Compact Stopband-Extended Microstrip Bandpass Filters with Folded Quarter-Wavelength Resonators

Compact stopband-extended microstrip bandpass filters based on folded quarter-wavelength resonators are proposed. By introducing the source-load cross coupling, two transmission zeros may be created to improve the filter selectivity without degrading the passband insertion loss. In order to extend the rejection bandwidth, suitable open stubs or meander stepped-impedance resonators are adopted in the filter design. In this study, two compact microstrip filters are implemented, the first one uses open stubs to suppress the spurious response around 3f₀ and the second one employs the meander quarter-wavelength stepped-impedance resonators to push the first spurious response up to 3.6 f₀, where f₀ is the passband center frequency. Good agreement between simulated and measured responses of these filters is demonstrated