Unbalanced-to-Balanced and Balanced-to-Unbalanced Diplexer With High Selectivity and Common-Mode Suppression

Two novel balun diplexers based on stub-loaded resonators are presented and carefully examined in this paper. Besides being diplexers, the two balun diplexers are also interface devices between a single-ended (differential) antenna and differential (single-ended) receiver and transmitter. The stub-loaded resonator not only has an extra degree of freedom to realize the anticipant coupling scheme, but it also can compact the circuit size and enlarge the frequency separation of the first differential- and common-mode resonant frequencies. The design procedure of the balun filter with three 50- Ω ports is introduced based on a symmetric four-port balanced-to-balanced bandpass filter. Two demonstrated balun diplexers, unbalanced-to-balanced and balanced-to-unbalanced diplexers, with Rx and Tx frequencies at 1.84 and 2.45 GHz are designed, fabricated, and measured. The implemented balun diplexers present an excellent in-band balanced performance with common-mode rejection ratio better than 38 dB over the passbands. High selectivity and isolation are also obtained by giving a cross-coupling scheme for both Rx and Tx balun filters.