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This paper describes a MEMS tunable bandpass filter operating in the S band region and featuring a low insertion loss of -2.3 dB and small size of 3.7 mm by 4.1 mm. The tunable filter is based on MEMS-varactors-loaded microstrip resonators and has been directly constructed on a high-k low-temperature-co-fired-ceramics (LTCC) wiring wafer whose dielectric constant is 50, using MEMS-on-LTCC technology. The RF signal paths are isolated from the MEMS driving paths by built-in high-resistivity-vias, leading to low insertion loss. RF power-proof performance was evaluated by applying OFDM signals to the device. The results confirm that MEMS tunable filters are sufficiently linear at cell phone power level.