What You Need to Know: Testing Standard EMC CISPR 25
Users of modern vehicles take for granted the many wirelessly connected devices that they have. Examples include the navigation systems that depend on signals from Global Positioning System (GPS) satellites, hands-free use of cell phones using Bluetooth, and communications related to a fleet management system being sent over cell phone networks. The functionality of each of these is entirely dependent on being able to reliably receive signals.
As will be familiar to those who have used analog radios and television, the quality of the received signal can be impacted by electromagnetic interference. This electromagnetic interference can come from a variety of sources, both natural, such as solar flares, and man-made from a wide variety of electrical devices. A vehicle will itself contain many electrical devices, a great number of which are in regular use. It is therefore important to ensure that only inconsequential electromagnetic radiation is emitted by them, thereby maintaining the quality of the received signal.
A robust design process for every component is therefore needed so that they function satisfactorily in their electromagnetic environment without introducing intolerable electromagnetic disturbances. This is known as electromagnetic compatibility (EMC). Once designed, the components, and the whole vehicle system, are tested to assure that their real-world electromagnetic compatibility meets the expectations of the engineers who design them.
In order to simplify the number of tests needed, and to ensure best practice amongst all manufacturers, various test standards exist for EMC that the vehicle and its components must adhere to. One such standard is CISPR 25, which contains limits and procedures for the measurement of radio disturbances. The standard is produced by the “Comité International Spécial des Perturbations Radioélectriques” (CISPR) or the International Special Committee on Radio Interference, who, since 1934, have been setting global standards for controlling electromagnetic interference in electrical and electronic devices, and who are part of the International Electrotechnical Commission (IEC).
CISPR 25 covers both the performance of the whole vehicle as a system, and of the individual components within it. This is to allow for discovery of any potential EMC concerns during the development phase of electrical components, saving time and money over only testing once the whole vehicle is ready. Therefore as a manufacturer of electrical components, Modine undertakes testing per CISPR 25 to ensure compliance of the components to the standard.
Within the testing, the components are tested in a specially designed chamber, known as an Absorber Line Shielded Enclosure (ALSE). The ALSE is required to both shield the components from sources of electromagnetic radiation from outside the enclosure, and to absorb any radiation from the components within the enclosure. In this way, the emitted electromagnetic radiation from the components can be precisely measured, free from reflections and outside interference sources. Different antennas are used to detect different emissions frequencies. All test equipment, and the components, are installed within defined dimensional limits to ensure reliability and repeatability of the tests. The component itself will then be connected to a wiring harness that is representative of the harness used in the vehicle installation. Power is supplied to the system under test through calibrated Line Impedance Stabilization Networks (LISN), and representative loads for the system are attached to the test harness. Once connected, the components are tested for both conducted emissions (unwanted signals traveling through the harness, which may be received by other vehicle modules and cause interference), and radiated emissions (unwanted signals being emitted by the component and being picked up by an antenna). During the test, frequencies in the range of 150kHz to 2.5GHz are assessed as defined in CISPR 25
After testing is complete, a test report will be generated, wherein the results are compared to the thresholds set for the various different frequencies defined within the standard. This allows for easy assessment of pass/fail of the component. Assuming the component passes, it will then later be tested again as part of the whole vehicle assessment to ensure the reliable operation of all devices on the vehicle that depend on receiving wireless communications signals. Additionally, electrical transients which may be produced by the component under test, are tested under ISO 7637.
As one would expect, the testing of the whole vehicle will then be conducted by the vehicle manufacturer. Modine supports this process not only by proactive testing of Modine components and systems to verify compliance, but also through defining installation guidelines to achieve optimum performance of the system, along with providing support and collaboration during the vehicle level testing, if required.
