Tunable balanced bandpass filter with wide frequency‐tuning range and high selectivity

Abstract

A tunable balanced bandpass filter (BPF) with a wide frequency-tuning range (FTR) and high selectivity is presented. The designed BPF is implemented by utilizing varactor-tuned square ring resonators (SRRs) with parallel-coupled feedlines. A wide FTR with an almost constant absolute bandwidth (CABW) can be realized by varactors C v2 connected between SRRs. Four transmission zeros (TZs) can be obtained by the mixed electric and magnetic coupling, source-load (S-L) frequency-variant coupling, and four shunt quarterwavelength open and short stubs. Meanwhile, two TZs on both sides of the passband keep the relative position unchanged to achieve high selectivity. The other two are generated to improve the out-band rejection. In addition, by employing additional stubs and capacitances on the symmetry plane, the common-mode (CM) suppression can be enhanced. For verification, a tunable balanced BPF with 0.62-1.27 GHz and four TZs was designed, manufactured, and tested. And experimental results agree well with the simulated ones. K E Y W O R D Sbalanced bandpass filter (BPF), constant absolute bandwidth (CABW), high selectivity, tunable, wide frequency-tuning range (FTR)