FCC Part 15 Class A vs B: Regulating EMI in Electronics

This article provides a quick overview of how the Federal Communications Commission (FCC) Part 15 rules regulate unintentional radio-frequency (RF) emissions from digital clocks and switched-mode power supplies (SMPS). It explains why these internal electronic components generate electromagnetic interference (EMI) and breaks down the specific regulatory differences between Class A limits for commercial environments and the stricter Class B limits for residential spaces. Finally, it covers the conducted and radiated emission testing standards that hardware manufacturers must meet to legally market their electronics in the United States.

Understanding Unintentional Radiators

Under FCC Part 15 rules, most modern electronic devices are classified as unintentional radiators. These are devices that intentionally generate radio-frequency energy for internal operations—such as processing data or converting power—but are not designed to broadcast that energy wirelessly through the air or over power lines.

By definition, any device utilizing digital logic that operates at frequencies above 9 kHz falls under these regulations. The primary goal of FCC Part 15 is to ensure that the unintentional RF noise generated inside a device does not escape at levels high enough to disrupt licensed radio services, emergency broadcasts, cellular networks, or nearby consumer appliances.

How Digital Clocks and SMPS Generate RF Emissions

Digital clocks and switched-mode power supplies are the two primary culprits of unintentional EMI in modern electronics.

Digital clocks generate square-wave signals to synchronize microprocessors and logic circuits. While a clock might operate at a fundamental frequency of 50 MHz, the rapid transition between low and high voltage states (fast rise and fall times) creates harmonic frequencies. These harmonics extend far into the hundreds of megahertz or gigahertz, turning circuit board traces into tiny, unintentional antennas that radiate RF noise.

Switched-mode power supplies (SMPS) convert electrical power efficiently by rapidly switching transistors on and off, typically at frequencies between 50 kHz and several megahertz. This high-speed switching of high currents and voltages generates significant lower-frequency noise. While SMPS noise can radiate through the air, its primary issue is pushing RF interference backward into the AC power grid, polluting the building's electrical wiring.

FCC Part 15 Class A vs. Class B Environments

The FCC divides unintentional radiators into two categories based on where the equipment is intended to be used: Class A and Class B.

Class A Devices are marketed exclusively for use in commercial, industrial, or business environments. Because industrial and office buildings typically have greater physical separation between equipment and fewer susceptible consumer receivers (like televisions and AM/FM radios) nearby, Class A emission limits are more relaxed.

Class B Devices are marketed for use in residential environments, even if they are also used in businesses. Examples include personal computers, smart home hubs, and phone chargers. Because homes place electronics in close proximity to broadcast receivers and shared wall outlets, Class B limits are significantly stricter. In general, Class B limits are roughly 10 decibels (dB) more stringent than Class A limits, meaning a Class B device is only allowed to emit about one-third of the electromagnetic field strength of a Class A device.

Conducted vs. Radiated Emission Limits

To control the specific types of noise generated by SMPS and digital clocks, the FCC enforces two distinct measurement thresholds: conducted emissions and radiated emissions.

Conducted Emissions (150 kHz to 30 MHz)

Conducted limits primarily regulate the noise generated by switched-mode power supplies. Testing measures the amount of RF voltage the device feeds back into the public AC power lines, using a Line Impedance Stabilization Network (LISN).

Class A limits allow up to 79 dBµV (decibels relative to one microvolt) in the lower frequency band (150 kHz to 500 kHz) and 73 dBµV from 500 kHz to 30 MHz (quasi-peak).
Class B limits drop the allowable noise to a sliding scale of 66 to 56 dBµV in the lower band, and cap emissions at 60 dBµV from 5 MHz to 30 MHz.

To pass these limits, engineers must design input line filters (using inductors and capacitors) into the SMPS to trap the switching noise before it exits the power cord.

Radiated Emissions (30 MHz and Above)

Radiated limits primarily regulate the high-frequency harmonic noise escaping from digital clocks and high-speed data lines. Testing is conducted in an semi-anechoic chamber or Open Area Test Site (OATS), where an antenna measures the electric field strength escaping the device through the air.

Class A limits are normally measured at a distance of 10 meters and allow field strengths ranging from 39 dBµV/m (at 30 MHz) up to 49.5 dBµV/m (above 960 MHz).
Class B limits are normally measured at a closer distance of 3 meters. Despite the closer measurement distance, the limits are still tightly restricted from 40 dBµV/m (at 30 MHz) to 54 dBµV/m (above 960 MHz). When mathematically adjusted to the same measurement distance, Class B allows drastically less radiated energy.

Engineering for Compliance

To meet FCC Part 15 Class A or Class B limits, commercial electronics manufacturers must actively suppress emissions from clocks and power supplies at the board level. For digital clocks, engineers often use Spread Spectrum Clocking (SSC), which slightly varies the clock frequency continuously so energy does not peak at a single harmonic. They also rely on proper printed circuit board (PCB) grounding, decoupling capacitors, and metallic Faraday shielding over noisy components. By combining these techniques with proper SMPS filtering, devices can operate reliably without violating federal radio-frequency spectrum regulations.