1. Low Level RF Design Outline Scope Requirements Options considered
Evaluation
System drawings
Conclusions

2. Scope
This document summarizes the evolution of the LLRF control system design from its original distributed VME chassis (1 per klystron) to the current design

3. Requirements Meet phase/amp noise levels
Achieve 120 Hz feedback to maintain phase/amp stability
Adhere to LCLS Controls Group standards: RTEMS, EPICS, Channel Access protocol
Begin RF processing of high-powered structures May, 2006

4. Options considered
Originally, various solutions were evaluated, from 100% COTS modules to hybrids of in- house designed boards. See Low Level RF Controls Design, LCLS Week, Jan 2005
Later, the options were narrowed down to two: an Off-the-shelf solution and an in- house solution. See Low Level RF, Lehman Review, May 2005

5. Evaluation The Off-the-shelf solution is: The in-house solution is:
Expensive ($25K per instance * 10 instances)
Noisy. ADCs are up to 150’ from what they measure so analog noise levels and ground loop problems would need to be dealt with
The in-house solution is:
Possibly longer to develop due to board design and fabrication time.

6. Evaluation (2)
Characteristics of the Off-the-shelf solution were seen as requiring more effort than those of the in- house solution
Potential offered by the lower cost of the in-house solution to replace 250 klystron controllers in the remainder of the LINAC is attractive
Hardware people were available as of 22aug2005 to work on board design if µcontroller was decided
Turned to the EPICS community for ideas and chose a µcontroller

7. Evaluation (3)
Lower cost alternatives to the $15K VME chassis and IOC were discussed in the session on hardware at the EPICS Collaboration Meeting. April 27-29, 2005
Of the options available, only the Coldfire uCdimm processor had the communication speed and power to meet our data requirements. Cost is $150 per processor plus the development of the board it sits on

8. Evaluation (4)
By choosing the Coldfire processor, we are able to make use of the port of the operating system, RTEMS, which has already been done.
RTEMS is the standard for the real-time operating system chosen for LCLS by the Controls Group
EPICS, the standard for the control system software for LCLS runs on RTEMS
With these choices, the LLRF control system will be fully integrated into the rest of the LCLS EPICS control system and can speak to other devices and applications such as control panels, alarm handlers and data archivers, using Channel Access protocol, the standard communication protocol for this project.

12. Conclusions This solution: meets the spec for speed and noise
avoids signal noise problems
avoids ground loop problems
meets LCLS control system requirments and standards running EPICS on RTEMS
provides a low cost path for future upgrade in the rest of the LINAC when the rest of the klystron control is replaced