Analysis and Design of a Differential Sampled-Line Six-Port Reflectometer

The analysis and design of a differential six-port reflectometer (SPR) based on a sampled-line structure is presented in this paper. The practical differential SPR is realized in Coplanar Strip (CPS) transmission line form, using two baluns for interfacing with the RF source and device under test (DUT) and four baluns connected to the RF logarithmic detectors. The performance of the proposed differential SPR structure is evaluated with a common-mode rejection ratio analysis and impedance measurement. A prototype differential SPR was designed and fabricated at 1 GHz in order to verify the analysis. Measurement results for various load impedances are compared with the measured results obtained from a commercial automatic vector network analyzer. It is shown that the differential structure gives a significant advantage in rejecting common-mode interference signals.