Architecture and Performance of the PEP-II Low-Level RF System

Abstract

Heavy beam loading in the PEP-II B Factory along with large ring circumferences places unique requirements upon the low-level rf (LLRF) system. RF feedback loops must reduce the impedance observed by the beam while ignoring the cavity transients caused by the ion clearing gap. Special attention must be placed on the cavity tuner loops to allow matching the ion clearing gap transients in the high energy ring and the low energy ring. A wideband fiber optic connection to the longitudinal feedback system allows a rf station to operate as a powerful ''sub-woofer'' to damp residual low order coupled bunch motion. This paper describes the design and performance of the VXI based, EPICS controlled, PEP-II low-level rf system(s). Baseband in-phase and quadrature (IQ) signal processing using both analog and modern digital techniques are used throughout the system. A family of digital down converters provide extremely accurate measurements of many rf signals throughout the system. Each system incorporates a built-in network analyzer and arbitrary rf function generator which interface with Matlab to provide a wide range of functions ranging from automated configuration of each feedback loop to cavity FM processing. EPICS based sequences make the entire system a turn-key operation requiring minimal operator intervention. In the event of a fault, fast history buffers throughout the system write selected rf signals to disk files which can be viewed later to help diagnose problems. Actual data from commissioning runs of PEP-II is presented.