A novel tunable single-to-balanced diplexer are proposed in this letter. The single-to-balanced diplexer consists of pairs of stepped-impedance resonators (SIRs), which are connected by back-to-back varactors in between. The diplexer can cover from 1.5 to 2.4 GHz, with constant absolute bandwidth. Moreover, the single-to-balanced diplexer exhibits differential outputs in each tuning state. Simulations and measurements are in excellent agreement.To the authors' best knowledge, it is the first tunable single-to-balanced diplexer based on printed circuit board (PCB) technology that has been reported so far for multi-standard communication systems.