FFT IP Core: User Guide

ID 683374
Date 11/06/2017
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3.3.1. Quad-Output FFT Engine

To minimize transform time, use a quad-output FFT engine. Quad-output refers to the throughput of the internal FFT butterfly processor. The engine implementation computes all four radix-4 butterfly complex outputs in a single clock cycle.
Figure 9. Quad-Output FFT Engine 

The FFT reads complex data samples x[k,m] from internal memory in parallel and reorders by switch (SW). Next, the radix-4 butterfly processor processes the ordered samples to form the complex outputs G[k,m]. Because of the inherent mathematics of the radix-4 DIF decomposition, only three complex multipliers perform the three non-trivial twiddle-factor multiplications on the outputs of the butterfly processor. To discern the maximum dynamic range of the samples, the block-floating point units (BFPU) evaluate the four outputs in parallel. The FFT discards the appropriate LSBs and rounds and reorders the complex values before writing them back to internal memory.