Identifying Sources of Conducted Emissions by Measuring the Coherence Function

Conducted emissions (CE) is one of the electromagnetic interference (EMI) issues that pose serious compliance problems for electronic devices. For a system with several sources, estimating the contribution of each source to conducted emission at different frequencies can be a challenge. In this article, a coherence function-based signal separation method is presented and validated on two commercial power supply units (PSU). The noise generated by the PSU and measured at the line impedance stabilization network (LISN) port contains two predominantly uncorrelated signals associated with the power factor correction (PFC) and H -bridge/rectifier circuits with unknown contributions at different frequencies. Two reference signals are obtained by probing the emission signals close to the sources. By calculating the coherence between the LISN noise signal and the reference signals, the contributions of these two signals to the noise are obtained. The measurement of the signal contributions can help engineers identify the dominant sources and mitigate the emissions more efficiently over a wide range of frequencies.