Novel 1.5-1.9 GHz Tunable Single-to-Balanced Bandpass Filter With Constant Bandwidth

A novel tunable single-to-balanced bandpass filter is proposed in this letter. It consists of pairs of stepped-impedance resonators (SIRs), which are connected by back-to-back varactors in between. Based on such a simple structure, the filter can cover from 1.5 GHz to 1.9 GHz with constant absolute bandwidth, and exhibit differential outputs in each tuning state. The amplitude and phase imbalances between each differential outputs are less than 0.7 dB and 3 degrees, respectively, in the passband of each tuning state.