Frequency converters are used to extend the frequency range of a classical vector network analyzer up to one THz and above.The complete setup becomes more susceptible to the phase stability of the corresponding cabling influencing especially the phase of the transmission Sparameters.One major error source is the inevitable movement of the converters between calibration and measurement.The solution is a calibration technique called UOSM with an unknown,imperfect Through calibration standard.Either an adapter with similar mechanical dimensions of the DUT can be used,or in case of a passive DUT,the DUT itself.This eliminates this kind of error and allows repeatable and precise phase measurements up to the THz range.