This paper presents a compact tri-band bandpass filter (BPF) with high selectivity. The proposed filter utilizes novel stub-loaded quarter-wavelength resonators and conventional uniform quarter-wavelength resonators. The latter is embedded in the former, and they are separated by a feed line. Due to these quarter-wavelength resonators, the total size is greatly reduced. Moreover, the passband frequencies can be controlled individually. To enhance its selectivity, source-load coupling is employed. For demonstration, an experimental filter is implemented. High skirt selectivity and suppression levels are observed in the measured results. The circuit area of the filter is 0.17λg×0.19λg, featuring compact size.
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