This paper describes an on-wafer characterization system and calibration technique for frequency up and down converters based on a superconductor–insulator–superconductor (SIS) tunnel junction. The measurement system uses a 4-K probe station in combination with a vector network analyzer which allows measurement of both up- and down-conversion gains and reflection coefficients. We employed and verified a scalar mixer calibration technique for accurate characterization of the conversion properties. We present the detailed measurement technique and verification using an SIS frequency converter sample.