Improvement of out-of-band rejected and spurious response for BPF using fourth-order cross-coupling resonator structure

This paper proposes a novel 2.4-GHz fourth-order cross-coupled bandpass filter (BPF) with an electric coupling open-loop stepped impedance resonator (SIR) configuration. The performance characteristics of three BPF configurations are compared, namely a conventional electric open-loop BPF, a conventional second-order SIR electric open-loop BPF, and the proposed fourth-order cross-coupled SIR BPF. Moreover, the coupling coefficients and external quality factors of the proposed filter are systematically derived. A good agreement is observed between the numerical and experimental characterization results. Overall, it is shown that compared to the conventional electric coupling and SIR electric coupling open-loop resonator structures, the proposed fourth-order cross-coupled SIR BPF increases the spurious frequency response to more than 8 GHz and improves the out-of-band rejection capability by more than 20 dB.