The performance of tunable microwave devices based on heteroepitaxial YBa_(2)Cu_(3)O_(x)/SrTiO_(3) films on (001) LaAlO_(3) substrates has been evaluated. It was ascertained that "out-of-plane" SrTiO_(3) lattice parameter is the relevant factor in determining both device agility and dielectric loss. After high temperature annealing (1100℃, 1 atm O_(2)), only SrTiO_(3) layers deposited under low oxygen pressure (～10~(-5) Torr) show an appreciable reduction of the dielectric losses while maintaining high agility. Annealed samples exhibit voltage independent losses of ～5×10~(-3) simultaneously with 55 dielectric agility at 6 GHz and 77 K.
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