Introduction to Conducted and Radiated Emissions in EMC Testing

Electromagnetic Compatibility (EMC) testing is a critical step in bringing any electronic product to market. It ensures that a device does not emit excessive electromagnetic interference (EMI) that could disrupt the operation of other equipment, nor is it unduly susceptible to interference from its environment. Among the various EMC tests, conducted emission and radiated emission measurements are fundamental for verifying that a product meets regulatory limits before it can be sold in most global markets.

Emission limits exist to protect the radio frequency spectrum—a shared resource—and to guarantee that devices can coexist without harmful interference. For example, a poorly designed power supply might inject noise back into the AC mains, disturbing nearby audio equipment (conducted emissions). Similarly, a high-speed digital circuit may radiate unintended signals that interfere with a nearby Wi-Fi receiver (radiated emissions). Understanding the differences, measurement methods, and applicable standards for each type of emission is essential for product developers, compliance engineers, and quality assurance teams.

This expanded guide provides an authoritative overview of conducted and radiated emission limits, including the regulatory landscape, testing methodologies, and practical implications for product design. Whether you are new to EMC or need a refresher on current requirements, the following sections break down the key concepts in a clear, actionable manner.

Understanding Conducted Emissions

What Are Conducted Emissions?

Conducted emissions are unwanted electrical noise that propagates along physical conductors, such as power cords, signal cables, or Ethernet lines. These noise currents travel from the equipment under test (EUT) back onto the AC mains or other connected cables, potentially affecting other devices sharing the same electrical network. Common sources include switching power supplies, clock signals, and motor drivers that generate high-frequency harmonics.

The conducted emissions phenomenon is most critical in the frequency range of 150 kHz to 30 MHz (as defined by most commercial standards). Below 150 kHz, power line hum and low-frequency harmonics are typically regulated by power quality standards rather than EMC emission limits. Above 30 MHz, interference tends to radiate rather than conduct efficiently through cables, shifting the focus to radiated emissions.

Measurement Setup for Conducted Emissions

Testing conducted emissions requires a Line Impedance Stabilization Network (LISN), which is inserted between the EUT and the AC mains. The LISN serves three purposes:

  • Impedance stabilization: It presents a defined, stable impedance (usually 50 µH / 50 Ω) to the EUT over the frequency range of interest, ensuring repeatable measurements.
  • Coupling of noise: It separates the noise voltage from the mains voltage and routes it to an EMI receiver or spectrum analyzer.
  • Isolation: It prevents external mains noise from skewing the measurement of the EUT’s own emissions.

Measurements are taken on both the phase and neutral lines (or on all lines in multiphase systems) and compared against the applicable limit. The test is typically performed in a shielded room to avoid ambient noise interfering with the low-level signals being measured.

Conducted Emission Limits and Standards

Regulatory limits for conducted emissions are specified in standards such as CISPR 32 (for multimedia equipment), CISPR 11 (for industrial, scientific, and medical equipment), and FCC Part 15 (for intentional and unintentional radiators in the United States). Most standards divide equipment into two classes:

  • Class A (Industrial) – Equipment intended for use in industrial environments, where conducted emission limits are more relaxed.
  • Class B (Residential) – Equipment intended for use in residential, commercial, and light-industrial environments, with stricter limits to protect radio and TV reception.

For example, CISPR 32 Class B limits for conducted emissions are around 66 dBµV (quasi-peak) at 150 kHz, falling to 56 dBµV at 500 kHz and remaining at 60 dBµV (quasi-peak) from 5 MHz to 30 MHz. Average limits are typically 10 dB lower. The exact limit values and frequency breakpoints vary slightly between standards and regions, but the overall shape is similar. It is essential to consult the latest version of the specific standard applicable to your product.

More information on CISPR standards can be found on the IEC EMC web page.

Understanding Radiated Emissions

What Are Radiated Emissions?

