FFT results from Matlab fft, Bit Accurate C model and SysGen FFT block

The usage of the FFT core is straightforward, however, it does have a lot more knobs to turn compared to a simple fft() function in Matlab. Sometimes when the output from the FFT core doesn't match Matlab fft() result you start to question if there is a bug in the FFT core. I will use a simple example here hopefully to

• provide some assurance that the result from the FFT core match well with Matlab fft() function and FFT bit accurate C model
• provide a starting point so you can look at configurations as well as timing of control and data signals of the core in case you didn't have time to read the FFT core data sheet

Below is the SysGen model that uses a 256-point FFT with pipelined streaming IO and unscaled output in natural order. The input vector is a simple ramp (-128:127)/128 in fixed 16.15 format.

The waveform below shows the FFT signals at the beginning of the simulation in WaveScope (a great tool in Xilinx Blockset for debugging, by the way). Note that in FFT v7.0 and newer, there is no longer a requirement of 3 cycle offset between xn_index and xn_re/xn_im inputs.

The waveform below is at the end of FFT computation and data start to unload. Note that xk_index is incrementing as the core output is in natural order.

The simulation results are collected in workspace. A m script is then run to compare the results from Matlab fft(), FFT bit accurate C model, and the Simulink simulation. Below is an overlay plot showing all three outputs, where the difference is negligible.

The script also prints all elements of outputs side by side for easy comparison.


idx    Matlab       Bit Accurate SysGen
0      1.000000     1.000000     1.000000
1      81.489376    81.489504    81.489595
2      40.747756    40.747746    40.747868
3      27.168581    27.168709    27.168739
4      20.380016    20.380048    20.380078
5      16.307697    16.307667    16.307587
...
251    16.307697    16.307667    16.307545
252    20.380016    20.380048    20.380017
253    27.168581    27.168709    27.168495
254    40.747756    40.747746    40.747960
255    81.489376    81.489504    81.489687

In case you want to play with the example design, you can download it from here. You will need to download the bit accurate C model from FFT core web site yourself because it requires registration.