hello; what i want to do is to get spectrum of an sinusoidal signal using the altera fft core i have generated a v8 fft with the following setting: fft point : 64 (to start with) architecture : brust data width : 16 twidle width : 16 i am including the matlab files in the attachment .. the input is a 64 point 2`compliment form. in the compare.m file i am doing three fft,,2 matlab fft and 1 altera core fft what i cant figure out is why the magnitude of the second and the the third power spectrum has bigger amplitude than the first fft at the signal fundmental frequency ? and why are those two are not so clean like the first one ? please have a look .. thanks to anyone who replies.

Hi Mesbah, if you plot(y2) or (y) you will see the distortion you are causing... good luck

i dont think its distortion i just raised the negitive part of the signal up... do u think i shouldnt do that ?..... should i feed y1(has pos & neg ) to the altera fft ? if so how can i express negative part of the signal in vhdl ? thanks

Hi Mesbah, you are causing fatal distortion to your nice sine waves by offsetting the negative halves up as this implies high frequencies due to sudden changes. Remember time domain and freq domain are related: sudden changes of amplitude or phase in time domain imply high frequencies, and high freq imply sudden changes in time domain... your ifft expects signed input, so I am told. The vhdl people allow you + representation. std_logic is read according to the way you decide(or ifft decides). If you want to store your sine data in lut then use mif and it accepts signed values. if you want to store 64 data on wires(as constants) then and to avoid the hassle of conversions use type "signed" then convert as required. Here is my adjustment of your code: clear; N = 64; Fs = 64; INVERSE = 0; y = round((2^15-1)*sin(2*pi*5*[0:63]/Fs)); Matlab_fft = fft(y); %[Y, exp_out] = fft_1_model(y,N,INVERSE); %Altera_fft = Y.*2.^(-exp_out); power_fft1 = abs(Matlab_fft); %power_fft2 = abs(Altera_fft); plot(power_fft1);hold %plot(power_fft2,'r') ************* if for any other reason you want to get rid of negative sine data then offset all values up, you will then get your frequency and some dc: y2 = y + abs(min(y)); figure;hold plot(y); plot(y2,'r')

I have a different but probably somewhat related question. I have a continuous digitized 1000Hz sine wave going to the FFT input. The FFT is configured with a streaming architecture. I get the expected two (almost identical) peaks from the output-one at the positive side and the other at the negative side. My question is how come the values for the peaks fluctuate constantly. In other words, the amplitudes for these two peaks keep changing, and the difference between the maximum and minimum amplitude is quite large.

Hi, You got a bug somewhere. One possible subtlety is this: Is your sine input continuous in phase. If you are reading sine LUT make sure there is phase continuity at the end of the cycle. You need to carefully choose your data points and not repeat the first value at the end. for a correct cycle I will do this: sin_data = sin(2*pi*[0:1023]/1024); plot(sin_data) then scale it for hardware. choose other than [0:1023]/1024 for your frequency case. You should see the first sample is 0 but the last sample is just under zero and shouldn't be zero. Having said that it is unlikley that phase will cause major amp fluctuations unless it is grossly wrong. So look for timing issues, your sampling clk and the way your input actually enters the fft.

Help with altera FFT | Altera Community

56 Replies

Altera_Forum
Honored Contributor
16 years ago
attached is a basic block diagram of a windowed FFT.
Altera_Forum
Honored Contributor
16 years ago
Thanks for the block diagram. Can you please tell me how to generate the numbers for the LUT?
Altera_Forum
Honored Contributor
16 years ago
if you have MATLAB you can type "help window" to get a listing of the window functions and learn their usage. for example:

my_window = hann(window_length);

it doesn't look like Octave has hann, but you can see which windows are supported here:

http://octave.sourceforge.net/doc/funref_signal.html

i attached a 256-point Hann window coefficient list.
hann256.txt4 KB
Altera_Forum
Honored Contributor
16 years ago
Did you try the vector wave form simulation or only the matlab simulation???????????
Altera_Forum
Honored Contributor
15 years ago
can someone tell me please what is that function :

??? Undefined command/function 'Sfftmodel'.

Error in ==> create_fft_model at 45
[roc,ioc,eoc] = Sfftmodel(rin,iin,N,THROUGHPUT,ARCH,DATA_PREC,TWIDDLE_PREC,INVERSE); ???

that is the function that altera creat for the fft , but matlab dont know that function...
Altera_Forum
Honored Contributor
15 years ago
there is a dll and mex file in $QUARTUS_ROOTDIR\ip\altera\fft\lib which you'll need

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Help with altera FFT

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