Wideband Slotline-to-Microstrip Transition for 210–375 GHz Based on Marchand Baluns

This article describes the design and cryogenic measurement of a novel slotline-to-microstrip transition based on Marchand baluns. The proposed transition is an attractive solution for numerous THz applications due to its remarkable broadband performance and compactness. For instance, such transition could be considered for wideband devices covering the frequency band 210–375 GHz. The suggested transition is designed on a thin silicon substrate and employs superconducting Nb as the electrode for the slotline and microstrip lines. In order to verify the performance of the designed transition, we fabricated a dedicated test structure consisting of two transitions connected back-to-back and integrated with E-probes at the waveguide interfaces. Due to the inherent bandwidth limitation of the E-probes, two different test structures for 210–295 GHz and 295–375 GHz were employed to characterize the proposed transition over the whole frequency band. The experimental verification performed at cryogenic temperatures showed results consistent with the simulation. Moreover, the cryogenic measurements indicated a remarkable 56% fractional bandwidth with an insertion loss as low as 0.3 dB for the fabricated slotline-to-microstrip transition.