  We study thermal instability in a driven superconducting NbN resonator
integrated with a microbridge. A monochromatic input drive is injected into the
resonator and the response is measured as a function of the frequency and
amplitude of the drive. Inside a certain zone in the frequency-amplitude plane
the system has no steady state, and consequently self-sustained modulation of
the reflected power off the resonator is generated. A theoretical model,
according to which the instability originates by a hot spot forming in the
microbridge, exhibits a good quantitative agreement with the experimental
results.
