  We have designed, fabricated and tested a frequency-tunable high-Q
superconducting resonator made from a niobium titanium nitride film. The
frequency tunability is achieved by injecting a DC current through a
current-directing circuit into the nonlinear inductor whose kinetic inductance
is current-dependent. We have demonstrated continuous tuning of the resonance
frequency in a 180 MHz frequency range around 4.5 GHz while maintaining the
high internal quality factor $Q_i> 180,000$. This device may serve as a tunable
filter and find applications in superconducting quantum computing and
measurement. It also provides a useful tool to study the nonlinear response of
a superconductor. In addition, it may be developed into techniques for
measurement of the complex impedance of a superconductor at its transition
temperature and for readout of transition-edge sensors.
