Multi-mode stub loaded resonator for multi-band bandpass filters

In multi-band and multi-service communication systems, multi-band bandpass filter (BPF) and multiplexer are important components in the RF/microwave front-end. However, most filters exhibit the disadvantage of poor insertion loss where the value can reach a loss of 2dB due to an increased number of resonators or via holes, large circuit size, or the resonant frequencies and bandwidth cannot be flexibly controlled. New configurations of multimode stub loaded resonator, SLR are presented in this thesis, which is classified into 4 phases. In Phase 1, two dual-mode SLRs are proposed to achieve quadruple-mode SLR, then the proposed SLRs are applied to design dual-band BPF and four-pole BPF operating at 1.80/2.45 GHz and 2.00 GHz respectively. Accordingly, the proposed dual-band BPF and four-pole BPF feature compact size and low insertion loss due to reduced number of via-holes and resonators. The second resonator, elaborated in Phase 2 is a triple-mode SLR with load step-impedance resonator, SIR. The simple diplexer structure is formed by two third-order BPFs, and each channel can be adjusted individually. The diplexer has the properties of compact size with high selectivity. For a demonstration, a diplexer with two BPFs centered at 1.50 and 1.70 GHz is designed and fabricated. In addition, the two transmission zeros are created close to the passband edges which extremely improve the skirt selectivity. As a result, the proposed diplexer occupies an extremely small area approximately 0.30λg × 0.35λg. In Phase 3, the third resonator is a quad-mode SLR based on two dual-mode SLRs, leading to quad-mode stub loaded resonator with basic triple- and quad-mode resonant frequencies are proposed and analyzed theoretically. The quadchannel diplexer, composed of two second-order dual-band BPFs of different operating frequencies is presented. The second-order BPFs are designed for multiband wireless communication system to operate at 1.85 GHz (4G LTE), 2.45 GHz (WLAN), at 2.15 GHz UMTS) and 3.50 GHz (WiMAX), the overall size of the fabricated circuit is 38.25 × 26.25 mm, i.e., about 0.32λg × 0.22λg. Additionally, the quad-mode SLR is combined with a dualmode SLR to achieve a triple-band BPF. The two resonators consist of quad-mode SLR and dual-mode share a common via hole at the midpoint of the short-ended stubs accordingly the controlled bandwidth of each passband is controllable and decrease loss due to number of via-hole. The three passband frequencies are located at 1.75/2.45/3.50 GHz for DCS/ WLAN/ WiMAX. The overall size of the fabricated circuit is 0.18λg × 0.20λg. Consequently, three passbands can be independently controlled bandwidth at each passband without affecting the performance of other passband frequencies. In Phase 4, the last resonator, the sextuple-mode with improved performances based on the tri-mode SLR is presented to achieve the demand for the circuit features compactness and high freedom design with controllable frequencies and bandwidths. Only one set of the sextuple-mode resonator can be easily tuned to desired values of the three passband frequencies which generate the first, second and the fourth passband while the third passband is achieved by combination with the dual-mode open stub-loaded resonator, centered at 1.75/2.45/3.50/5.25 GHz for DCS/ WLAN/ WiMAX. The size of the fabricated filter is about 0.13λg × 0.22λg. All the fabricated circuits, the measured results agree well with the simulated results, verifying the proposed configuration and design procedure.