High selectivity quarter‐wavelength resonator bandpass filter utilizing source‐load coupling

This paper presents a compact, narrowband, high‐selectivity, two‐pole mixed‐coupled quarter‐wavelength (λ/4) resonator filter utilizing source‐load (S/L) coupling. The mixed coupling between resonators is modeled by the separated J and K inverters in the form of parallel‐coupled transmission lines and coupled metallic vias, respectively. Capacitive dominant mixed coupling along with the coupling path phases are used to create a lower stopband transmission zero (TZ). In addition, the upper stopband selectivity is enhanced by employing parallel S/L coupling in the form of J inverter which produces three TZs above the passband. Source‐load coupling is modeled by parallel‐coupled tapped feed lines, which also provide the necessary electrical length ratios (L r), for the positioning of TZs. For verification, a prototype filter is designed, fabricated and measured at fo = 2.75 GHz (center frequency). The measured filter response gives fo = 2.755 GHz, fractional bandwidth equal to 3.6% and an insertion loss of 2.1 dB at fo. Additionally, the four stopband TZs are produced at 2.47, 3.19, 4.18, and 5.98 GHz. The measured group delay in passband is less than or equal to 3.6 ns. Overall a good agreement between measured and simulated results is observed, which validates the proposed synthesis technique.