RF over WR allows real-time transmission of frequency tuning words over a WR network. These control words are then used to synthesize phase-aligned radio-frequency (RF) signals in different locations. These RF signals drive accelerating cavities in the Super Proton Synchrotron (SPS) at CERN. In this system, WR is used to distribute the base phase-aligned WR clock signal and the frequency tuning words representing the RF signals. Together, they are used to synthesize the analogue phase-aligned radio-frequency signals. Additionally, WR ensures that the transmission latency over the WR network is deterministic to within 1ns.
Background
CERN, the European Organization for Nuclear Research, operates a number of particle accelerators to perform fundamental research. To accelerate particles, the accelerators are fitted with metallic chambers known as radio-frequency (RF) cavities, wherein an electromagnetic field is established. Charged particles injected into this field receive an electrical impulse that accelerates them. The field in an RF cavity is made to oscillate (switch direction) at a given frequency, so timing the arrival of particles is important. This frequency is adjusted during the acceleration as well, to cater for the change in speed of particles and to manipulate the beam itself. These adjustments are particularly demanding in the SPS.
In order to meet the requirements of beam performance for the High Luminosity LHC, the Low Level RF (LLRF) system, which controls the RF cavities of the SPS, needed an upgrade.
Challenge
- Distribute multiple RF frequencies that can almost arbitrarily change within a specified range
- Ensure sub-10ps phase alignment reproducibility over system reboots between output RF signals generated in spatially distributed nodes
- Provide a distributed RF frequency with a jitter of less than 100fs RMS.
Solution
- Use Direct Digital Synthesis (DDS) techniques to generate RF signals based on the WR clock
- Use White Rabbit Network to distribute WR clock signal and information needed for the DDS
- The WR Switch with low-jitter daughter board is used to minimize phase noise
- Low Phase Drift Calibration (LPDC) was introduced to improve phase alignment reproducibility over reboots
- Dedicated WR Nodes were developed:
- Performance:
- <+/-10ps phase reproducibility over reboots
- <100fs rms jitter
Figures and performance quoted from “WHITE RABBIT AND MTCA.4 USE IN THE LLRF UPGRADE FOR CERN’S SPS”, Tomasz Włostowski et al.
Other relevant articles:
- “TRIGGER AND RF DISTRIBUTION USING WHITE RABBIT”, Tomasz Włostowski et al.
- “THE CERN SPS LOW LEVEL RF: THE CAVITY-CONTROLLER”, G. Hagmann et al
