This paper presents a new type of six-port instrument to meet the specific needs of electrical characterization tools for impedances clearly different than the standard 50 Omega reference impedance. A method is developed for the precise adjustment of the reference impedance of the measurement system to any complex impedance value. A proof-of-concept system based on the association of a four-port network analyzer associated to an original three-paths additive mixing architecture is developed in the frequency range 1-10 GHz to validate the approach proposed.