Radiated emissions testing is the measurement of the electromagnetic emissions that are intentionally and unintentionally generated by the equipment under test (EUT). This test is one of the EMC family's most common and essential tests. Not all but most electronic devices emit electromagnetic radiations unintentionally. These radiations often create hindrance in the operations of other devices in its vicinity. Hence, many international regulatory organizations have set limits for these radiated emissions in their regions. All electronics and electrical products need to comply with the standards.

Fig: Radiated Emission Testing

What are the EMC Standards for Radiated Emissions Testing?

The following are some EMC standards that specify limits on radiated emissions:

• CISPR 32: This standard applies to multimedia equipment (MME) as defined as having a rated r.m.s. AC or DC supply voltage not exceeding 600 V. It establishes the requirements which provide an adequate level of protection of the radio spectrum, allowing radio services to operate as intended in the frequency range 9 kHz to 400 GHz;
• FCC 15: This standard sets limits on the amount of electromagnetic radiation allowed from digital and electronic devices such as wristwatches, musical instruments, computers, telephones, and low-power transmitters;
• FCC 18: This standard sets the limits on the emission of electromagnetic energy produced by industrial, scientific, and medical equipment (ISM);
• IEC 61000-6-3: This standard sets the electromagnetic emission limits for equipment in residential environments;
• IEC 61000-6-4: This standard sets the electromagnetic emission limits for equipment for industrial environments;
• IEC 61000-6-8: This standard sets the electromagnetic emission limits or professional equipment in commercial and light-industrial locations.

What are the Test Equipment and Test Site Required for Radiated Emissions?

Test Equipment:

The radio emissions produced by the devices are not spherical and symmetric. Rather, they are pretty directed in one direction. Hence, the test lab has to vary the height of the receiving antenna between 1 and 4 meters as well as adjust the rotation of a turntable on which the EUT will be placed. Following are other the test equipment required for radiated emissions testing:

• Close-field probes
• Current probes
• Antennas
• Oscilloscopes
• Spectrum Analyzers (development, diagnostic, and QA testing)

Test-Sites:

For testing radiated emissions from electrical devices, there are two main types of test sites: Open Area Test Site (OATS) and Semi-Anechoic Chamber (SAC). A fully compliant test site includes a ground plane that stretches between the EUT’s position and the measuring antenna and beyond them, a means of rotating the EUT, and a mast that allows the antenna to be scanned in height from 1 to 4 m as well as giving both horizontal and vertical polarization. The commercial standards allow three measurement distances, 3 m, 10 m, and 30 m. 

• OATS: They are usually designed to meet the specifications of ANSI C63.4 and CISPR 16-1-x. OATS are set up in an open outdoor with a ground reference plane stretching from the EUT to the antenna.  

Fig: Open Area Test System

• SAC: SAC is housed within a shielded (metal) space. This test site is more accurate because it reduces the background (ambient) radio signals, making it much easier to differentiate between what is coming from the product and what is coming from the background signals.

Fig: SAC