An On-Line Measurement Approach for EMI Filter Characterization

A measurement system to analyze and characterize impedance and transfer function of EMI-filters is presented. The system allows to examine filters at mains voltage and under load, thus ensuring realistic conditions and measurement results. Despite up to date standards the specification of filters under real operational conditions is necessary as these standards use unrealistic testing environments under exclusion of the mains voltage and possible influence of the mains impedance. While injecting a signal directly on line at given frequency points, a modified Discrete Fourier Transformation is used to convert the measured values into the frequency domain. Instead of averaging over the integration time a raised cosine window is used to minimize interharmonic signal disturbances. Additionally a zero crossing detection algorithm is used to align the phasors with the mains cycles, allowing subcycle measurements in multiple time slots per mains cycle. Working in the frequency domain allows compensation of the measurement equipment and the circuitry components which provides minimal measurement errors and repeatability.