The Input Impedance of Common-Mode and Differential-Mode Noise Separators

Abstract

This paper discusses the Δ-networks and other circuits designed to separate the conducted electromagnetic interference (EMI) into its common mode (CM) and differential mode (DM) components. The input impedances of CM/DM separators must be 50 Ω resistive in the measurement frequency range, and they must be independent of the values of the noise signals and noise source impedances. The conditions for achieving such input impedances are derived. It is shown that many of the proposed separators, including the Δ-network suggested in the International Special Committee on Radio Interference (CISPR) 16-1-2 standard, do not fulfill the input impedance requirement. This leads to unreliable CM and DM measurements and, consequently, to the oversizing of EMI filters and design by trial and error.Index Terms-Common mode (CM), delta network, differential mode (DM), electromagnetic compatibility (EMC), electromagnetic interference (EMI), International Electrotechnical Commission (IEC), measurement, noise separator, power filters.

Analysis and practical relevance of CM/DM EMI noise separator characteristics

“…In addition to limited separation capability of the separator itself, however, also interactions at the interfaces between the AMNs' output ports and the separator's input ports in Fig. 4 may deteriorate the accuracy of the measured output voltages [16]. This Paper evaluates the practicability of different CM/DM separators and, first, reveals common properties of CM/DM separators in Section 1, i.e.…”

“…Figure 2: Measurement set-up used to determine the conducted EMI noise levels V AMN,l = |V AMN,l | and V AMN,n = |V AMN,n | (on the basis of [4]). The Device Under Test (DUT), modeled according to [16], denotes the source of conducted EMI. 2.…”

“…Fig. 5, which introduces errors to V sep,l and V sep,n [16]. Further measurement errors arise from inaccurate processing of the measured voltages, commonly accounted for with gain errors [Differential Mode Transmission Ratio (DMTR) and / or Common Mode Transmission Ratio (CMTR) different to 0 dB] and cross coupling [Common Mode Rejection Ratio (CMRR) and / or Differential Mode Rejection Ratio (DMRR) greater than zero, i.e.…”

“…Characterization of the input ports of a CM/DM separator based on impedance matrix coefficients[16].…”

Analysis and Practical Relevance of CM/DM EMI Noise Separator Characteristics

“…1 In addition to limited separation capabilities of the separators themselves, however, also interactions at the interfaces between the LISNs' output ports and the separator's input ports in Fig. 2 may deteriorate the accuracy of the measured output voltages [6].…”