  European Spallation Source will be the brightest neutron source in the world.
It is being built in Lund, Sweden. Over 120 superconducting cavities will be
installed in the facility, each regulated by an individual LLRF control system.
To reduce the risk associated with testing the systems on real cavities a
Cavity Simulator was designed. It reproduces the behaviour of superconducting
cavities used in the medium and high beta sections of ESS's Linac. The high
power RF amplifier and piezo actuators parameters are also simulated. Based on
the RF drive and piezo control signals the Cavity Simulator generates the RF
signals acquired by the inputs of the LLRF control system. This is used to
close the LLRF feedback loop in real time. The RF front end of the Cavity
Simulator consists of vector modulators, down-converting circuits, and a set of
fast data converters. The cavity response simulation is performed in a high
speed FPGA logic by a dedicated firmware, that was optimized to minimize the
processing time. The device also generates clock, LO, and the 704.42 MHz
reference signals to allow for system tests outside of the accelerator
environment. In this paper the design of the Cavity Simulator, description of
the algorithms used in its firmware, and measurement results of the device are
presented.
