Abstract: This paper presents a tunable bandpass filter (BPF) with the wide tuning range of center frequency and high selectivity. The wide frequency tuning range is achieved by a pair of simple varactors-tuned parallel coupled line resonators. Since the electromagnetic mixed coupling between resonators and frequency-variant source-load coupling are incorporated in this configuration, three self-adaptive transmission zeros (TZs) close to the tunable passband are obtained. Also, 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. Meanwhile, by selecting a proper coupling region, a constant fractional bandwidth (CFBW) in the frequency tuning process can be realized. For demonstration, a tunable 0.83-2.15 GHz BPF with a 9%±0.3% CFBW is designed, fabricated and measured. The experimental results show the proposed filter has the advantages of high selectivity and good out-of-band rejection.