  We present a detailed analysis of the quantum description of electro-optical
phase modulation. The results define a black-box type model for this device
which may be especially useful in the engineering steps leading to the design
of complex quantum information systems incorporating one or more of these
devices. By constructing an explicit representation of the phase modulation
scattering operator, it is shown that an approach based entirely on its
classical description leads to unphysical modes associated to non-positive
frequencies. After modifying this operator, phase modulation is described, for
the first time to the best of our knowledge, in terms of a unitary scattering
operator defined over positive-frequency modes. The modifications introduced by
in the process of sideband generation by phase modulation are shown to be not
significant when the radiation belongs to optical bands, thus being consistent
with the classical description. Finally, the model is employed to characterize
the important case of multitone radiofrequency modulation of an optical signal.
