We have seen in previous posts that FPGA data acquisition systems, which may seem very specific in nature, can in fact be used for a wide array of applications such as:
- Pre-clinical imaging (PET, MRI, MicroPET)
- Radio astronomy
- Cargo security inspection
- MIMO radar
We also looked at the use of FPGA data acquisition systems in linear accelerators. FPGA data acquisition systems are used for two key functions within an accelerator: beam position monitoring (BPM), and low-level radio frequency (LLRF) control systems. As background, LLRF control systems manage the electromagnetic field amplitude and phase inside an accelerator. Proper electronics are essential here to ensure the highest possible beam quality. Similarly, beam position monitoring systems are essential to maintaining beam stability.
For this post, we will take a closer look at LLRF systems, and see how innovations in FPGA data acquisition systems might bring further involvement of digital electronics over analog in the years to come.
Typical Components Of A Low-Level Radio Frequency (LLRF) System
The following diagram illustrates the key components of an LLRF control system:
As we can see, an LLRF system is composed of five major elements:
- The cavity in which the particle is accelerated
- An analog front end to perform up/down conversion of the radio frequency (RF) signal into an intermediate frequency (IF) signal for interfacing with the FPGA data acquisition system.
- The FPGA data acquisition system, which in a way is the ‘brain’ of the entire LLRF system. It’s a digital board featuring all the ADC and DAC required for signal conversion from the analog to the digital world, along with an FPGA processor on which the control algorithms are implemented.
- A host PC, for communication with all the other components of the accelerator (often based on EPICS software – see here).
- A clock generation system, to provide the various clocks required by the ADC/DAC converters and RF front-end modulators. All clocks are derived from a single reference clock to ensure proper synchronization across all the elements.
If you are new to LLRF, but familiar with system-level notions of a wireless system, you can see that the two are actually quite similar. This goes as well for the operations performed inside the FPGA, which typically include digital filtering, and ‘baseband’ IQ processing.
On The Way To A Fully Digital LLRF?
It’s interesting to note that most LLRF systems were purely analog-based not so long ago. The innovation drivers that came into play over the last years in the FPGA data acquisition system business (arrival of faster ADCs, increased processing power of Xilinx FPGAs, and arrival of FMC and AMC form factors) certainly had an impact on the way LLRF systems are now designed, supporting a transition from the analog to the digital world. Similar to what has happened with the arrival of “Zero-IF” converters for the wireless industry, new innovations are likely to push the involvement of digital electronics even closer to the cavity. Ultimately, this will allow direct sampling of the RF signals, and a simpler system architecture, as shown in the following diagram:
In their paper, “Beam Position Monitor System of the ESS Linac”