Extended-stopband bandpass filter using both half- and quarter-wavelength resonators

A novel quasielliptic microstrip bandpass filter (BPF) using both half- and quarter-wavelength resonators is proposed. With the quarter-wavelength (lambda/4) resonators placed in the interstage, the filter spurious passband can be pushed up to 3 f0 where f0 stands for the passband center frequency. To improve the stopband characteristics, a modified stopband-extended filter is implemented, utilizing the multiple transmission zeros placed at specified frequencies to achieve good frequency selectivity and out-of-band rejection. The modified filter provides a 22.5-dB rejection level from 1.14 f0 to 5.2 f0

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