Tunable balanced BPF with wide tuning range and high selectivity

Abstract

A highly-selective tunable balanced bandpass filter (BPF) with wide tuning range of center frequency is presented. The balanced BPF is designed by using compact varactor-tuned parallel coupled-line resonators with the direct-feed structure. It can realize a wide tuning range with an almost constant fractional bandwidth (CFBW). Three differential-mode (DM) transmission zeros (TZs) close to the tunable passband are obtained by mixed electromagnetic coupling and frequency-variant source-load (S-L) coupling. Meanwhile, the three TZs can almost keep the same relative location of passband to achieve continuous high selectivity and good out-of-band rejection over the whole frequency-tuning range. For verification, a tunable 1.02–3.25 GHz balanced BPF with three self-adaptive TZs is designed, fabricated and measured. And experimental and simulated results are in good agreement.