  A general mechanism for the generation of frequency combs referenced to
atomic resonances is put forward. The mechanism is based on the periodic phase
control of a quantum system's dipole response. We develop an analytic
description of the comb spectral structure, depending on both the atomic and
the phase-control properties. We further suggest an experimental implementation
of our scheme: Generating a frequency comb in the soft-x-ray spectral region,
which can be realized with currently available techniques and radiation
sources. The universality of this mechanism allows the generalization of
frequency-comb technology to arbitrary frequencies, including the hard-x-ray
regime by using reference transitions in highly charged ions.
