results of HDL streaming FFT block is not same as FFt block in dsp toolbox

hello,
   i have created an ofdm to implement in fpga for my project in simulink. then HDL coder doesn't support the FFT block so i used *HDL streaming FFT* (fft for length n=64 and N=256).
   but now i am facing the problem that the value produced by HDL streaming FFT block is different from FFT block in dsp toolbox, due to this i am getting wrong values. 
    when i studied the documentation of HDL streaming FFT it says that this hdl streaming fft will produce same values similar to  DSP toolbox FFT block.
    To verify and compare the value of both the blocks i designed simple design which has
# random number generator
# qpsk modulation
# parallel to serial  (connected to qpsk output)
# HDL streaming fft   
# serial to parallel
# fft  (connected to qpsk output)
   so how can i correct this error

[Kiran Kintali]

hi idhya,

can you please share the model with us? we will get back to you on the reason for numerical differences. thanks.

-kiran

[e-mail]

Answer 1

[Kiran Kintali]

dsp.HDLFFT is optimized for HDL Code generation.

dsp.HDLFFT

Fast Fourier transform — optimized for HDL code generation

The HDL FFT System object™ provides two architectures to optimize either throughput or area. Use the streaming Radix 2^2 architecture for high-throughput applications. This architecture supports scalar or vector input data. You can achieve giga-sample-per-second (GSPS) throughput using vector input. Use the burst Radix 2 architecture for a minimum resource implementation, especially with large FFT sizes. Your system must be able to tolerate bursty data and higher latency. This architecture supports only scalar input data. The object accepts real or complex data, provides hardware-friendly control signals, and has optional output frame control signals.