Wide-stopband microstrip bandpass filters using dissimilar quarter-wavelength stepped-impedance resonators

Wide-stopband and compact microstrip bandpass filters (BPFs) are proposed using various dissimilar quarter-wavelength (λ/4) stepped-impedance resonators (SIRs) for multiple spurious suppression. The use of λ/4 SIRs is essential in widening the filter stopband and reducing the circuit size. By properly arranging the individual λ/4 SIR, which has the same fundamental resonance frequency f₀, but has different spurious (harmonic) resonance frequencies, and also carefully misaligning the maximum current density nodes, several higher order spurious resonances may be suppressed so that a BPF with wide stopband may be realized. In this study, the basic concept of multiple spurious suppression is demonstrated by thoroughly investigating the spurious characteristics of the fourth-order interdigital BPFs, which consist of two different types of λ/4 SIRs. To widen the rejection bandwidth, the fourth-order coupled-resonator BPFs based on three and four different types of λ/4 SIRs are implemented and carefully examined. Specifically, a very wide-stopband microstrip BPF composed of four dissimilar λ/4 SIRs is realized and its stopband is extended even up to 11.4f₀ with a rejection level better than 27.5 dB.