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Tunable parallel-plate capacitors employing Bi1.5Zn1.0Nb1.5O7/Ba0.5Sr0.5TiO3 (BZN/BST) thin films for RF applications are reported. The intermediate frequency measurements indicate that the BZN/BST-based varactors demonstrate large tunability of 39% at 40 V and high device quality factor of 300 at 1 MHz. The devices maintain quite low leakage current density even under a high applied bias. The quality factor analysis shows that the device quality factor is highly dependent on conductor loss of electrodes at frequencies above 1 MHz. The phase shifter employing BZN/BST-based varactors exhibits lower insertion loss than does employing semiconductor diodes at a designed frequency of 445 MHz, demonstrating the potential of tunable capacitors employing BZN/BST thin films for RF applications.