RF control system development at CEBAF

The CEBAF (Continuous Electron Beam Accelerator Facility) RF control system uses a heterodyne scheme to convert the cavity frequency of 1497 MHz down to a more manageable intermediate frequency (IF) of 70 MHz. At this IF frequency, all the signal processing is done. Because of the tight system requirements of ±0.2° phase stability and 2×10^-5 amplitude stability, CEBAF uses new technologies such as high-speed analog circuits at 70 MHz. Recent tests on the RF control system at CEBAF have shown that it is possible to control phase to ±0.2° and amplitude fluctuations to the 4×10^-4 level. If the errors are uncorrelated, the requirements concerning beam quality would be met. If the errors are correlated, the amplitude control has to be improved by an order of magnitude. The present design makes use of a vector modulator as phase shifter, and it increased gain at low frequencies (up to 100 Hz) by changing the frequency response of the gain stages