A balanced-to-unbalanced with bandpass responses

This paper presents a balanced-to-unbalanced (balun) with bandpass responses. Two ±K-inverters are utilized to replace the quarter-wavelength transmission lines or three quarter-wavelength lines in the conventional balun. The ±Kinverters consist of two half wavelength resonators, which can function as two filtering network. To connect the ±K-inverters in parallel, a filtering balun is realized. Theoretical analysis is carried out and design equations are derived. For demonstration, a filtering balun operating at 1.9 Ghz is implemented. The band width and insertion loss are 6% and 1dB,respectively.