Radiated emissions are electromagnetic fields that propagate through the air from a device, much like radio waves from a transmitter. Any electronic circuit capable of generating high-frequency currents—especially those with fast edge rates, large current loops, or insufficient shielding—can act as an unintended antenna. These emissions can interfere with radio, television, cellular, Wi-Fi, and other wireless services within a radius of several meters or more.

The typical frequency range for radiated emission testing is 30 MHz to 1 GHz (and up to 6 GHz or higher for products incorporating high-speed digital interfaces or wireless transmitters). Below 30 MHz, radiation is less efficient, and conducted emissions are the primary concern; above 1 GHz, emissions are measured as radiated field strength in a similar manner but with different antenna types and measurement distances.

Measurement Setup for Radiated Emissions

Radiated emission measurements are performed in a fully anechoic chamber (FAC) or on an open area test site (OATS) that meets specific site attenuation requirements. The EUT is placed on a turntable at a specified height (usually 0.8 m for tabletop equipment), and a receive antenna is positioned at a distance of 3 m, 10 m, or 30 m (depending on the standard). The antenna is scanned in both horizontal and vertical polarizations while the EUT is rotated to capture the maximum emission levels.

Key equipment includes:

  • Broadband antennas: Biconical (30–300 MHz) and log-periodic (300–1000 MHz) antennas are common for the 30–1000 MHz range; above 1 GHz, horn antennas are used.
  • EMI receiver: A calibrated receiver measures the field strength in dBµV/m.
  • Pre-amplifiers: Low-noise amplifiers boost weak signals to improve the measurement dynamic range.

Ambient signals (e.g., broadcast radio) are either avoided by using a shielded chamber or subtracted through site validation procedures.

Radiated Emission Limits and Standards

Radiated emission limits are defined in the same standards as conducted emissions, but the units are different: dBµV/m at a specified distance (typically 3 m or 10 m). For example, FCC Part 15 Class B limits at 3 m range from 40 dBµV/m at 30 MHz (quasi-peak) to 46 dBµV/m at 100 MHz, and 54 dBµV/m above 230 MHz up to 1 GHz. CISPR 32 Class B limits at 10 m are somewhat tighter due to the larger distance.

As with conducted emissions, radiated limits are stricter for residential (Class B) equipment and more lenient for industrial (Class A) equipment. Some standards also impose peak and average limits for emissions above 1 GHz to protect satellite and radar services.

A useful reference for FCC limits is the official FCC Part 15 electronic code of federal regulations.

Key Differences Between Conducted and Radiated Emissions

While both types of interference stem from the same underlying noise sources within a device, their propagation paths and measurement methods differ significantly. The following table summarizes the primary distinctions:

Attribute Conducted Emissions Radiated Emissions
Propagation medium Through cables and power lines (metallic paths) Through the air (electromagnetic waves)
Frequency range (typical) 150 kHz – 30 MHz 30 MHz – 1 GHz and above
Measurement device LISN + EMI receiver (voltage measurement) Antenna + EMI receiver (field strength measurement)
Test environment Shielded room with LISN Anechoic chamber or OATS
Impact on other devices Interference through shared power or signal cables Interference to wireless receivers in proximity
Primary mitigation techniques Line filters, ferrites, proper grounding, shielding of cables Shielding enclosures, PCB layout, filtering of I/O lines, absorption

Understanding which type of emission dominates your product’s EMI profile helps in selecting the right diagnostic approach. For instance, a product with long external cables is more likely to have conducted issues, while a compact, high-speed digital device may primarily radiate from the PCB itself.

Regulatory Standards and Compliance

International and Regional Standards

EMC regulations exist worldwide, and most are based on the CISPR (Comité International Spécial des Perturbations Radioélectriques) standards developed under the International Electrotechnical Commission (IEC). The most common commercial standards include:

  • CISPR 11 – Industrial, scientific, and medical (ISM) equipment
  • CISPR 32 – Multimedia equipment (TVs, computers, audio systems)
  • CISPR 25 – Automotive components (used in vehicles)
  • FCC Part 15 – US requirements for intentional and unintentional radiators
  • EN 55032 – European harmonized standard (equivalent to CISPR 32)
  • EN 55011 – European standard for ISM equipment

In the European Union, EMC compliance is mandated by the EMC Directive 2014/30/EU, which requires products to meet the essential protection requirements. Manufacturers must draw up a Declaration of Conformity and affix the CE mark. Similarly, in the US, the Federal Communications Commission (FCC) requires testing and authorization procedures depending on the product’s risk category (verification, declaration of conformity, or certification).

