FFT Simulation Errors—MATLAB Model Mismatch after block floating scaling

Hi everybody,

first of all thx to all the people who will supply an answer! Let me explain the context: I implemented a FFT block using megawizard tool of Quartus, with the following parameters:

■ Transform length: 2048

■ I/O data flow: burst architecture

■ FFT engine architecture: quad output

■ Number of parallel engines: 2

Input of FFT module is a sine at 50 Hz, amplitude = 5 and fixed point data. I stored imaginary, real and exponent data on 3 different files. After that, using the same input file, I calculated FFT using Matlab. I multiplied (scaling operation) Matlab output data by 2^exponent ( -5 in my case).

I realized an excel file where I compare results from Modelsim and Matlab. Many of all ( maybe 97%) are similar: the difference among Modelsim and Matlab data, is at most equal to +/- 2 . Anyway I saw that for some of them there is no correspondance: difference amoung these particular case is equal to +123,-36, -118.

Looking at "Release Notes and Errata" of Megacore IP library, on paragraph "Simulation Errors—MATLAB Model Mismatch", I saw that there could be some problem.

( the link to document is)

http://www.altera.com/literature/rn/rn_ip.pdf

Am i wrong???