# How can I separate differential mode and common mode noise of measured noise spectra on the line and neutral?

The following spectra are the measured EMI conducted noise by a switchable LISN. As this LISN has only one BNC output, the noise separator can not be used. The following spectra are of the noise on the line and neutral which are a combination of differential mode and common mode noise. How can I separate them and calculate differential mode and common mode spectra?

i don't have access to practical tests anymore, there are two equations provided for this purpose. VDM=(VL-VN)/2 and VCM=(VL+VN)/2 is it possible to use these equations on presented spectra by numerical methods? And how?

• You could observe the effect on particular bands/peaks by adding more CM or DM filtering near the LISN, perhaps; crude, not really a quantitative method. If this is not permissible, probably not much can be done. Jan 11, 2023 at 14:38
• @TimWilliams i don't have access to practical tests anymore, there are two equations provided for this purpose. VDM=(VL-VN)/2 and VCM=(VL+VN)/2 is it possible to use these equations on presented spectra by numerical methods? Jan 11, 2023 at 14:54
• No, not from magnitude data, you need phase. Jan 11, 2023 at 14:56
• @TimWilliams, how should i apply the phase to calculations? Jan 11, 2023 at 15:39
• As you quoted, just add or subtract the values. But that's the thing, it has to be done in the complex domain (phase+magnitude or I+Q). Or the raw acquisition waveforms (time domain). Jan 11, 2023 at 16:06