  We study theoretically and experimentally the response of a microwave
superconducting stripline resonator, integrated with a microbridge, to a
monochromatic injected signal. We find that there is a certain range of driving
parameters, in which a novel nonlinear phenomenon immerges, and self-sustained
modulation of the reflected power off the resonator is generated by the
resonator. A theoretical model which attributes the self modulation to a
thermal instability yields a good agreement with the experimental results.
