 2: final-scan in quasi-peak
Section titled “Pass 2: final-scan in quasi-peak”For each candidate frequency identified in pre-scan, the receiver switches to quasi-peak mode and the measurement is repeated point by point. Dwell time here is 1 to 2 seconds per frequency (550 ms discharge constant plus stabilisation). The number of frequencies to measure is typically 5 to 50 across the whole band, against thousands in a continuous sweep.
For each QP frequency, the operator sweeps:
- Both antenna polarisations (vertical, horizontal),
- Antenna height range (1 to 4 m below 1 GHz),
- Turntable azimuth in steps (typically 22.5 or 45 degrees),
- EUT operating modes relevant to the frequency (a peak at 480 MHz appears with USB 2.0 active, not necessarily at idle).
The value retained per frequency is the maximum across all those parameters. That value is compared against the limit.
Sequence summary
Section titled “Sequence summary”| Step | Detector | Typical time | Output |
|---|---|---|---|
| Fast pre-scan | Peak | 5 - 30 min per EUT mode | Candidate-frequency list |
| Threshold filter | - | Immediate | Frequencies above QP_limit - 6 dB |
| Final-scan QP | Quasi-peak | 1 - 2 s per candidate point | Compliance measurement |
| Maximisation | QP | Included | Worst case across polarisation / height / azimuth |
| Limit comparison | - | Immediate | Pass / fail per frequency |
EUT setup and design margin
Section titled “EUT setup and design margin”The physical configuration of the EUT during the test is not a detail. CISPR 16-2-3 and ANSI C63.4 set a rule: the EUT must be tested in the most-emissive configuration reasonably expected in normal use. That covers:
- Cable arrangement (excess length folded in a 30 to 40 cm serpentine per the standard),
- Representative connected peripherals (keyboard, screen, power supply, sensors),
- Software operating mode (data transfer, screen active, processor load),
- Enclosure orientation on the turntable.
A small change in cable routing can shift a peak by 6 to 10 dB. That is the single biggest cause of divergence between in-house pre-compliance and lab compliance.
Pre-compliance vs full-compliance
Section titled “Pre-compliance vs full-compliance”Pre-compliance is run in-house or in a non-accredited chamber, with simplified instrumentation (spectrum analyser plus biconilog antenna, without a calibrated ground plane). Its purpose is to flag gross overshoots before paying for lab time. Its typical uncertainty is 6 to 10 dB. Pre-compliance carries no presumption of conformity.
Full-compliance is carried out in an ISO/IEC 17025-accredited lab recognised by the competent authority (a notified body for CE, a TCB for FCC). The report it produces is legally binding.
Design margin
Section titled “Design margin”Engineering practice is to target 6 dB below the standard limit in pre-compliance, which absorbs:
- Lab measurement uncertainty (CISPR 16-4-2 quotes up to 5.2 dB for radiated emissions 30 MHz - 1 GHz),
- Unit-to-unit dispersion in manufacturing,
- Lifetime drift (component ageing, shielding-gasket contamination),
- Risk of a peripheral change (mains cable, third-party power supply).
A margin below 3 dB is treated as a significant non-conformity risk at serial production.
Common pitfalls in radiated-emissions measurement
Section titled “Common pitfalls in radiated-emissions measurement”Six errors recur in failed campaigns.
- Cable routing modified between pre-scan and final-scan. An operator re-arranges cables to ease access to the EUT, and the peak found in pre-scan no longer reproduces in QP. The frequency is not measured in final, and the product fails on a later serial unit or under market surveillance. Photographing the setup before each scan is the minimum prevention.
- Missed polarisation. A quick pre-scan covers only one polarisation. If final-scan does not include cross polarisation, a major peak can be ignored. CISPR 16-2-3 mandates both polarisations per retained frequency.
- Mis-calibrated antenna. The antenna factor (in dB/m) is specific to each antenna and to each polarisation, and must be traceable to a recent calibration. An antenna whose certificate has expired, or whose factor is applied with the wrong sign, introduces a 10 to 20 dB error without any warning.
- EUT operating mode not exhaustive. The EUT is tested idle, while the real application involves data transfer, active radio communication, or sensor scanning. The emission frequencies differ. The rule: identify all possible modes before entering the chamber, and measure in each.
- Confusion between 120 kHz and 100 kHz RBW. A generic spectrum analyser defaults to 100 kHz. An EMI receiver runs at 120 kHz per CISPR. Conversion between the two is not linear and depends on signal nature (CW, modulated, pulsed). Measuring at 100 kHz and applying a correction factor is not accepted in compliance.
- Insufficient margin in pre-compliance. A product clearing pre-compliance at -1 dB from the limit is a product that will not pass. Cumulative uncertainty exceeds that gap. The pragmatic rule: if the margin is below 6 dB, treat the peak as a fail and apply mitigation before paying the lab.
See also CE pitfalls and FCC pitfalls for per-regime detail.
