Quantization Analysis of the Infrared Interferometer of the TJ-II Stellarator for its Optimized FPGA-Based Implementation

Infrared interferometers are used in magnetic fusion devices for measuring the line-integrated electron density of the plasmas. An FPGA-based processing system is currently being used in the TJ-II infrared interferometer to compute the line-integrated electron density. In high performance Digital Signal Processing (DSP) applications, the computations carried out in the FPGAs are usually performed in fixed point. The floating-point values of the algorithm description must be quantized to their fixed-point counterparts, introducing some deviations with respect to the un-quantized case. These deviations are modeled as Round-Off Noise (RON) sources, whose effects are propagated through the different parts of the system. Thus, these quantization operations significantly affect the maximum attainable system performance. In the TJ-II system, the amount of RON is a limiting factor. Therefore, its analysis and reduction is essential to perform the control operations in real time.