A compact reconfigurable microstrip dual-band filter using varactor-tuned stub-loaded stepped-impedance resonators

A new varactor-tuned microstrip dual-band band-pass filter (BPF) based on the tri-mode stub-loaded stepped-impedance resonators (SL-SIRs) is investigated. By employing the dual coupling paths, the two passbands can then be fully controlled and designed independently. Meanwhile, a third-order bandpass response can be realized by properly incorporating the first three resonator modes of a tri-mode SL-SIR at adjacent frequencies into the passband. Thus, the third-order dual-band BPF can be designed with only two resonators, resulting in an extremely small size. In addition, for the electronic tuning, three varactors are loaded onto the open-ends of each resonator and, thus, the passband frequency can be changed by properly tuning the first three resonant modes of the SL-SIR. For the demonstration, a compact reconfigurable dual-band BPF with a size of 0.19 λ₀ by 0.04 λ₀ has been designed and fabricated. Good agreement is also achieved between simulation and measurement.