Compact tunable dual/Tri-band bandpass filters with multiple zeros

Here, compact dual/tri-band bandpass filters (BPFs) with tunable passbands are proposed. The multiband BPFs are designed by combining a BPF with tunable passband and a BPF with fixed passband using a common external coupling line. The former consists of a dual-mode resonator loaded with varactors (DMVLR) and the latter is composed of a quad-mode resonator (QMR). The classical analysis method of odd/even mode is employed to analyse the characteristics of the resonators. The centre frequency (CF) of the tunable passband can be easily tuned by varying the voltage imposed on the varactors. A constant absolute bandwidth (CABW) is obtained over the whole tuning range of frequency by properly adjusting loading positions of the varactors. The single and dual-passband BPFs with fixed passband are obtained by designing the parameters of QMR appropriately. Simulation and measurement results show that the proposed BPFs are of high selectivity, and the resultant filters are of compact size. In addition, the proposed multiband BPFs have four transmission zeroes (TZs). There are two self-adaptive TZs on both sides of the tunable passbands, which are generated by the multipath coupling structure.