Compact Filtering Balun-Diplexer Using Coupled-Line-Loaded Hairpin Ring Resonator

A filtering balun-diplexer (FBD) with compact size and high channel isolation is proposed based on the dual-mode coupled-line-loaded hairpin ring resonator (C-HRR). The special features of C-HRR are presented and concluded. The out-of-phase current distribution of the C-HRR at the odd-mode resonant frequency is noticed and utilized in the balun design. By carefully arranging the resonant frequencies and coupling topology, the two channels of the FBD can share one common C-HRR to reduce the circuit size. The lower and higher channel filtering baluns are designed separately and integrated directly into the FBD without extra impedance matching network because of the low loading effect. For validation, an FBD is fabricated with circuit size of 0.24
${\lambda }_{\mathrm{ g}}\,\, {\times }\,\,0.14{\lambda }_{\mathrm{ g}}$
, where
${\lambda }_{\mathrm{ g}}$
is the guide wavelength at the center frequency of the lower passband. The isolation between the lower and higher channels is greater than 39 dB with the common mode rejection better than 34 dB over the passbands.