  We demonstrate a high-frequency generator operating at ~200GHz based on
flipping a semifluxon in a Josephson junction of moderate normalized length.
The semifluxon spontaneously appears at the $\pi$ discontinuity of the
Josephson phase artificially created by means of two tiny current injectors.
The radiation is detected by an on-chip detector (tunnel junction). The
estimated radiation power (at the detector) is ~8nW and should be compared with
the dc power of ~100nW consumed by the generator. The measured radiation
linewidth, as low as 1.1MHz, is typical for geometrical (Fiske) resonances
although we tried to suppress such resonances by placing well-matched microwave
transformers at its both ends. Making use of a phase-locking feedback loop we
are able to reduce the radiation linewidth to less than 1Hz measured relative
to the reference oscillator and defined just by the resolution of our
measurement setup.
