  The voltage oscillations which occur in an ideally current-biased Josephson
junction, were proposed to make a current standard for metrology. We
demonstrate similar oscillations in a more complex Josephson circuit derived
from the Cooper pair box: the quantronium. When a constant current I is
injected in the gate capacitor of this device, oscillations develop at the
frequency fB=I/2e, with e the electron charge. We detect these oscillations
through the sidebands induced at multiples of fB in the spectrum of a microwave
signal reflected on the circuit, up to currents I exceeding 100 pA. We discuss
the potential interest of this current to frequency conversion experiment for
metrology.
