How Conducted EMI Is Measured

Note that from the viewpoint of the noise generators in the power supply, it is the LISN that forms their load. 

Let us assume that the values of L and C used in the LISN are chosen so that the following statements hold true, unequivocally  

As we flick the switch on the front panel of the LISN, we will measure the following noise voltages  

Both the Vl scan and the Vn scan obviously need to comply individually with the limits. 

But how different can the Vl and Vn scans be In fact, the above two equations have inspired a rather misleading statement often found in related literature — if the noise emission is predominantly DM, the Vl and Vn scans will look almost the same. The scans also look identical if the noise is predominantly CM. And if the Vl and Vn scans look very different, that implies that both CM and DM emissions are present. However, in the case of an off-line power supply, this statement is clearly not true. Because, that would imply that somehow the emissions on the L and N lines are different. However, we know that in any typical off-line power supply (with an input bridge rectifier), the L and N lines are 

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