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 (lambda/4) stepped-impedance resonators (SIRs) for multiple spurious suppression. The use of lambda/4 SIRs is essential in widening the filter stopband and reducing the circuit size. By properly arranging the individual lambda/4 SIR, which has the same fundamental resonance frequency f(o), 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 lambda/4 SIJRs. To widen the rejection bandwidth, the fourth-order coupled-resonator BPFs based on three and four different types of lambda/4 SIRs are implemented and carefully examined. Specifically, a very wide-stopband microstrip BPF composed of four dissimilar lambda/4 SIRs is realized and its stopband is extended even up to 11.4f(o) with a rejection level better than 27.5 dB.