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In this paper, a novel integrated bandpass filter balun based on the double-sided parallel-strip line (DSPSL) with an inserted conductor plane is proposed. The conductor plane, inserted into the middle of the substrate, can function as virtual ground plane to convert the top and bottom identical resonators into two back-to-back microstrip resonators. The out-of-phase feature between the two output ports using the microstrip lines is frequency-independent because the proposed balun is fed by the DSPSL at input port. No transition or matching network between the DSPSL and back-to-back microstrip lines is required. The proposed balun has been demonstrated and verified by the simulated and measured results with good agreement.