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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?

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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?

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  • \$\begingroup\$ 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. \$\endgroup\$ Jan 11, 2023 at 14:38
  • \$\begingroup\$ @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? \$\endgroup\$
    – WeTech
    Jan 11, 2023 at 14:54
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    \$\begingroup\$ No, not from magnitude data, you need phase. \$\endgroup\$ Jan 11, 2023 at 14:56
  • \$\begingroup\$ @TimWilliams, how should i apply the phase to calculations? \$\endgroup\$
    – WeTech
    Jan 11, 2023 at 15:39
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    \$\begingroup\$ 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). \$\endgroup\$ Jan 11, 2023 at 16:06

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Generally, at low frequencies (e.g. up to 1 MHz), differential-mode dominates (more specifically, bigger portion of the total noise is differential-mode noise). Likewise, it's generally the common-mode noise that dominates at high frequencies.

Another way that generally works is to check the bandwidths: DM noise has generally lower spectrum while CM noise has generally broader.

When it comes to calculation of the amounts, you need separators for each type i.e. one to suppress CM and another one to suppress DM. That'll give you proper CM and DM noise measurement. Conducted emissions measurement on its own is not enough for that purpose.

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  • \$\begingroup\$ 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? \$\endgroup\$
    – WeTech
    Jan 11, 2023 at 14:58
  • \$\begingroup\$ @WeTech No. You need the phase data as well. \$\endgroup\$ Jan 11, 2023 at 15:16
  • \$\begingroup\$ How can i calculate dm and cm with magnitude and phase diagrams? Do you mean converting the frequency domain to time domain then using those equations? \$\endgroup\$
    – WeTech
    Jan 11, 2023 at 15:42

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