EMI Signature in electronics
When you fill out a form and sign the document at the end with a pen, for instance, the graphical characteristics of your &quot;drawing&quot; - the signature - identify you as the author.
In power electronics, but also in other domains, a signature is a specific set of characteristics (voltage, current, frequency etc.) which let you identify a behavior, a type of converter, a specific source etc. For instance, the drain-source ringing occurring in the primary-side of a flyback converter after the secondary diode has stopped conducting represents the typical signature of a converter operated in discontinuous conduction mode or DCM that you can observe with an oscilloscope:

What is of interest for the designer is the harmonic content of the converter operated in this mode: what is its electromagnetic interference (EMI) signature is what matters to dimension and size the front-end filter. What frequency dominates the response and how much shall it be attenuated are design criteria.
The equivalent circuit to determine the signature of a typical converter requires a line impedance stabilization network (a LISN) which sets the source output impedance to 50 ohms along a certain frequency range (10 kHz to 150 kHz). If you connect the switching converter to this LISN, you will obtain a frequency spectrum linking the current signature with its specific EMI signature. This is excerpted from an application note I wrote when releasing the NCP1200 in 2000:

This EMI signature, which is the EMI spectrum (conducted or radiated) generated by a specific converter in a particular operating conditions (input and output) will tell you what compensation strategy to adopt to pass the standard you target (CISPR 22 or EN55022 for instance). Below is the typical EMI signature of a 100-kHz flyback converter:

From this signature, you see a second harmonic at 200 kHz delivering 90 dBµV. If you want to pass the limit with some margin, you will have to size your filter so that it attenuates at least by 30 dB at 200 kHz.