Low-mode coupled bunch feedback channel for PEP-II

Both the HER and LER of PEP-II use broadband longitudinal multi-bunch feedback systems which process all coupled-bunch modes in the machine spanning a 119 MHz bandwidth. Roughly 1 MHz of this bandwidth includes modes driven by impedance related to the RF cavity fundamental. The longitudinal modes within the cavity bandwidth are processed by the all-mode broadband systems, though the correction signal is applied to the beam via a path through the broadband kicker, as well as through a special woofer channel which uses the RF system to apply low-mode correction signals to the beam. As there are two correction paths, with differing group delay and frequency response, yet only one adjustable processing channel, it is difficult to get an optimal low-frequency ("woofer") response if the broadband feedback path is configured to best control HOM driven instabilities. A new low-mode processing channel has been designed to provide an independent means of providing the low-mode correction signal. It is a digital channel, operating at a 10 MHz sampling rate, and incorporating programmable 12 tap FIR control filters. This channel, implemented using EPLD technology, allows more optimal gain and phase adjustment of the woofer control path, with lower group delay allowing higher gain. This extra flexibility and higher gain will be useful in future high-current PEP-II operation. The design of the control channel is illustrated, and a possible control filter with system dynamics is described