Overshoot mitigation
Section titled “Overshoot mitigation”When a peak exceeds the limit in final-scan, three families of levers are available, from fastest to most structural.
Short term: setup and accessories
Section titled “Short term: setup and accessories”- Cable re-arrangement (separating power and signal, shorter length, twisted pairs),
- Clip-on common-mode ferrites on external cables (selected for the peak frequency),
- Additional conductive gaskets on enclosure apertures,
- Local capacitive decoupling added in series on suspect lines.
These actions do not require a PCB revision and can be validated in a few hours of chamber time. They typically yield 3 to 8 dB of additional margin.
Medium term: shielding and filtering
Section titled “Medium term: shielding and filtering”- Local Faraday cage on the emissive module (oscillator, DC/DC converter),
- Pi filters (capacitor - inductor - capacitor) in series on inputs / outputs,
- Cable-shielding upgrade (from single braid to double shield),
- Reduction of enclosure apertures below lambda/20 at the peak frequency.
These changes affect the mechanical enclosure and the BOM, but not the board. Typical lead time: 2 to 4 weeks.
Long term: PCB revision
Section titled “Long term: PCB revision”- Re-routing of clocks and high-speed traces to reduce current loops,
- Continuous ground plane with no slot under critical signals,
- Pairwise local decoupling (100 nF + 10 nF + 1 nF) at each power pin,
- Series termination on outgoing clocks (22 to 33 ohm series resistor),
- Spread-spectrum-clocking oscillator on main clocks.
This is the most effective mitigation but also the heaviest, with a 4 to 8 week PCB cycle. It becomes unavoidable when the required margin exceeds 10 dB.
See also
Section titled “See also”- Radio: RX blocking, selectivity and intermodulation tests
- HEMP and IEMI: IEC 61000-4-25 and hardened electronics
- SAR procedures: absorption rate (IEC 62209, EN 50360)
- IEC 61000-4-3: radiated RF field immunity
- IEC 61000-4-6: conducted RF immunity
- Pre-compliance EMC: TEM cell, near-field probes, LISN
- EMC chamber types: SAC, FAR, OATS, GTEM, reverberation
- Calibration and measurement uncertainty (GUM, CISPR)
- PCB design for EMC: return paths, decoupling, stackup
Articulation with other EMC tests
Section titled “Articulation with other EMC tests”Radiated emissions are only one test in a full EMC campaign. The table below places the test within the typical sequence.
| Family | Test | Generic standard | Reference |
|---|---|---|---|
| Emissions | Radiated emissions | CISPR 32 / FCC 15 | This guide |
| Emissions | Conducted emissions on LISN | CISPR 32 / FCC 15 | Guide pending |
| Immunity | Electrostatic discharge (ESD) | EN 61000-4-2 | See CE tests |
| Immunity | Radiated RF field | EN 61000-4-3 | See CE tests |
| Immunity | Fast transients (EFT) | EN 61000-4-4 | See CE tests |
| Immunity | Surge | EN 61000-4-5 | See CE tests |
| Immunity | Conducted RF | EN 61000-4-6 | See CE tests |
See RED tests for additional tests on radio products, and FCC tests for the US-side sequence. The glossary covers the acronyms (RBW, QP, SAC, NSA, EUT) with their CISPR definitions.
Key takeaways
Section titled “Key takeaways”- The 30 MHz - 1 GHz vs above-1 GHz split dictates instrumentation (antennas, chamber, detector). No single antenna covers the range.
- RBW is fixed by CISPR 16-1-1, not by the operator: 120 kHz below 1 GHz, 1 MHz above.
- CISPR Class B is tighter than FCC Class B above 230 MHz, and the reverse below. Size to the tighter envelope per band.
- Pre-scan in peak then final-scan in QP divides chamber time by ten without loss of rigour, provided the cable setup stays identical.
- The 6 dB margin absorbs measurement uncertainty (up to 5.2 dB per CISPR 16-4-2), serial dispersion and ageing.
- Cable routing changes between passes are the number-one cause of pre-compliance / compliance divergence.
For implementation on the EU side, see CE tests. On the US side, see FCC tests. On radio products, see RED tests.
Sources & references
- CISPR 32:2015+A1:2019, Multimedia equipment, emission requirements , IEC webstore.iec.ch/publication/26241
- CISPR 16-1-1:2019, Specifications of radio disturbance and immunity measuring apparatus , IEC webstore.iec.ch/publication/63465
- CISPR 16-2-3:2016, Methods of measurement, radiated disturbance measurements , IEC webstore.iec.ch/publication/26326
- CISPR 11:2024, Industrial, scientific and medical equipment, radio-frequency disturbance characteristics , IEC webstore.iec.ch/publication/68645
- 47 CFR Part 15, Radio frequency devices , FCC www.ecfr.gov/current/title-47/chapter-I/subchapter-A/part-15
- ANSI C63.4-2014, American National Standard for methods of measurement of radio-noise emissions , IEEE / ANSI standards.ieee.org/ieee/C63.4/5536/