Compact LTCC Balun With Bandpass Response Based on Marchand Balun

In this letter, we present a compact low temperature co-fired ceramic (LTCC) balun with bandpass responses. It is designed based on a Marchand balun. The halfand quarterwavelength transmission lines in Marchand balun are utilized as resonators in this design. Three feeding lines, functioning as input and outputs, are coupled to the resonators. The 180° phase difference between two output ports is achieved due to the inherent out-of-phase characteristic at the two open ends of the half-wavelength resonator. The bandpass responses are enabled by only adding three coupling lines and thus compact size is obtained. The coupling matrix is utilized in the design for realizing the good filtering responses and the design guideline is presented. For demonstration, a LTCC balun filter centered at 5.5 GHz is implemented. Good agreement between the simulated and measured results validates the proposed idea. The core circuit size is only 2 mm × 1.7 mm × 1.6 mm or 0.089λg × 0.076λg × 0.071λg.