Improved Multimode TRL Calibration Method for Characterization of Homogeneous Differential Discontinuities

The multimode thru-reflect-line (TRL) calibration technique is widely applied to characterizing differential discontinuities. However, its accuracy is highly dependent on the inhomogeneity degree of differential transmission lines. In particular, in the case that the calibration structures are immersed in a homogeneous medium where different modes travel with the same propagation constant, the conventional multimode TRL is no longer applicable. To overcome this limitation, a new symmetrical reflect standard with distinct pure-mode reflection coefficients is proposed, and the conventional multimode TRL method is improved based on the assumptions applied to the reflect standard. A group of strip-line calibration kits are tested, and the proposed method is implemented to calculate the de-embedded S-parameters of a symmetrical device-under-test. The experimental results show that, from 20 MHz to 16 GHz, the calibrated pure-mode parameters agree well with the simulation, and the cross-mode parameters are mostly lower than -30 dB. Moreover, using the proposed method, the average measurement error achieves at least 14.8-dB reduction relative to that before calibration, and a maximum improvement of 21.2 dB compared with that of the conventional multimode TRL.