Stopband-Extended Balanced Filters Using Both λ/4 and λ/2 Sirs with Common-Mode Suppression and Improved Passband Selectivity

TLDR: In this article, a via-free balanced bandpass fllter (BPF) with direct-coupled scheme is presented, where the TZs can be made structure-inherent based on the coexisted out-of-phase couplings among a coupled-resonator pair.

Abstract: Benefltting from the simultaneous utilization of quarter- wave (‚/4) and half-wave (‚/2) microstrip resonators, a via-free balanced bandpass fllter (BPF) with direct-coupled scheme is presented in this study. In the beginning, a single-ended fllter with transmission zeros (TZs) is newly proposed and the mechanism of creating two TZs around the passband without necessitating cross couplings is adopted. The TZs can be made structure-inherent based on the coexisted out-of-phase couplings among a coupled-resonator pair. On the foundation of the presented single-ended fllter, a balanced fllter featuring extended difierential-mode (DM) stopband, good common- mode (CM) suppression, and improved passband selectivity has been designed and implemented. The DM stopband extension is achieved by misaligning the higher-order harmonic frequencies of each resonator in the DM bisected circuit while the CM suppression is accomplished by both harmonic misalignment and careful designed coupled structure in the CM bisected circuit. Eventually, a demonstrated balanced fllter centering at 1.5GHz possesses DM stopband extended up to 8f d 0 , where f d 0 denotes the DM operation frequency, and its CM rejection ratio (CMRR) within DM passband better than 51.9dB is attained. For measurement convenience, the DM characterizations have been accomplished by 2-port network analyzer with simple rat-race baluns and are found relatively accurate within the i15dB bandwidth of the utilized baluns.