Three-Pole 1.3–2.4-GHz Diplexer and 1.1–2.45-GHz Dual-Band Filter With Common Resonator Topology and Flexible Tuning Capabilities

In this paper, a third-order tunable diplexer and a third-order tunable dual-band bandpass filter with flexible tuning capabilities and high selectivity are proposed. Both the diplexer and dual-band filter consist of two passbands that share a common resonator for size reduction. By adding tuning elements in the middle and ends of the common resonator, the resonant frequencies of the even and odd modes can be independently tuned, thus offering flexible tuning for multi-band operations. Also, by adding extra resonators next to the common resonator, high-order tunable multi-band filters can be obtained with higher stopband suppression. The 1.3-2.4-GHz tunable diplexer shows that the two channels can be either independently tuned or simultaneously tuned as desired. The two channels cover a fractional tuning range of 34% and 18.6%, respectively, while keeping high stopband suppressions, higher isolation, and good input impedance matching. Similarly, the 1.1-2.45-GHz tunable dual-band filter shows independent and simultaneous tuning capability operating with good impedance matching and high stopband suppression.