An FPGA-based multi-rate interpolator with real-time rate change for a JET test-bench system

Eight independent multi-rate signal interpolators, with real-time change of rate capability, were implemented on a field programmable gate array. The interpolator main building blocks are a cascaded integrator-comb (CIC) filter and the respective compensation filter. The latter performs a fixed rate change of 4 and was implemented as a 129 taps finite impulse response (FIR) filter. The FIR filter coefficients were attained from the MATLAB/spl reg/ simulation, based on the inverse sinc(x) function. The CIC was designed to have six stages (N), a differential delay (M) of 1, and a variable rate change factor (R) ranging from 10 up to 10 000. Each interpolator over-samples the multiple data rate digital signals stored at the Joint European Torus (JET) pulse database to a fixed sampling rate of 40 MSPS. These signals are subsequently converted to the analog domain by 16 bit digital-to-analog converters to be used as stimulus for testing real-time control tools and systems at JET.