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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.
Microwave Theory and Techniques, IEEE Transactions on (Volume:54 , Issue: 3 )
Date of Publication: March 2006