The two most common EMC test failures are conducted emissions and radiated emissions. Many products go through repeated remediation cycles – the root cause is often not understanding the physical principles, limit rules, or test‑setup‑induced variations. This article covers conducted and radiated emissions separately – from test principles, standard systems, to common engineering issues.

1. First, Distinguish Conducted from Radiated Emissions

·Conducted emissions: measures electromagnetic noise transmitted through cables – power lines, wiring harnesses, high‑speed signal lines – measured as noise voltage or current – typical frequency range: 150 kHz – 30 MHz.

·Radiated emissions: measures electromagnetic energy radiated through space – the product, internal cables, and PCB traces can all act as antennas – measured as field strength in space – start frequency: 30 MHz – upper limit depends on product category and standard version.

Simple distinction: noise transmitted along wires = conducted. Noise propagating through air as electromagnetic waves = radiated. Remediation approaches differ:

·Conducted: focus on power‑supply filtering, common‑mode chokes, Y‑capacitor matching.

·Radiated: shielding optimisation, grounding, PCB layout, and high‑speed interface placement.

  2. Conducted Emissions – Standards

2.1 Standards by product category – choose correctly:

·Household appliances, power tools: CISPR 14‑2 (CISPR 14‑1 covers voltage fluctuation/flicker – not RF conducted/radiated).

·Industrial, scientific, medical equipment: CISPR 11.

·Multimedia audio‑video equipment: CISPR 32.

·In‑vehicle components: CISPR 25.

Different standards have different limits and test setups – choosing the wrong one invalidates results.

2.2 Domestic GB equivalents:

·GB 4824 ↔ CISPR 11.

·GB/T 9254.1/9254.2 ↔ CISPR 32.

·Household appliance RF emission GB ↔ CISPR 14‑2 series.

When applying for domestic or export certification, always check the standard year – old‑version limits are not accepted by current certification systems.

Conducted emissions are measured using LISN (Line Impedance Stabilisation Network – also called artificial mains network). Two core functions:

·Provides standardised power‑source impedance to the EUT.

·Suppresses external interference from the mains – ensuring the receiver measures only noise from the EUT.

Limits: conducted emission voltage limits are judged by quasi‑peak – average values are only monitoring indicators – not pass/fail criteria.

  3. Conducted Emissions – Measurement Details

Test setup is strictly specified:

·EUT placed 0.8m above the ground reference plane.

·Power‑cable length between LISN and EUT ≤0.8m – excess cable bundled neatly – no loose trailing.

·Setup significantly affects noise coupling – improper setup causes data distortion.

Products with multiple power ports – each port must be tested independently. For products with detachable power cables – use the original manufacturer‑supplied cable.

If signal‑line coupling significantly affects conducted results – follow standard requirements for cable routing. Ground and shield terminations must follow standard specifications – confirm all setup conditions with the lab before formal testing.

Limits – divided by frequency bands – lower bands are more relaxed – higher bands tighten. Common failure: low‑band exceedance – usually from switching‑power‑supply switching frequency and harmonics. Remediation: check common‑mode choke effective inductance, Y‑capacitor matching – and optimise power‑entry PCB layout.

Distinction: AC‑powered consumer equipment uses AC‑LISN. 12V/24V in‑vehicle DC components must use DC‑LISN – the two are not interchangeable. Most conducted failures are caused by common‑mode noise – don't rely only on X‑capacitors.

  4. Radiated Emissions – Standards

4.1 Standards by product category:

·Household appliances: CISPR 14‑2.

·Multimedia equipment: CISPR 32.

·In‑vehicle components: CISPR 25.

·Industrial equipment: CISPR 11 – with Class A/Class B – Class B (residential) has stricter limits.

4.2 Test environment: full anechoic chamber – absorber‑lined walls/ceiling – simulating free‑space conditions – eliminating reflections. Consumer electronics: commonly 3m or 10m test distance. In‑vehicle components: industry practice uses 1m setup.

Note: 1m is not a CISPR 25 mandatory requirement – it's industry practice – different OEMs may specify 0.5m or 3m – do not directly apply fixed conversion formulas for field‑strength comparison.

Measurement bands:

·Consumer multimedia (CISPR 32): 30 MHz – 1 GHz.

·In‑vehicle (CISPR 25:2021): mandatory band 30 MHz – 1 GHz – 1 GHz – 6 GHz is optional in the standard – but most OEM specifications require it.

  5. Radiated Emissions – Measurement Details

Radiated emissions are measured using receiving antennas – different bands use different antennas:

·30–200 MHz: biconical antenna.

·200 MHz – 1 GHz: log‑periodic antenna.

·Above 1 GHz: horn antenna.

Both horizontal and vertical polarisations are scanned.

The EUT is placed on a turntable – rotating 360° during testing to capture maximum radiation. The receiving antenna scans from 1m to 4m – recording peak field strength.

For products with multiple external cables – cable layout and bundling directly affect radiation characteristics – cable length and droop height must be kept consistent throughout.

Radiated emissions remediation is generally more difficult than conducted. First, locate the source: is it from power circuits, high‑speed clocks, differential data lines, enclosure gaps, or connector shield gaps? Then apply shielding, filtering, grounding improvements, and PCB layout optimisation. We recommend using near‑field probes to locate interference sources – then remediate – much more efficient than blind debugging.

  6. In‑Vehicle EMC – Special Requirements

The electromagnetic environment in vehicles is far harsher than consumer electronics – ignition systems, drive motors, and multiple DC‑DC converters are all strong interference sources. OEM internal EMC requirements are generally stricter than international standards.

In‑vehicle component RF EMC: CISPR 25.

·Conducted emissions: 150 kHz – 80 MHz (consumer electronics: 30 MHz max).

·Radiated emissions: 30 MHz – 1 GHz mandatory – 1–6 GHz optional extension.

Classes: Class 1 (strictest) to Class 5 (most relaxed). Mainstream OEMs typically require at least Class 3 or higher.

In addition to RF emissions, in‑vehicle EMC testing adds transient immunity – pulse trains, load‑dump, voltage surges – following ISO 7637 series. For in‑vehicle electronics, don't only reference consumer EMC standards – both RF emissions and transient immunity must be addressed – missing either risks OEM rejection.

  7. The Value of Pre‑Testing

First‑pass EMC formal testing is rare – most products require multiple remediation rounds. Formal testing is expensive and tightly scheduled – repeated submissions extend timelines. Pre‑testing is an effective way to control costs and compress timelines.

Pre‑testing is conducted in a formal EMC chamber – with streamlined test procedures and relaxed scanning conditions – quickly identifying exceedance frequencies – providing baseline data for internal remediation. Both conducted and radiated emissions can be pre‑scanned.

Pre‑test data cannot be used for formal certification – only as remediation reference. After pre‑test metrics are stable, then schedule formal certification testing.

  8. Test Reports and Compliance Documents

The EMC test report is a core technical document for product certification. A compliant report must clearly state:

·Test standard and limit class.

·Full test‑setup diagram.

·Original test curves.

·Each test conclusion.

All raw data must be fully traceable – test site photos, valid instrument calibration certificates, and spectrum records must be archived.

CE certification (EU): EMC testing uses the corresponding EN harmonised standard – the report is included in the product technical file.

US:

·Unintentional radiators: FCC Part 15B – SDoC.

·Products with wireless transmission: not unintentional radiators – Part 15B does not apply.

For Korea KC, Japan MIC, Australia RCM, and other markets – each regulator has its own acceptance rules for EMC report versions and lab accreditation – confirm requirements before starting.

For EMC testing standards, contact BlueAsia at 13534225140 (King) or email king.guo@cblueasia.com.