FFT / iFFT / RS - Basys3

[donwazonesko]

Hello, 

A while ago i've managed to receive data from XADC using my board Basys3 and now i need to use FFT, iFFT and proceed the output from these to via RS to my computer. Could you please show me examples of FFT / iFFT and RS connection? I've been trying to find something online, but there is nothing as simple as that.

 

Best Regards,

Michael

[donwazonesko]

Unfortunately i need to use FFT inside of an FPGA. I need to save data received from 2 XADC ports - use FFT on one and iFFT on the second one and multiply each other. Then proceed the data via RS to PC.

[xc6lx45]

Hi,

if you aren't required to do it on the FPGA, you'll save yourselves a lot of implementation and debugging effort if you do the FFT offline.

An Octave script for the job can be found here. It's basically this

        b = b .* blackmanharris(numel(b));
        
        % calculate spectrum
        mag = abs(fft(b.')) .^ 2;
        
        f_Hz = fd_freqbase(numel(mag)) * rAdc_Hz;    
        
        % === keep only positive frequencies ===
        mask = (f_Hz >= 0) & (f_Hz < rAdc_Hz / 2);
        mag = mag(mask);
        f_Hz = f_Hz(mask);

        % === convert to dB ===
        mag = 10*log10(mag/max(mag)+eps);
		h = plot(f_Hz/1e3, mag); %set(h, 'lineWidth', 2);
        xlabel('f/kHz'); ylabel('dB');

  with those two functions

function fb = fd_freqbase(n)
    fb = 0:(n - 1);
    fb = fb + floor(n / 2);
    fb = mod(fb, n);
    fb = fb - floor(n / 2);
    
    fb = fb / n; % now [0..0.5[, [-0.5..0[
end

function win = blackmanharris(n)
    a0 = 0.35875;
    a1 = 0.48829;
    a2 = 0.14128;
    a3 = 0.01168;
    N = n-1;
    n = [0:N]';
    win = a0 - a1.*cos(2.*pi.*n./N) + a2.*cos(4.*pi.*n./N) - a3.*cos(6.*pi.*n./N);
end

There's many ways to skin a cat... this one has regulatory approval ?

This is just about postprocessing. You still need to transfer the data e.g. via a UART. I'd recommend 16 bit binary format. ASCII or hex will be very slow in comparison.