BY BRUCE ROSE, Applications Engineer
CUI
www.cui.com
Electromagnetic-conducted and -radiated emissions are the radio frequency (RF) energy emitted by a product along connecting cables (including power) and through free space. Governments worldwide regulate the allowable level of such emissions to ensure that a device does not cause unreasonable harm to other electronic products or interfere with communications. Every designer making electronic products for sale thus needs to know something about electromagnetic compatibility (EMC).
Most industrial and consumer electronic products sold in the United States are required to meet conducted and radiated emissions standards as described in FCC regulations Title 47 Part 15, often referred to as FCC Part 15. Similar standards for products sold in Europe are governed by European regulations CISPR 22/EN 55022. Both sets of regulations describe limits for conducted and radiated emissions as a function of frequency. While these two sets of regulations are created and administered by separate organizations, they have been constructed to be similar or “harmonized.” One benefit of harmonizing these regulations is that designing a product to meet one set of regulations typically ensures that it will also satisfy the requirements set forth in the other set of regulations.
Conducted and radiated emissions limits.
Conducted radiation specifications cover emissions in the frequency range of 150 kHz through 30 MHz. A separate set of radiated emissions specifications covers the spectrum of 30 MHz and greater. Test procedures and tools are slightly different for conducted versus radiated emissions, and the filter components used to mitigate the EMC issues are similar but differ in electrical values. Because the conducted emissions frequency band is lower than the radiated emissions frequency band, the filter components used to address conducted emissions will be electrically and physically larger than those required to address radiated emissions.
Both types of emissions need consideration. At low frequencies (less than about 30 MHz), the conductors and cables of electronic devices can conduct RF energy through shared power sources or loads and cause issues with other electronic products. However, at these low frequencies, the conductors and cables are ineffective as antennas, and thus radiated emissions in these frequencies are typically not an issue. At high frequencies (above about 30 MHz), on the other hand, the impedances of the conductors and cables attenuate the conducted energy sufficiently to prevent that from being an issue. But at these higher frequencies, the conductors and cables can instead serve as effective antennas and radiate the RF energy with the ability to cause interference with nearby electronic products.
It is worth noting that tests of a system to demonstrate compliance with emissions standards must be applied to the final system, regardless of any compliance testing that individual subsystems may have undergone. Consider, for example, an internal or external power supply, which many designs use off the shelf. While most internally mounted power supplies are themselves designed and tested to meet EMC regulations, that testing is performed with the supply configured as a standalone product. Similarly, most wall-plug and desktop versions of external power supplies are also designed and tested to meet EMC regulations as standalone units.
After the power supply has been installed into a system, however, the completed system must also be tested to ensure that it meets EMC regulations. Thus, while incorporating compliant power supplies (or any other pre-tested subsystem) into a design will minimize the potential for EMC-related issues during system testing, it does not guarantee that the completed system will pass emissions testing. So if a power supply customer is a manufacturer combining that power supply with a load, then they will be required to perform testing to ensure that the complete system meets EMC regulations and take whatever corrective action is needed. This will be more challenging for wall-plug and desktop versions as compared to internally mounted power supplies as the circuitry is housed in an enclosed case, so adding external components will be needed in order to address EMC issues.
To help, many subsystem vendors will provide recommended circuits to address EMC issues encountered during system integration. Because the requirements vary with each application, these recommendations are left to the discretion of the end system’s designer. This way, each design incorporates only the components required for that specific application.
Starting a new design? Read this follow-up article to see how considering EMC early in the design process can help avoid big problems later.
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