What Is EMI Testing?

EMC testing covers both emissions and immunity, with conducted and radiated tests used for each. The table compares these 4 main test types.

Conducted emissions testing measures unwanted electromagnetic signals that travel from a device along its power lines, signal cables, or other conductors. Excessive emissions can disrupt other equipment on the same power circuit.

• Common applications: Products that connect to AC power, either directly or through an adapter or charger.
• Test setup: A line impedance stabilization network (LISN) connects the device under test (DUT) to the supply and gives the measurement a defined impedance. The LISN passes the unwanted emissions to a spectrum analyzer or EMI receiver. FCC Part 15.107, for example, sets conducted limits from 150 kHz to 30 MHz and specifies a 50 µH/50 Ω LISN. Signal and telecom ports are measured with other methods, depending on the product standard.

Radiated emissions testing evaluates the electromagnetic energy a device emits through the air. A device that radiates too much can interfere with nearby equipment, such as communication systems.

• Test setup: The DUT sits in a semi-anechoic chamber or on an open area test site (OATS). Antennas measure emissions at a set distance. FCC Part 15.109, for example, gives its main limits from 30 MHz and uses 3 meters for most unintentional radiators and 10 meters for Class A digital devices.
• Regulations: Standards such as CISPR 32 and FCC Part 15 set radiated emission limits for different product categories. CISPR 32 replaced CISPR 22 at the IEC, although FCC Part 15.109 still references CISPR 22.
   

Conducted immunity testing assesses whether a device can withstand electromagnetic disturbances carried on its power or signal cables. Surges and fast transients are tested separately (see Transient testing below).

• Test setup: A defined disturbance is injected into the device's power or signal lines while the device is monitored. IEC 61000-4-6 covers immunity to conducted disturbances induced by radio-frequency fields, from 150 kHz up to 80 MHz.

Radiated immunity testing evaluates how well a device resists electromagnetic fields in its operating environment.

• Test setup: The DUT is exposed to a field from a transmitting antenna in a controlled test space. The field strength is set to a test level and the frequency is swept while the device is watched for malfunctions.
• Regulations: IEC 61000-4-3 (2020 edition) establishes test levels and test procedures for immunity to radiated electromagnetic energy.

Beyond conducted and radiated RF testing, other types of electromagnetic immunity testing may apply, depending on the device and relevant standards:

• Electrostatic discharge (ESD) testing: Tests a device against static discharges from people, directly or to nearby objects, often following IEC 61000-4-2.
• Magnetic field immunity testing: Evaluates a device's tolerance to magnetic fields at power frequencies (50 and 60 Hz), as covered by IEC 61000-4-8.
• Transient testing: Tests immunity to repetitive fast transients (IEC 61000-4-4) and to surges caused by switching and lightning (IEC 61000-4-5).

Each instrument below plays a different role in finding, measuring and tracing interference.

Spectrum analyzers measure signal amplitude across a range of frequencies. They help you identify and measure a DUT's emissions to determine whether further investigation is needed or whether measured levels exceed applicable limits.

• Applications: Conducted and radiated emissions testing, especially when you are troubleshooting a compliance failure.

EMI receivers are designed for electromagnetic compatibility (EMC) testing. CISPR 16-1-1 specifies requirements for EMI receivers and spectrum analyzers used to measure electromagnetic disturbances from 9 kHz to 18 GHz. When an applicable product standard references CISPR 16-1-1, the measuring instrument must meet its requirements. The Keysight N9048B PXE EMI test receiver, available through the Official Keysight Used Equipment Store, covers 1 Hz to 44 GHz and is listed by Keysight as compliant with CISPR 16-1-1:2019.

• Functionality: EMI receivers typically provide peak, quasi-peak and average detectors. The Keysight EMI testing guide explains how these measurements are used in radiated emissions testing. FCC Part 15.107 specifies quasi-peak and average limits for conducted emissions.

Oscilloscopes show voltage signals in the time domain, which makes them useful for analyzing transient EMI events. Although oscilloscopes are not typically used for formal emissions compliance measurements, they help engineers troubleshoot circuits and identify possible sources of interference.

• Use cases: Observing power supply noise and capturing transient spikes from switching or electrostatic discharge.
• Advanced capabilities: Many oscilloscopes include Fast Fourier Transform (FFT) analysis, which converts a time-domain signal into a frequency-domain view.

Near-field probes help you locate EMI sources on a circuit board, such as noisy components or traces.

• How they work: A handheld probe picks up the magnetic or electric field close to the board and feeds the signal to an oscilloscope or spectrum analyzer.
• Advantages: Probing needs no electrical connection, so you can examine a circuit without modifying it.
• Common applications: Debugging printed circuit boards (PCBs), finding hotspots and checking shields and seams for leaks.