How do I take the fft of a signal after I using the blackman window?

I have written the following code for putting the blackman window on the data :

window = blackman(length(strain_seg)); %the length of the strain_seg is 256^2
windowed_strain = strain_seg*window;

How do I perform an fft of the windowed strain? Do I need to use the rfft function?

[SCuri]

Updated code:

M = length(strain_seg);%this length is 65536 x1
w = blackman(M);%  length 64 x 1 
C = [strain_seg,zeroes(65536,63)]; % added zeroes to the strain data to match the dimension of the window to I could multiply
xw = C.*w;
X = fft(xw)

How do I plot this signal as Strain vs Frequency ?

Answer 1

[Enthusiastic Student]

I think you might have issue with dimensions early on.

Have you tried to see what

size(strain_seg)
length(strain_seg)

actually output? I'm guessing the strain_seg has a size of [65536 1] and the blackman has a size of [1 64], which is why you have to add zeroes. I would suggest transposing one of them using the ' operator:

M = length(strain_seg);
w = blackman(M);
xw = C.*w';
X = fft(xw)

With regards to plotting the fft of the windowed strain segment you would need to know the time the segment spans. Lets say the segment you are examining was taken over 10 ms:

Ts = 10e-3; %samples taken in a 10 ms long period of time /[s]
dt = 1/Ts; %/[Hz]

freq = dt:dt:(dt*length(xw)+dt); %Frequencies
figure
loglog(freq,abs(X));
xlabel('Frequency /[Hz]')
ylabel('Magnitude of FFT')