A Compact Dual-Plane Bandpass Filter Based on Grounded Spiral-Shaped Resonators and Quarter-Wavelength Stepped-Impedance Resonators

A novel compact wideband bandpass filter with grounded spiral-shaped resonators and quarter-wavelength stepped-impedance resonators is proposed. In this study, the dual-metal-plane structure is adopted to achieve the required strong coupling, so that a wide bandwidth filter may be realized. Then, by designing the spiral-shaped inductor to form a miniaturized resonator, a very compact filter may be implemented, having a size of 0.169 lambda_g, times 0.127 lambda_g, where lambda_g is the microstrip guided wavelength on the substrate at center frequency. Specifically, the implemented filter has the merits of wide 3-dB fractional bandwidth of 35%, wide stopband up to 3.5 f₀, better selectivity, and good insertion/ return losses, where f₀ is the passband center frequency of the filter. Agreement between the measured and simulated results is also observed.