High selectivity wideband balanced filter based on modified coupled lines structures

A novel wideband balanced filter based on modified coupled lines structures is proposed. The presented modified coupled lines structures are implemented by loading open/ shorted stubs at the end of the traditional open coupled lines to perform bandpass and bandstop response. To obtain the high selectivity and good out-of-band suppression for the differential mode, four full-wavelength open stubs are introduced to achieve multiple transmission zeros and further improve the suppression level over a wide frequency band due to the all-stop transmission characteristic for the common mode. A prototype with 3-dB fractional bandwidth of 38% for the differential mode is designed and fabricated for demonstration.