Tri-band bandpass filter using two short stubs and an open stub loaded resonator

Novel Tri-Band Bandpass Filter With High Selectivity

“…Many design approaches have been published. They are divided into several categories: (1) tri-section stepped-impedance resonators (TSSIRs) [1][2][3], whose the centre frequencies of triple passbands depend on the impedance ratios when electric length (θ i ) of each section is equal, (2) stub loaded resonators (SLR), including single or multistub loaded resonators [4][5][6][7] and square ring loaded resonator (SRLR) [8], because stubs are used to control the passband and open stubs independently to produce transmission zeros, (3) transmission zeros to construct triple passbands or split a single passband to tri-band [9][10][11], (4) three single-band sub-filters with matching junctions [12], (5) combination of dual-mode resonator and half-wavelength resonator [13,14], (6) multilayer substrate technique, such as LTCC [15], (7) special structures, such as Hilbert-fork resonators [16]. However, it is difficult for many structures to tune centre frequencies, and the isolation between adjacent passbands needs to be improved.…”