Meaning
Federal regulations define the maximum allowable radiated emission limits for intentional and unintentional radiators in the United States. Compliance with fcc part 15.209 ensures that electronic devices do not interfere with licensed radio services or other equipment. The standard specifies field strength limits across a wide range of frequencies, from 9 kilohertz up to the highest harmonic of the device.
Radiated Limit
Emissions must remain below a specific microvolts per meter threshold measured at a distance of three or thirty meters. At lower frequencies, the limits are more permissive but tighten as the frequency enters the VHF and UHF bands. For example, between 30 and 88 megahertz, the limit is 100 microvolts per meter.
Testing identifies spikes caused by oscillators or high speed data buses. Any peak exceeding the curve requires immediate design modification to the enclosure or PCB. Ferrite beads often act as the first line of defense in these cases.
Compliance Margin
Manufacturers aim for several decibels of headroom below the regulated ceiling. This margin accounts for unit to unit variation during mass production. A device that sits right on the limit may fail during a spot check if a component supplier changes.
Shielding and filtering provide the necessary attenuation to maintain this buffer.
Test Environment
Measurements occur in an open area test site or a semi-anechoic chamber. The receiving antenna moves vertically to find the maximum emission level from the device under test. Rotating the product on a turntable ensures that radiation is checked from every angle.
Calibration of the test equipment is mandatory for a valid certification filing.
