Problem with FFT of 32768 points and more

Hello,

I have created a FFT with MegaWizard Plug-in Manager of the 13.0 Quartus version, with the following parameters :

• Target Device family : Stratix III

• Transform Length : 16384, 32768

• Data Input Precision : 18 bits

• Data Output Precision : 18 bits

• FFT Engine Architecture : Single Ouput (number of parallel engines : 2)

• I/O Data Flow : Burst

• Use of M144K Blocks enabled

• Structure 4 Mults/2 adders

The problem is : the Altera FFT created works well under Matlab with a length of 16384 points (16384.png screen), whereas with a length 32768 points the result of the FFT is zero for both real and imaginary parts excepted for the first point, as you can see in the 32768.png screen.

https://www.alteraforum.com/forum/attachment.php?attachmentid=7372 https://www.alteraforum.com/forum/attachment.php?attachmentid=7371

On the pictures, there is the real part, imaginary part and absolute result of the Altera FFT and of the Matlab FFT, and the difference between them.

The signal used in input of Altera and Matlab FFT is the same, it is a complex exponential with some noise.

So, would you have any idea of the origin of this problem? Is it a bug in the FFT model?

Thank you in advance.

FR