  We study the effect of an ac drive on the current-voltage (I-V)
characteristics of a tunnel junction between two fractional Quantum Hall fluids
at filling $\nu ^{-1}$ an odd integer. Within the chiral Luttinger liquid model
of edge states, the point contact dynamics is described by a driven damped
quantum mechanical pendulum. In a semi-classical limit which ignores electron
tunnelling, this model exhibits mode-locking, which corresponds to current
plateaus in the I-V curve at integer multiples of $I= e\omega /2\pi$, with
$\omega$ the ac drive angular frequency. By analyzing the full quantum model at
non-zero $\nu$ using perturbative and exact methods, we study the effect of
quantum fluctuation on the mode-locked plateaus. For $\nu=1$ quantum
fluctuations smear completely the plateaus, leaving no trace of the ac drive.
For $\nu \ge 1/2$ smeared plateaus remain in the I-V curve, but are not
centered at the currents $I=n e \omega /2\pi$. For $\nu < 1/2$ rounded plateaus
centered around the quantized current values are found. The possibility of
using mode locking in FQHE point contacts as a current-to-frequency standard is
discussed.
