Compact Balun Filter Using Coupled Resonators

In this paper, we present a compact balun with bandpass responses, which is designed based on a Marchand balun. One half-wavelength and two quarter-wavelength transmission lines in Marchand balun are utilized as resonators in this design. Three ports are coupled to the resonators. The lines are folded to miniaturize the circuit size. By controlling the coupling region, the phase difference of the two output ports is nearly 180°. For demonstration, a balun filter is designed, fabricated and measured. Good agreement between the simulated and measured results validate the proposed idea.