Discussion of conversion theory between mixed-mode S-parameters and T-parameters for differential circuit with and without common-mode suppression

Recently, mixed-mode S-parameters are becoming more and more important due to the widely used of differential circuits in the RF and microwave area. In the single-end network, designing process based on the S-parameters is one of the most common and convenient approaches, and the Chain Scattering parameters allows the ease of design for the cascade topology. A parameter conversion theory was reported for the transformation between mixed-mode scattering parameters (S-matrix) and chain scattering parameters (T-matrix) for the differential two-port networks. In this paper, measurement results of a practical differential UWB bandpass filter with Common-mode suppression is applied to the said conversion theory and it is found that the reported method is not applicable to the differential circuit with well common-mode rejection.