Miniaturized high-performance UWB bandpass filter consisting of only one microstrip three-mode resonator

A novel ultra-wideband bandpass filter is developed by using one microstrip three-mode resonator. The new resonator is configured by adding a short-circuited stub and an open stub to a half-wavelength resonator, and its first three modes are designed to cover a very wide passband. The resonator is fed by parallel-coupled quarter-wavelength lines. As a consequence, five reflection zeros are obtained in the passband covering approximately 3.4-10.15 GHz. As the filter consists of only one resonator, it is very small and low loss, with measured insertion loss better than 0.6 dB in the passband. The group delay is very flat over the passband, and is about 0.5 ns at the center frequency. Moreover, the filter has multiple transmission zeros near its passband, and exhibits thereby sharp skirt property. The minimum strip-width and gap-width in the filter is 0.2 mm and 0.1mm, respectively, and no complicated backside-aperture-fabrication process is required. So the filter can be fabricated easily at low cost. The measured frequency response of the filter agrees favorably with the predicted one, and the FCC´s indoor limit is satisfied well.