High-Selectivity Wideband Balanced Filters Using Coupled Lines With Open/Shorted Stubs

Two novel high-selectivity wideband balanced filters using symmetrical/asymmetrical coupled lines with open/shorted stubs are proposed in this letter. Multiple transmission zeros are achieved to obtain sharp rejection and good out-of-band suppression for the differential mode, in which fixed transmission zeros can be realized by half-wavelength open stubs and adjustable transmission zeros can be realized by loaded open/shorted coupled lines. In addition, all the transmission zeros can be allocated to suppress the common mode from 0 GHz to 3 f0( f0is the center frequency of the passband). Two balanced filters with 3-dB fractional bandwidths of 22% and 21% are, respectively, designed and fabricated.