The influence of VHF-LISN component and ferrite position to the common mode current on emission measurement according to CISPR 22

Radiated emission measurement requires that good correlation is achieved between data from different test sites. The common mode impedance of the AC power line as seen from the EUT should have same the frequency response at each test site. The use of VHF LISN (line impedance stabilisation network) was proposed to realize the condition (K. Osabe et al, 14th Int. Zurich Symp. and Tech. Exhibition on EMC, 66K3, 2001). The proposed VHF LISN consists of two disk resistors, two feed-through capacitors and several ferrite cores. However, it is not clear how the component values, AC cord length between EUT and VHF LISN, and mains power supply impedance influence the common mode impedance of the AC power line. It is a design problem for the VHF LISN. Thus, we have analyzed how the imbalance of component value in the VHF LISN influences the conversion ratio from differential mode current to common mode current and how the position of ferrite cores and the length of AC cord influence the common mode impedance of the AC power line.