A New Approach to Design Microstrip Wideband Balun Bandpass Filter

A new design method of microstrip wideband balun bandpass filter (BPF) is presented in this letter. By utilizing the standing-wave property of voltage distribution along an open-circuited half-wavelength
$(\lambda/2)$
microstrip transmission line, the out-of-phase signals can be obtained at two output ports of a balun. Meanwhile, proper coupling topologies between the open-circuited
$\lambda/2$
microstrip transmission line and the multi-mode resonators are selected to achieve a balun BPF with both characteristics of balance performance and wideband filtering properties. To validate our proposal, a balun BPF operating at 1 GHz with a 3 dB FBW of 30% is designed, fabricated, and measured. Both the simulation and measurement results of the designed balun BPF are provided with a good agreement. Results indicate that this balun BPF exhibits not only a high selectivity filtering performance with two common transmission zeros (Tzs) of
${\mathrm S}_{21}$
and
${\mathrm S}_{31}$
outside the passbands, but also a good balance performance with 0.5 dB amplitude imbalance and 5
$^{\circ}$
phase imbalance.