In this paper, a novel tunable single-to-balanced bandpass filter is proposed. It consists of three stepped-impedance resonators (SIRs), which are loaded and connected by varactors in between. With the very simple structure, the proposed reconfigurable filter can not only achieve center frequency control, but independent bandwidth tuning in the whole tuning state. The filter can cover from 1-1.3GHz, with 1-dB bandwidth tuning from 40-110MHz. The in-band amplitude and phase imbalances between each differential outputs crossing the entire tuning range are less than 0.3dB and 1 degree, respectively. Good agreement is obtained between the simulated and experimental results.