Miniaturized dual-band bandpass filter using λ/2 spiral-resonator and loaded open-stub

In this paper, a miniaturized dual-band bandpass filter using the half-wavelength (λ/2) spiral-resonators and loaded open-stubs with a spiral-coupled scheme is proposed. The mechanism of the dual-band operation is shown as follows. The first passband resonance is employed by the fundamental resonant-mode (i.e., f1) of the λ/2 spiral-resonator. Meanwhile, the loading effect introduced by the loaded open-stub could shift the first-spurious (i.e., f2) of the λ/2 spiral-resonator downward, which leads to a finely adjusted second passband resonance. Then, based on the λ/2 spiral-resonators with loaded open-stubs mentioned above, the spiral-coupled scheme is implemented, which employs a strong enough passband enhancement around the dual-resonances to achieve the dual-band bandpass filter. Besides, the loaded-tapping scheme could produce five transmission zeros, which can not only enhance the dual-band passband selectivity, but also extend the upper stopband bandwidth with a good rejection level. To verify the mechanism above, a dual-band bandpass filter operated at 1.035 and 1.56 GHz is designed, implemented, and fabricated. A good agreement between the simulation and measurement is achieved.