  The ISIS synchrotron at the Rutherford Appleton Laboratory in the UK now
routinely uses a dual harmonic RF system to accelerate beam currents in excess
of 230uA to run two target stations simultaneously. The acceleration in the
ISIS synchrotron is provided by six fundamental frequency (1RF) and four second
harmonic (2RF) RF cavities. The 1RF systems are required to sweep from 1.3MHz
to 3.1MHz during the 10ms acceleration period, repeated at 50Hz, with the 2RF
systems sweeping from 2.6MHz to 6.3MHz. The existing analogue LLRF control
system has been in service for over 30 years and is now showing some signs of
old age and spare parts are becoming difficult to source. In order to overcome
this and to give more stable control of the phase of the RF voltage at each of
the cavities, changes have been made to the LLRF control system. A new FPGA
based combined frequency law generator . master oscillator has been implemented
using 'off the shelf' National Instruments PXI-express based FlexRIO modules.
This initial design has been successfully used during the ISIS operational
cycles for over three years. This paper reports on the commissioning of the
FlexRIO system, the implementation and recent testing of the cavity control
loops and plans for the gradual replacement of remaining parts of the LLRF
system.
