Extract FFT of every column in frequency domain

Hello,

So I have analysed the data and put it into an FFT like this:

A_G_fft = fft(A_G,[],1);

Now I get a complex double of dimensions 250000x151, now, all I want to do is to plot the FFT of for example column 1 in the frequency domain, so I get the magnitude on the y-axis and frequency on the x-axis.

So the files I work with have 250.000 measurement points and 151 different measurements in this case. The variable A_G is a double of size 250000x151 and is the vibration measured through labVIEW and written into csv files.

I tried it in several ways but cannot seem to work it out.

How would you guys do it?

Thanks in advance!

Answer 1

[KALYAN ACHARJYA]

A_G_fft = fft(A_G,[],1);

Is that :

y_data=vibration measured through labVIEW ?

for i=1:size(A_G_fft,2)
col_data=A_G_fft(:,i);
plot(col_data,....)
% You have put ^other y variable here which must have same length 
hold on;
end 

Hope I got the question!

[Robin L]

The value of the vibration is in mV/g on the y-axis (already converted to the correct value of g) into the variable A_G. Whereas measurement point 1 is the start of the measurement and measurement point 250.000 is the end of measurement 1 (here the measurement time was 5 seconds). Then there are 151 measurements with each 250.000 measurement points.

From that I took an FFT as shown.

Judging from your code, I would get all the plots on 1 graph and each plot would be the complex value given by the FFT? Not converted to the frequency domain and dB?

[KALYAN ACHARJYA]

I have not considered any such unit conversion. I answered based on the following statement.

"all I want to do is to plot the FFT of for example column 1 in the frequency domain, so I get the magnitude on the y-axis and frequency on the x-axis.

You have a matrix A_G_fft with size 25000x151, I have shown the plot of individual columns of data with respect to any other y data (which should have the same length, which is 25000).

"Judging from your code, I would get all the plots on 1 graph and each plot would be the complex value given by the FFT? Not converted to the frequency domain and dB?'

I cannot comment on the subjective issue without understanding the full details, however, FFT is an algorithm for calculating DFT (Fourier analysis converts data from its original domain into frequency dammin).

[Robin L]

I'm sorry for not being clear. I wanted to plot it using the magnitude in dB and the frequency in Hz. Can I just use abs and phase functions to convert the complex double?

[KALYAN ACHARJYA]

Please refer the link, it may help

https://dsp.stackexchange.com/questions/35992/what-are-the-units-of-my-data-after-an-fft

Consider a little example with pen and paper, it may be more clear. Note, most of the members may not be from the same subjective domain, hence it is preferable to interpretation in the universal scientific language, which is mathematics.

:)