  We begin by giving a description of the radio-frequency generator-cavity-beam
coupled system in terms of basic quantities. Taking beam loading and cavity
detuning into account, expressions for the cavity impedance as seen by the
generator and as seen by the beam are derived. Subsequently methods of
beam-loading compensation by cavity detuning, radio-frequency feedback and
feedforward are described. Examples of digital radio-frequency phase and
amplitude control for the special case of superconducting cavities are also
given. Finally, a dedicated phase loop for damping synchrotron oscillations is
discussed.
