## 1 Introduction  
Due to the widespread integration of renewable energy facilities, electromagnetic interference (EMI) issues are significantly growing in both AC and DC power systems [1, 2]. Specifically, electromagnetic disturbances from power electronics converters that interface renewable energy sources to the power grid propagate along lines and metallic earth-return structures [3, 4]. Passive EMI filters are commonly used to suppress common mode (CM) and differential mode (DM) emissions exiting power converters to mitigate the noise propagating along distribution lines [5]. Filter performance is usually assessed in terms of insertion loss (IL), defined in CISPR 17 as the ratio of the voltages across a specific reference load in the absence and presence of the filter [6]. For IL measurement, a reference test setup is used, which is characterized by source and load impedances defined by the standard (50 \(\Omega\)/50 \(\Omega\), 100 \(\Omega\)/0.1 \(\Omega\), and 0.1 \(\Omega\)/100 \(\Omega\)) [7]. However, in real-world applications, the actual IL of the filter (hereinafter called \"in-circuit IL\") depends on the actual frequency-depending impedances of the devices connected with the filter, which scarcely replicate the reference impedances in the CISPR 17 test setup. Therefore, assessing the in-circuit IL based on the actual load/source impedance is of great importance to selecting or designing proper filters for a specific system. Several contributions to the assessment of IL focused on the worst-case scenario (minimum IL). For instance, the theoretical minimum IL of the filter derived from its network parameters under extreme reference impedances revealed a substantially lower level than the so-called 'approximate worst case' in the CISPR 17 setup, which considers only two scenarios of mismatched load/source impedances (100 \(\Omega\)/0.1 \(\Omega\) and 0.1 \(\Omega\)/100 \(\Omega\)) [8]. Taking into account the impedance of the EMI source, a more realistic IL was predicted, which, at some frequencies, was smaller than the approximate worst-case estimated by CISPR 17 [9]. Some works are devoted to thorough characterization of filters intended for IL prediction. For instance, in [10,11], the standard or in-circuit IL was predicted using the EMI filter scattering parameters (S-parameter) obtained by modeling or experiments. In [12], a black-box model of EMI filters compatible with circuit solvers was proposed using the vector fitting technique, which can predict the IL in both time and frequency-domain simulation [12,13]. In [14], a single setup was presented to evaluate all modes of insertion loss and extended to an approximate worst-case measurement. Besides predicting the IL, some contributions have focused on designing EMI filters based on the actual source/load impedances in the targeted applications [15]. For instance, in [16], the EMI filter design took into account the terminal impedances of an inverter-driven motor system that were measured offline by an LCR meter. However, recent research has shown that the impedance of power electronics converters measured offline may not coincide with the actual impedance when the system is powered on [17,18]. Hence, the online impedance measurement is more appropriate for optimal filter design. Note that, in this context, the online measurement is to be intended: a) a measure performed without physical modification of the circuit under test, just clamping a suitable probe on one or more cables, and b) a measure that can be performed while the circuit under test is working. Examples of filter design exploiting online impedance measurement were presented in [19] and [20] for inverter-driven motor systems and switched-mode power supplies, respectively. The available methods for online impedance measurement can be classified into voltage-current strategy and capacitive/inductive coupling approaches [21].