Hi Wan The overflow block that generates the overflow output does not currently work properly, fixing it keeps slipping my mind. I will fix it as soon as I can.
The FFT needs the same number of bits as there are stages i.e 5 stage = 5 bits. If you give it more it will just slice off the ones it needs and ignore the rest so 32 bits should be more than enough. Regards Andrew 2009/6/22 <wan.ch...@csiro.au> > 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:martens.and...@gmail.com] > *Sent:* Monday, 22 June 2009 4:23 PM > *To:* Cheng, Wan (ATNF, Marsfield) > *Cc:* casper@lists.berkeley.edu > *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 <wan.ch...@csiro.au> > >> 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 >> > >
