  The complex impedance of a semiconductor superlattice biased into the regime
of negative differential conductivity and driven by an additional GHz ac
voltage is computed. From a simulation of the nonlinear spatio-temporal
dynamics of traveling field domains we obtain strong variations of the
amplitude and phase of the impedance with increasing driving frequency. These
serve as fingerprints of the underlying quasiperiodic or frequency locking
behavior. An anomalous phase shift appears as a result of phase synchronization
of the traveling domains. If the imaginary part of the impedance is compensated
by an external inductor, both the frequency and the intensity of the
oscillations strongly increase.
