  The goal of a LLRF system is to control an actual RF cavity with beam. While
digital simulations have a place, having an analog circuit to stand in for the
cavity can be tremendously helpful in validating hardware+firmware+software
under development. A wide range of cavity emulators have been developed in
collaboration with SLAC, and LBNL. Cavity emulators are typically based on
quartz crystals and frequency conversion hardware. The choice of crystal
frequency and coupling mechanism depends in part on the bandwidth and coupling
of the cavity it's intended to emulate. Examples of bandwidth range from 800 Hz
(SLAC) as a stand-in for a SRF cavity, to 31 kHz (LBNL) for a room-temperature
accumulator ring cavity. An external LO is used to tune the emulated cavity
frequency. The coupling properties are also of interest if the scope includes
emulating reverse power waveforms. LLRF system checks such as closed-loop
bandwidth, and determining cavity detuning can be performed interactively and
as part of a Continuous Integration (CI) process. This paper describes the
design, implementation, and performance of the cavity emulators.
