Large-area BaₓSr₁₋ₓ TiO₃ (BSTO-x) thin films, doped with Fe, Mg, and Y, have been grown by pulsed laser deposition (PLD) on polycrystalline alumina based ceramics. The capacity and Q-factor of planar Pt / BTO:Fe / Pt capacitors were investigated within a temperature range from -35 to +85° C for applied DC-bias voltages up to 10V at 1kHz. A homogeneous tunability of the capacity of about 60 % was achieved for applied electrical field strengths between 0 and 5V/ mm. Furthermore, by Fe-doping of BTO films and post-deposition annealing of the Pt / BTO / Pt structure the Q-factors could be increased by a factor of four up to 200 compared to the undoped films. However, at a microwave frequency of 30 GHz, higher values of the Q-factor of more than 300 at room temperature were measured for bare BSTO-0.45 films. These results show the considerable influence of the interface, doping material and post-deposition treatment on the dielectric thin film properties. The physical reasons for this behavior are not completely understood up to now. Supported by the BMBF under FKZ 13N8158.