  Oscillators, which produce continuous periodic signals from direct current
power, are central to modern communications systems, with versatile
applications such as timing references and frequency modulators. However,
conventional oscillators typically consist of macroscopic mechanical resonators
such as quartz crystals, which require excessive off-chip space. Here we report
oscillators built on micron-size, atomically-thin graphene nanomechanical
resonators, whose frequencies can be electrostatically tuned by as much as
14\%. The self-sustaining mechanical motion of the oscillators is generated and
transduced at room temperature by simple electrical circuitry. The prototype
graphene voltage controlled oscillators exhibit frequency stability and
modulation bandwidth sufficient for modulation of radio-frequency carrier
signals. As a demonstration, we employ a graphene oscillator as the active
element for frequency modulated signal generation, and achieve efficient audio
signal transmission.
