Quasi-Lumped Analysis of Wideband Bandpass Filter With High Out-of-Band Rejection Rate

In this paper, a wideband bandpass filter (BPF) is proposed with three transmission zeros (TZs) that are created using immittance inverters. Impedance- (K-) and admittance- (J-) inverters are integrated with the high-pass and low-pass sections, respectively, and both sections are integrated to obtain a bandpass response. Controllable TZs are created near the passband for improving the skirt factor (SF). For the theoretical investigation and validation, a lumped equivalent circuit model of the proposed BPF is extracted with the quasi-lumped element techniques. The simulation results of electromagnetic (EM)/distributed model, equivalent lumped circuit model, and measured results of fabricated prototype BPF are in good agreement. Fabricated EM/distributed BPF has a center frequency of 2.17 GHz, with a fractional bandwidth of 67.3% (1.4–2.9 GHz). The measured insertion loss and group delay at the center frequency are 0.57 dB and 5.91 ns, respectively. A high SF of 0.97 is achieved at both the cutoff frequencies with the compact physical circuit size 20.5 $\text {mm} \times 28$ mm and normalized circuit size $0.24\lambda _{g} \times 0.33\lambda _{g}$ .