Continuous Time Fourier Transform of a signal matrix

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Anubhav Rohatgi
Anubhav Rohatgi 2012-7-28
回答： Juhi Maraskole 2020-9-16
I have a matrix of 100 rows and 2 columns. Column 1 consists of time signal at frequency 50Hz (0.02 0.04 0.06...) column 2 is signal whose fft is to be determined. I would like to have a function that determines the Fourier transform of the signal at the frequency determined by the 1st column of the matrix.

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Wayne King
Wayne King 2012-7-28
编辑：Wayne King 2012-7-28
If the first column of your matrix is just the time vector with increments of 0.02 seconds, then just take the Fourier transform of the 2nd column
Let X be your matrix
xdft = fft(X(:,2));
If the signal is real-valued, you only need 1/2 the DFT to examine the amplitude spectrum.
The frequency vector can be formed as follows:
freq = 0:50/100:25;
For example:
t = 0:0.02:(100*0.02)-0.02;
x = cos(2*pi*10*t)+randn(size(t));
X(:,1) = t';
X(:,2) = x';
% Now X is your matrix
xdft = fft(X(:,2));
xdft = xdft(1:length(xdft)/2+1);
freq = 0:50/100:25;
plot(freq,abs(xdft))
xlabel('Hz'); ylabel('Magnitude')
  1 个评论
Anubhav Rohatgi
Anubhav Rohatgi 2012-7-28
Thanks for your response, but I am not getting the results still here is my code:
x = data(:,2);
len = length(x)
t = 0:0.02:(len*0.02)-0.02;
xdft = fft(x);
xdft = xdft(1:length(xdft)/2+1);
freq = 0:50/len:25;
plot(freq,abs(xdft))
xlabel('Hz'); ylabel('Magnitude')
Output is a line of 0 magnitude and on the X axis
and getting a warning:::: Warning: Integer operands are required for colon operator when used as index > In test3 at 9
Datafile can be accessed from :::
https://docs.google.com/open?id=0BzuiGHpNJIxCTXZ1ek9Ob1c5azA

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更多回答（3 个）

Wayne King
Wayne King 2012-7-28
编辑：Wayne King 2012-7-28
You stated in your original post that your matrix had an even number of elements, 100x2. In fact your matrix is 9079x2. You data also has a mean which is nonzero so that will make the 0 frequency component very large, it's better to remove the mean first.
x = data(:,2);
x = detrend(x,0);
len = length(x)
t = 0:0.02:(len*0.02)-0.02;
xdft = fft(x);
xdft = xdft(1:(length(xdft)+1)/2);
freq = 0:50/len:25;
plot(freq,abs(xdft))
xlabel('Hz'); ylabel('Magnitude')
Anubhav Rohatgi
Anubhav Rohatgi 2012-8-28
Hi Wayne,
I have tried to solve the problem but the plot I get is not what I want. I have breathing data that is very periodic. I have attached the data link to this
https://docs.google.com/open?id=0BzuiGHpNJIxCTXZ1ek9Ob1c5azA
I want to get frequency peaks at around 3Hz.
Please help me.
Juhi Maraskole
Juhi Maraskole 2020-9-16
x(t) = e-AT

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