Novel miniaturized bandpass filter with wide stopband using spiral resonator

A miniaturized bandpass filter with wide stopband is presented in this letter. It utilizes two spiral quarter-wavelength resonators for size reduction. By adding extra lines to the feeding structure, mismatching at harmonic frequencies can be realized and wide stopband can be obtained. To verify the proposed idea, an experimental filter is implemented. The measured results show that better than 25 dB rejection levels in the stopband up to 6.8f₀ are achieved. The circuit area of the filter is only 0.1λ_g×0.1λ_g, where λ_g is the guided wavelength at the operating frequency of 1 GHz.