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A novel wideband balun filter based on a symmetric four-port differential circuit is proposed in this paper. A pair of open coupled lines is used to realize DC suppression and in-band balance improvement for the balun filter. The bandwidth can be easily adjusted by changing the characteristic impedance of transmission lines in the differential circuit. For the proposed balun bandpass filter, excellent in-band balance performance (amplitude and phase imbalance are less than 0.25 dB and 1.3° respectively) over the passband are achieved. A wideband balun bandpass filter prototype with center frequency 3.75 GHz and 3-dB bandwidth 33.8% is designed and fabricated. Good agreement can be observed between the measured results and theoretical expectations.