2 CISPR 25 Requirements

CISPR 25 is an automotive EMI standard that is required for many manufacturers. This testing is titled as “Vehicles, boats and internal combustion engines – Radio disturbance characteristics – Limits and methods of measurement for the protection of on-board receivers.” The purpose of this standard is to limit the amount of emissions from the device under test (DUT) in different frequency bands to ensure that the DUT does not interfere with other systems that operate in those bands. The testing and limits are split into two separate types of emissions: conducted and radiated. Conducted emissions are coupled onto supply lines directly through conductors (such as traces or wires), and radiated emissions are emitted as EM waves and can be picked up by intentional and unintentional antennas on other systems.

The test procedures, relevant-frequency bands, and limits are different for both types of emissions, but the basics are similar: the device under test (DUT) is placed in an isolated room or chamber and set up in a well-defined, reproducible-electrical setup. All other possible emitters are removed from the chamber and the DUT is turned on and then allowed to operate normally. The DUT is powered through an artificial network (LISN) and loaded through its normal operation. A spectrum analyzer is used to measure the DUT emissions across different frequencies (through the LISN or from an antenna) and compares the emissions against the CISPR 25 limits. Both the peak and average values of the emissions are measured, and both must pass. Finally, the level of passing falls into several categories, or classes, that have different limits. OEMs define which class a specific subsystem must satisfy. For more information on the test procedure and limits, please see the official CISPR25 documentation.