Class A vs. Class B

As mentioned, emission limits are categorized by intended environment. Class B (domestic/residential) limits are the strictest because residential areas often have higher sensitivity to interference—people expect to use radios, television, and wireless devices without disruption. Class A limits are relaxed because industrial environments already contain higher ambient noise, and equipment is usually protected by separate power distribution and planning.

It is important to note that in many jurisdictions, products that meet only Class A limits must be labeled with a warning stating that the equipment may cause interference in residential areas. For most consumer electronics, compliance with Class B is mandatory.

Testing Procedures and Best Practices

Pre-compliance vs. Full Compliance Testing

Because full compliance testing in an accredited lab can be expensive, many manufacturers begin with pre-compliance testing using simpler setups such as a near-field probe or a temporary LISN in a bench environment. While pre-compliance cannot guarantee passing the final test, it helps identify problematic frequencies and validate early design changes. Once the design is stable, a full compliance test is performed at a certified laboratory to obtain the official measurement report.

Design Tips for Reducing Emissions

Whether conduction or radiation is the concern, following good EMC design practices from the outset pays dividends. Key guidelines include:

  • Proper PCB layout: Minimize loop areas for high-speed signals, use solid ground planes, and segregate analog and digital circuits.
  • Filtering: Install ferrite beads on power inputs and sometimes on I/O cables; use mains filters for conducted emissions; add LC filters at critical clock/outputs.
  • Shielding: Encase noise sources in metal enclosures, paying attention to seams and apertures; use conductive gaskets.
  • Cable management: Keep cables as short as possible; route cables away from noisy components; use shielded twisted-pair cables for data lines.
  • Component selection: Choose components with slower edge rates (if speed allows), spread-spectrum clocking, and reduced output drive strength.

These techniques often reduce both conducted and radiated emissions simultaneously, though the dominant coupling path may differ.

Importance of Meeting Emission Limits

Failing to meet conducted and radiated emission limits can have severe consequences for product manufacturers:

  • Market access denial: Products cannot be legally placed on the market without EMC compliance. In the EU, a CE mark cannot be affixed; in the US, FCC equipment authorization may be withheld.
  • Returns and customer complaints: Even if a product is sold without proper testing, end users may experience interference, leading to returns, poor reviews, and damage to brand reputation.
  • Legal and financial risks: Regulatory bodies can issue fines, recall orders, or compel redesign. Product liability claims may also arise if interference causes safety issues (e.g., medical device malfunction).
  • Increased development costs: Redesigning after a failed compliance test is far more costly than incorporating EMC considerations early in the design phase.

Conversely, investing in proper EMC design and testing provides a competitive advantage: faster time-to-market, fewer last-minute surprises, and a robust product that operates reliably in its intended environment.

Conclusion

Understanding the differences between conducted and radiated emission limits is fundamental to EMC testing and compliance. Conducted emissions travel via cables and are measured with LISNs and receivers in the low-frequency range, while radiated emissions propagate through the air and are measured with antennas in anechoic chambers at higher frequencies. Both types are controlled by internationally recognized standards such as CISPR 32, CISPR 11, and FCC Part 15, with strict Class B limits for residential equipment.

Effective EMC management requires careful design, pre-compliance checks, and ultimately formal testing in an accredited laboratory. By proactively addressing both conducted and radiated emissions, manufacturers can achieve smoother certification, reduce the risk of interference complaints, and ensure their products succeed in the global marketplace. For further guidance, consult the latest editions of the relevant standards and consider working with experienced EMC test engineers early in the product development cycle.

Additional resources on EMC testing and standards are available from the ETS-Lindgren EMC learning center and the FCC EMC information page.