Libera LLRF, the industrial digital RF stabilization system, offers a unique combination of hardware, digital signal processing and software. It satisfies the requirements of less to more demanding types of accelerators. This is achieved by employing the following features implemented in Virtex5 FPGA:

Signal monitoring

• values (voltage, phase, power, frequency, I/O status)
• slow acquisition (sampling rates 1 Hz – 10 kHz)
• fast acquisition (fixed sampling rates up to 130 MHz)

 

Cavity tuning

• detune measurement algorithm based on forward, probe (CW mode)
• detune measurement algorithm based on cavity voltage decay (pulsed mode)
• display of resonance frequency
• display of phase difference between forward and probe signals
• feedback with a PID controller
• feedforward for the Lorentz force detune compensation
• stepper motor driver interface (serial commands, step and direction)
• Piezo amplifier interface

 

Control loops for the RF field:

• fast feedback loop (amplitude and phase)
• COMB loop
• pulse-to-pulse
• klystron feed-back loop (gain, phase)
• beam based feedback
• stabilization and control of the accelerator’s variable RF frequency
• equalization of the accelerating gradient in the several cavities driven by same klystron

 

Operation mode

• CW
• CW with pulsed operation mode
• pulsed
• combined CW and pulsed

 

RF diagnostics tools for super and normal conducting cavities

• RF system's transfer function characterization (to the measured frequency sweep applied mathematical model)
• Feed-back loop nyquist stability plot and stability characterization

 

Compensation of the Libera LLRF receiver modules' temperature drifts

Slow analog and digital interfaces

Fast interlock interface with active low logic (programmable interlock levels)

 

The Libera LLRF system can process up to 38 RF input signals per unit. Probe, forward and reflected signals are connected to the RF inputs of one or more Libera LLRF units. Each unit can populate up to 4 front-end modules. The front-end modules are equipped with temperature stabilization and drift compensation systems. Each RF front-end module converts 9 RF input signals to an IF frequency. The converted signals are then sampled by the 16-bit ADCs. The sampled IF signals are subsequently processed in real time in a powerful Virtex 5 FPGA. The partial vector sum is transferred to the vector modulator module by means of a low-latency link. In the vector modulator module, another Virtex 5 FPGA implements the LLRF control algorithm, and an RF output drive signal is generated and applied to the transmitter.

The Libera LLRF high-level application software performs RF system response characterization and therefore uses the results for automatic loop stability optimization. The system can also be used to provide optimal RF system frequency tuning and optimal RF distribution system tuning by means of the implementation of slower tuning loops. During operation, the system provides diagnostics information to the user and triggers the interlock if a failure is detected.

 

 

Data Paths

 

The powerful features of Libera LLRF enable RF system monitoring, configuration and RF system diagnostics. The diagnostic algorithms automatically characterize the RF system transfer function by means of built-in vector network analysis. The RF parameters are then passed to a cavity field control loop stability analysis algorithm, which optimally configures the Libera LLRF parameters for closing a stable loop. Cavity decay analysis is then used in the case of pulsed applications to continuously monitor the cavity behavior for tuning and interlock purposes.