On Measuring the Response to Differential Mode Noise of Common Mode Chokes

This work analyzes different techniques to measure the attenuation of differential mode noise provided by common mode chokes. The proposed setups are alternatives to the direct and symmetrical setups described in CISPR-17, which are also investigated in this work. This study is based on a modal analysis of a high-frequency circuit model of the common mode choke that allows for obtaining analytical expressions for the insertion loss of the common mode choke when excited in different setups in terms of the admittances of the modes excited in each setup. This provides additional insight to understand which modes are excited in each setup. We demonstrate that the setups are equivalent at low frequencies and we identify the key differences between them at high frequencies, in particular regarding their different frequencies of resonance. To validate our analysis, we have measured and characterized different commercial common mode chokes, and we have verified that in all the cases the measured transmission coefficients exhibit the behaviour predicted by the theoretical analysis.