Electronics Guide

Sources of Radiated Emissions

Electronic circuits generate radiated emissions through several fundamental mechanisms. Differential-mode currents flowing in signal traces and cables can radiate directly when the trace or cable dimensions become significant compared to the signal wavelength. However, common-mode currents induced on cables, enclosure surfaces, and heat sinks are typically responsible for the majority of radiated emissions problems in practical systems.

Clock signals and their harmonics are often the dominant sources of radiated emissions in digital systems. The fast edge rates of modern clock signals generate frequency components extending into the gigahertz range, and even small loop areas carrying these signals can produce significant radiation. Switching power supplies contribute additional emission sources through their fundamental switching frequencies and associated harmonics. I/O cables frequently act as efficient antennas for common-mode currents generated within the equipment, making cable management a critical aspect of emissions control.

Radiation Mechanisms

Radiated emissions arise from two primary antenna structures: electric dipoles and magnetic loops. Differential-mode signals flowing in traces act as small dipole antennas when their physical length approaches a quarter wavelength at the frequency of interest. The radiation efficiency increases dramatically as the electrical length increases, explaining why emissions typically become more problematic at higher frequencies.

Common-mode currents on cables and enclosures create more effective radiating structures because the involved lengths are typically much larger than PCB traces. A cable just one meter long becomes an efficient half-wave dipole antenna at 150 MHz. Return current paths that do not directly follow signal traces create loop areas that radiate as magnetic dipoles, with radiation efficiency proportional to the loop area and the square of frequency. Understanding these mechanisms is essential for identifying and correcting emission problems.

Measurement and Testing

Radiated emission measurements quantify the electromagnetic field strength at a specified distance from the equipment under test. Standard measurements are performed in controlled environments such as open area test sites (OATS), semi-anechoic chambers, or fully anechoic rooms that minimize reflections and ambient interference. The equipment is typically placed on a turntable and rotated while a receiving antenna scans vertically to capture the maximum emission at each frequency.

Pre-compliance testing using near-field probes, current probes, and simplified far-field measurements helps identify emission sources during development before formal compliance testing. Spectrum analyzers and EMI receivers with appropriate detectors (peak, quasi-peak, and average) measure emission levels against regulatory limits. Understanding the correlation between pre-compliance and compliance measurements enables engineers to predict final test results and address problems early in the design cycle when changes are least costly.

Emission Control Strategies

Controlling radiated emissions requires a systematic approach addressing sources, coupling paths, and radiation mechanisms. At the source level, minimizing high-frequency spectral content through edge rate control, spread spectrum clocking, and careful component selection reduces the available energy that could be radiated. Proper PCB layout with continuous reference planes, controlled impedance traces, and minimal loop areas prevents the formation of efficient radiating structures.

Shielding provides a physical barrier to electromagnetic radiation when source-level controls prove insufficient. Effective shielding requires attention to seams, apertures, and cable penetrations that can compromise enclosure integrity. Filtering at I/O ports prevents common-mode currents from reaching cables that would otherwise act as antennas. Cable shield termination techniques, ferrite suppression, and bonding practices all contribute to a comprehensive emission control strategy that addresses problems at multiple levels.

Regulatory Framework

Radiated emission limits are established by regulatory bodies worldwide to ensure electromagnetic compatibility in the shared radio frequency environment. In the United States, the Federal Communications Commission (FCC) Part 15 rules specify limits for unintentional radiators. International standards from CISPR (International Special Committee on Radio Interference) form the basis for European, Asian, and other regional requirements, enabling products to achieve global market access.

Different product categories face different standards and limits depending on their intended environment and application. Information technology equipment, industrial scientific and medical devices, automotive components, military systems, and medical devices each have specific applicable standards. Class A limits apply to commercial and industrial environments, while more stringent Class B limits govern residential applications. Understanding which standards apply to a product and designing to meet those requirements from the outset prevents costly redesigns and delays in market access.

Radiated Emissions Topics