Design of a Compact Microstrip Quint-Band Filter Based on the Tri-Mode Stub-Loaded Stepped-Impedance Resonators

A simple and efficient method has been presented to design multi-band microstrip bandpass filters (BPFs) without a significant increase in circuit size. By utilizing multiple coupling paths, all passbands can then be fully controlled and, thus, the design is flexible so as to independently realize different bands. In addition, by adjusting the structural parameters of the proposed trimode stub-load stepped-impedance resonator, its first three resonant frequencies can be controlled. Thus, it can be utilized to implement a third-order BPF with a transmission zero at the upper side of the passband when these resonant frequencies are suitably assigned. For our demonstration, a compact quint-band BPF formed by only five tri-mode resonators has been designed and fabricated. It is shown that the quint-band filter occupies an extremely small area, i.e., 0.34 λ₀ by 0.03 λ₀ (0.52 λ_gby 0.045 λ_g). Good agreement is also achieved between simulation and measurement.