FFT

Hi All,

I have 3 different separate I and Q signals at 48K. Can any one explain how to do a weighted complex FFT on the three different I and Q Signals.

Answer 1

[Wayne King]

Then you just do

 t = 0:0.001:1-0.001;
 I = cos(2*pi*100*t);
 Q = sin(2*pi*100*t);
 sig1 = I+1j*Q;
 dftsig1 = fft(sig1);

Do the same for I2 and Q2, etc.

Answer 2

[Wayne King]

Why do you want to weight them? And by weight, do you simply mean window the signals?

fft() accepts a complex-valued input with no problem

   t = (0:0.001:1-0.001)';
   x = exp(1j*2*pi*100*t)+complex(randn(size(t)),randn(size(t)));
   % and if by weight, you mean window the I and Q channels
   x = x.*complex(hamming(length(x)),hamming(length(x)));
   xdft = fft(x);

Answer 3

[Vivekanandh]

Hi Wayne,

Thanks for your reply. I think my question is not clear. I am doing I and Q modulation and i have the I signal and Q signal separately (which is a sine wave). Let's say I1 = {0,1,2,1,0,-1,-2,-1,0} and Q1 = {-2,-1,0,1,2,1,0,-1,-2}. And similarly i have I2,Q2 and I3 and Q3 (all are sine wave). I need to combine I1 and Q1 signals and create a complex FFT. Similarly for I2,Q2 and I3 and Q3 signals.

Answer 4

[Vivekanandh]

Hi Wayne,

Thank you very much. This helps a lot. Is that possible to look for a DC component in FFT. If so how is it possible? Is that possible to remove the DC component from FFT and compare the result with the FFT image?

[Dr. Seis]

The DC will just be your dftsig1(1) value. You can remove the DC component by removing the mean of your input signal to the FFT (i.e., sig1).