A GaAs/AlAs superlattice with a large miniband (120 meV) showed self-sustained current oscillation at a frequency of 103 GHz giving rise to microwave emission (power 0.5 mW). The emission line had a linewidth of about 1 MHz and was tuneable by about 800 MHz. An analysis suggests that the transport in the superlattice was mainly due to electrons in the lowest miniband and that the oscillation was caused by traveling dipole domains. We also observed frequency locking of the current oscillation attributed to a synchronization of domain propagation by the external high-frequency field.