  This report presents a robust composite resonator known as the high overtone
bulk acoustic wave resonator (HBAR) for demonstrating experimentally the
ability of the resonator to tune the envelope of the spectrum by applying dc
bias. The reported HBAR exhibits exceptionally high-quality factor in the
frequency range of 700 MHz to 3 GHz and high effective coupling coefficient
over the broad range of microwave frequencies. The HBAR is based on thin
Ba0.5Sr0.5TiO3 film sandwiched between two electrodes forming a transducer,
supported by a thick sapphire substrate which is a low acoustic loss material.
The resonator works on the principle of induced piezoelectricity due to the
applied dc bias. The minima of the envelope changes from 1.16 GHz with S11 of
-3.51 dB at 100 kV/cm bias to 1.90 GHz with S11 of -42.23 dB at 700 kV/cm bias,
giving a relative tunability of the envelope to be around 64%. The spacing of
the parallel resonance frequency (SPRF) for different bias voltages are also
presented in this report. The quality factor of the resonator is over 22,000 at
around 2GHz.
