What is the difference between radiated and conducted emissions?

Radiated emissions are electromagnetic fields the product releases through the air, measured with antennas as field strength (dBuV/m) at a defined distance, typically from 30 MHz upward. Conducted emissions are RF voltages the product couples onto its power cable, measured with a LISN as dBuV, typically from 150 kHz to 30 MHz. Both are limited by standards such as CISPR 32 and FCC Part 15.

What is the difference between pre-compliance and full compliance testing?

Full compliance testing is performed at an accredited laboratory on a validated test site using calibrated CISPR 16 / ANSI C63.2 instrumentation, producing a report usable for certification. Pre-compliance testing uses simplified setups - near-field probes, a spectrum analyzer, a small chamber or an ordinary room - to find problems early and cheaply. Pre-compliance results carry measurement uncertainty of many dB and cannot replace the accredited test.

Why does EMC testing use a quasi-peak detector?

The quasi-peak detector, defined in CISPR 16-1-1, weights signals by repetition rate to approximate the annoyance an interference source causes to analog radio reception: frequent impulses read close to their peak, infrequent ones read lower. Most emission limits below 1 GHz are specified with quasi-peak values, with average detector limits alongside for conducted measurements and above 1 GHz.

Do I need an anechoic chamber to test my product?

For accredited radiated emission results you need a validated site: an open area test site (OATS) or a semi-anechoic chamber meeting normalized site attenuation or site VSWR criteria. For development work you do not - near-field probes and a spectrum analyzer on a bench catch many problems, and a GTEM cell offers a compact intermediate option with correlation algorithms to open-site results.

EMC Testing Basics: Emissions, Immunity, Test Sites and Equipment

EMC testing verifies two things: that a product’s electromagnetic emissions stay under the limits of the applicable standard, and (outside the US regulatory regime) that the product keeps working when subjected to defined disturbances. A complete compliance campaign combines radiated and conducted emission measurements with an immunity suite, run on validated test sites with calibrated instrumentation per CISPR 16 and ANSI C63 methods.

Interior of an RF anechoic chamber with walls and ceiling covered in white absorber panels and an antenna mast standing at the center of the floor — Photo: Togabi, CC BY-SA 4.0, via Wikimedia Commons

What do emission measurements involve?

Emission testing measures what the product sends out, in two coupling paths.

Radiated emissions are measured with calibrated antennas on a validated site, typically 30 MHz to 1 GHz and above depending on the product’s internal clock frequencies. The equipment under test (EUT) sits on a turntable and is rotated 360 degrees while the antenna scans in height (1 m to 4 m per ANSI C63.4) in both horizontal and vertical polarization, hunting the maximum emission. Results in dBuV/m are compared against limits such as FCC Section 15.109 or CISPR 32 Class B.

Conducted emissions are measured on the power cable, typically 150 kHz to 30 MHz. The EUT is powered through a line impedance stabilization network (LISN), which presents a defined 50 ohm / 50 uH impedance and delivers the noise voltage on each mains conductor to an EMI receiver. Quasi-peak and average detector readings in dBuV are compared with limits such as FCC Section 15.107. Our article what is a LISN explains this instrument in depth.

What does immunity testing involve?

Immunity (also called susceptibility) testing deliberately disturbs the product and observes whether it keeps working. The standard suite comes from the IEC 61000-4 basic standards: electrostatic discharge (61000-4-2), radiated RF fields (61000-4-3), electrical fast transients (61000-4-4), surge (61000-4-5), conducted RF (61000-4-6) and voltage dips (61000-4-11), each described on our IEC 61000 page. The product standard assigns severity levels and performance criteria: criterion A means the product must operate normally throughout, B allows temporary degradation with self-recovery, C allows a loss of function that an operator can restore. Immunity is mandatory for the EU market under the EMC Directive; FCC Part 15 does not require it.

Which test environments are used in EMC testing?

Environment	What it is	Typical use
OATS (open area test site)	Outdoor site with a conductive ground plane and no reflecting objects within a defined ellipse	Reference site for radiated emissions; historically the baseline against which chambers are validated
Semi-anechoic chamber (SAC)	Shielded room with RF absorber on walls and ceiling, reflective ground plane	Everyday radiated emission and immunity testing, immune to weather and ambient signals
Fully anechoic room (FAR)	Shielded room with absorber on all six surfaces	Free-space measurements, antenna calibration, some military and radiated immunity work
GTEM cell	Tapered transmission-line cell creating a defined field in a compact volume	Pre-compliance and small-EUT emissions/immunity with correlation to open-site results
Reverberation chamber	Shielded cavity with a mode-stirring paddle creating a statistically uniform field	High-field immunity, shielding effectiveness, large or cable-rich EUTs

Emission sites must prove their quality: normalized site attenuation (NSA) within +/-4 dB of theory per ANSI C63.4, or the site validation methods of CISPR 16-1-4.

What equipment does an EMC lab rely on?

Four instrument groups do most of the work:

EMI receiver - a measuring receiver meeting CISPR 16-1-1 / ANSI C63.2, providing calibrated quasi-peak, peak and average detection across 9 kHz to 40 GHz.
LISN - the coupling network for conducted emissions, defining mains impedance and isolating the measurement from grid noise.
Antennas - biconical, log-periodic or combined bilog antennas for 30 MHz to 1 GHz, horns above 1 GHz, each with a calibrated antenna factor per ANSI C63.5.
Immunity generators - ESD simulator (“ESD gun”), EFT/burst and surge generators, RF power amplifiers with field probes for radiated immunity.

Definitions for these and related terms (antenna factor, common mode, ambient) are collected in the EMC glossary.

EMC unit converter (50 ohm system)

dBuV uV dBm

Type in any field - the others update. dBm = dBuV - 107 (in a 50 ohm system); uV = 10^(dBuV / 20). These are the everyday conversions of EMI receiver work.

What does a typical compliance test plan look like?

A workable test plan fixes, before anyone enters the chamber: the applicable standards and editions (for example CISPR 32:2015+A1:2019 Class B plus FCC Part 15 Subpart B), the EUT configuration and operating modes to exercise (worst-case clocks, all interfaces active), the cable layout and support equipment, the emission measurements with frequency ranges and detectors, the immunity tests with severity levels and pass criteria per port, and the definition of a failure. Good plans also record EUT software versions and monitoring methods, because an immunity test is only as meaningful as your ability to detect a malfunction during it.

When is pre-compliance testing worth doing?

Always before the first accredited test, and after any significant design change. Pre-compliance means using affordable tools - near-field probes, a spectrum analyzer, a benchtop LISN, possibly a GTEM cell - to find and fix the dominant emission sources while the design can still change cheaply. Expect absolute accuracy no better than several dB; the goal is margin hunting and rank-ordering of problems, not a certifiable number. Products that arrive at the accredited lab with 6 dB of pre-compliance margin usually pass; products that were never pre-scanned usually do not. The full standards landscape these tests serve is mapped on our EMC standards hub.