Balanced dual-band bandpass filter using stub-loaded stepped-impedance resonators

A balanced dual-band bandpass filter (BPF) based on stub-loaded stepped-impedance resonators (SIRs) is proposed in this paper. With the aid of the two open stubs loaded to the SIR symmetrically, the employed stub-loaded SIR possesses more design freedom for controlling differential-mode fundamental resonant frequency and second harmonic, as well as the ratio between them in the design of the proposed BPF. In addition, as the loaded stubs’ length increases, both fundamental resonant frequency and second harmonic can be shifted down, implying that the size reduction in this design is achieved. Finally, a two-order balanced dual-band BPF is designed and fabricated for demonstration, and the simulated and measured results are presented, showing good agreement. In addition, the centrally loaded resistor is added and optimized to achieve high common-mode suppression, especially in the two differential-mode passbands.