  We begin by giving a description of the rf 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, rf feedback, and feed-forward are described.
Examples of digital rf 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.
