High-Isolation Multi-Function Duplexer Using Varactor Loaded SIR and Feed-Line

In this paper, a multi-function duplexer using the varactor-loaded stepped-impedance resonators (SIRs) is proposed. Such duplexer utilizes two sets of SIRs to construct the two channels. Meanwhile, loaded-varactors are tapped at the SIRs for adjusting the frequency response. Besides, the varactor is embedded in each pair of SIRs for further enhancing the passband performance. Moreover, the T-junction is introduced to achieve higher output isolation. Thus, a multi-function duplexer is fabricated and measured. The measured tuning range of the dual channel is 2.7-3 GHz and 3.5-3.8 GHz, respectively, with an output isolation greater than 48 dB.