EMI, EMC, and Shielding Shielding and Enclosure Design Informational

What is the difference between conducted emissions and radiated emissions in an RF system?

Conducted emissions and radiated emissions are the two categories of unintentional electromagnetic energy that an electronic device generates. They are measured separately and have different compliance limits: (1) Conducted emissions (CE): unwanted RF signals that travel along the cables and wires connected to the device (power cords, data cables, control lines). The emissions propagate as AC currents and voltages on the conductors. Measurement: a Line Impedance Stabilization Network (LISN) is connected between the power source and the device under test. The LISN provides a defined impedance (50 ohms) and filters the external noise, presenting only the DUT emissions to the measurement receiver. Frequency range: typically 150 kHz to 30 MHz (FCC Part 15), or 9 kHz to 30 MHz (CISPR 32). Below 150 kHz: emissions are generally not regulated. Above 30 MHz: emissions transition to radiation and are covered by radiated emission limits. (2) Radiated emissions (RE): unwanted electromagnetic fields that radiate from the device, its cables, and its enclosure. The device acts as an unintentional antenna, with the cables often being the primary radiating elements (they are the longest conductors and the most efficient antennas). Measurement: performed at a specified distance (3 m, 10 m, or 30 m) using calibrated antennas in an anechoic chamber, OATS (open area test site), or semi-anechoic chamber. Frequency range: 30 MHz to 1 GHz (FCC Part 15 Class B). Extended to 6 GHz, 18 GHz, or 40 GHz for intentional radiators and 5G devices. Key relationship: conducted emissions on cables create radiated emissions. A 1 MHz conducted emission on a 1 m cable: the cable radiates as a monopole antenna. At frequencies where the cable is > lambda/10: the radiation is significant.

Category: EMI, EMC, and Shielding

Updated: April 2026

Product Tie-In: Enclosures, Gaskets, Absorbers, Filters

Conducted vs Radiated Emissions

Understanding the distinction between conducted and radiated emissions is fundamental for EMC design and compliance testing. Both must be controlled to pass regulatory requirements.

Conducted Emissions Detail

(1) Common-mode vs differential-mode: conducted emissions have two components: differential-mode (DM): current flows on the power line and returns on the neutral wire. Created by the normal switching current of power supplies, motor drives, and digital circuits. Filtered by differential-mode filters (series inductors, X-capacitors). Common-mode (CM): current flows in the same direction on both power and neutral wires, returning through the ground plane or parasitic capacitance. Created by high dV/dt switching coupled to ground through parasitic capacitance (e.g., the drain-to-heatsink capacitance of a MOSFET). Filtered by common-mode chokes (wound on a common ferrite core) and Y-capacitors (from line/neutral to ground). (2) In RF systems: the local oscillator, clock, and PA switching frequencies are primary sources of conducted emissions. The switching harmonics travel along the power supply cables and radiate. A 100 MHz clock with 1 V amplitude and 1 ns edges: harmonics extend to > 1 GHz. The harmonics conducted to the power cable can cause radiated emission failures above 30 MHz.

Radiated Emissions Detail

(1) Sources: PCB traces carrying high-frequency signals (clock, LO, digital buses). Cables connected to the device (act as transmission-line antennas). Enclosure apertures (slots, seams leak internal fields). Heatsinks (connected to switching transistors, radiate at switching frequency harmonics). (2) The radiation efficiency depends on the source dimensions relative to wavelength: for a cable or trace of length L: at f where L < lambda/10: very inefficient radiation (near-field only). At f where L = lambda/4: maximum radiation (quarter-wave monopole). Above lambda/4: the radiation pattern becomes complex with multiple lobes. For a 1 m cable: lambda/4 at 75 MHz. Maximum radiation near 75 MHz. (3) Electric field at distance d from a monopole antenna: E = 60 × I × (2×pi×L/lambda) / d (V/m). For I = 1 mA, L = 0.3 m, f = 300 MHz (lambda = 1 m), d = 3 m: E = 60 × 0.001 × (2×pi×0.3/1) / 3 = 60 × 0.001 × 1.885 / 3 = 0.038 V/m = 91.5 dBuV/m. FCC limit at 300 MHz (Class B, 3 m): class B limit at that frequency is about 43.5 dBuV/m. The emission exceeds the limit by 48 dB. This shows how even small currents on long cables can cause radiated emission failures.

Compliance Strategy

(1) Design for compliance: start EMC design early (not after the product is built). RF engineers: ensure all intentional RF signals are properly contained (shielding, filtering, controlled impedance routing). Digital designers: minimize clock harmonics (use spread-spectrum clocking, minimize rise times to the minimum required, terminate transmission lines properly). Power supply: use a properly filtered DC-DC converter or linear regulator. Filter the output for conducted emissions and shield the converter for radiated emissions. (2) Testing sequence: test conducted emissions first (easier to fix, less expensive to test). Fix any CE failures before testing RE (CE failures often cause RE failures through cable radiation). Then test radiated emissions. Fix any RE failures (usually by improving shielding, adding cable ferrites, or fixing layout issues). (3) Pre-compliance testing: use a near-field probe set and a spectrum analyzer to identify emission sources before formal testing. Map the emissions on the PCB and cables to find the dominant sources. Fix the dominant sources first (the highest emissions are usually caused by one or two design flaws).

EMC Emission Parameters

CE range: 150 kHz - 30 MHz (FCC Part 15)
RE range: 30 MHz - 1+ GHz
LISN impedance: 50Ω (standardized)
E field: E = 60·I·(2πL/λ)/d V/m
Cable radiation: significant when L > λ/10

Common Questions

Frequently Asked Questions

Which is harder to fix, conducted or radiated?

In general: radiated emissions are harder to fix because: (1) The sources are less obvious (any trace, cable, or aperture can radiate). Conducted emissions come from specific conductors (easier to isolate). (2) Radiated fixes often require hardware changes (shielding, PCB re-layout, gaskets), while conducted fixes can often be done with added components (ferrites, capacitors, filters on the existing PCB). (3) Radiated testing is more expensive and time-consuming (requires an anechoic chamber). However: some conducted emission problems (particularly common-mode) can be very difficult to diagnose and fix because the current paths are through parasitic capacitances that are hard to identify and control.

Do I need to test both for every product?

Yes, for FCC/CE (EU) regulatory compliance: both conducted and radiated emissions must be tested and must pass the applicable limits. FCC Part 15: CE 150 kHz - 30 MHz, RE 30 MHz - 1 GHz (Class B for consumer, Class A for industrial). CISPR 32 (EU): CE 150 kHz - 30 MHz, RE 30 MHz - 6 GHz. MIL-STD-461G: CE and RE from 10 kHz to 18 GHz (military). Some product categories have additional or different requirements (automotive: CISPR 25, medical: IEC 60601-1-2). Intentional radiators (Wi-Fi, Bluetooth, cellular): must also meet specific intentional emission limits and spurious emission limits under FCC Part 15.247/15.407 or the applicable rule part.

What about conducted immunity and radiated immunity?

In addition to emissions: most regulatory standards also require immunity (susceptibility) testing: conducted immunity: the device must continue to operate correctly when RF signals are injected onto its cables (IEC 61000-4-6: 150 kHz - 80 MHz, 1-10 Vrms). Radiated immunity: the device must operate correctly in an RF field (IEC 61000-4-3: 80 MHz - 6 GHz, 1-10 V/m). These tests verify that the device is not susceptible to external interference. Immunity testing is required in the EU (CE marking) but not currently required by the FCC in the US (the FCC tests only emissions, not immunity).

Keep Reading