Hi Andrew: Now I can find the OF output from FFT module is high. Does this mean the FFT is overflow? But I already set all shift bit to '1'.
I an not very sure how many shift bits the FFT module has. I just give it 32 bits width '1'. Thanks Wan ________________________________ From: Andrew Martens [mailto:[email protected]] Sent: Monday, 22 June 2009 4:23 PM To: Cheng, Wan (ATNF, Marsfield) Cc: [email protected] Subject: Re: [casper] about the shift input of FFT Hi Wan The FFT applies gain to any repeating input signal. You can view this happening with the following MATLAB commands; x = 0:1:32; y = sin(4*x*2*pi/32); plot(abs(fft(y,32))); You will see that the output has spikes of height 16 instead of 1 which you would expect from a sinusoid of amplitude 1. The FFT scales the signal by 2^(n-1), where n is the number of stages in the FFT (a 32 bin fft has log2(32) = 5 stages etc). This scaling has the potential to introduce overflow errors. The CASPER FFT allows you to remove the risk of overflows by shifting the data down by one bit at each stage in the FFT. The shift input allows you to set how much shifting is done. The default is normally to leave this all '1's but it depends on what your input data looks like. I hope this answers your question. If you have MLIB_ROOT properly defined in your environment, you should have access to some documentation on the FFT (and most blocks). Press the 'Help' button in the mask dialog. Regards Andrew 2009/6/22 <[email protected]> Hi: Could anyone give me some idea on the function of shift input pin of FFT? I find some example design set it to all 1s. This means every stage are set to shift 1. What does this mean? Thanks Wan
