Fourier transform of text data

Hi everyone,

I have the Text data which part of my data is shown below:

           -197           -196           -187           -172           -174
           -173           -157           -135           -112           -106
           -106            -98           -100           -106           -107
           -119           -122           -100            -86           -109
           -122            -95            -80            -84            -75
            -37            -20            -29            -16              4
             23             33             44             67             80
            100            113            103            107            128
            138            126            116            112             98
             76             50             44             
             -1            -23            -48            -76           -102
           -121           -142           -159           -188           -221
           -245           -274           -284           -291           -309
           -305           -296           -312           -331           -320
           -313           -322           -311           -288           -262
           -252           -251           -237           -230           -229
           -220           -209           -189           -170           -169
           -177           -189           -188           -188           -194
           -196           -207           -209           -205           -205
           -206           -205           -210           -211           -189
           -174           -172           -161           -155           -162
           -169           
           -189           -183           -165           -143           -129
           -120           -108            -84            -59            -42
            -26             -4             28             54             71
             96            121            137            131            118
            120            123            131            128            100
             71             64             63             49             38

The sampling rate of data was 20Hz that started from 00:00:00UT to 23:59:59 UT. I don't know how to calculate Fourier Transform (FT) of the data using MATLAB. Moreover, I need to plot the power spectrum of the data on a graph. The x-axis should be frequency (Hz) and y-axis should be power spectrum. Could you help me calculate FT and plot a graph of power spectrum? Thank you very much.

Regards, Aike

Answer 1

[Fangjun Jiang]

help fft

Answer 2

[Honglei Chen]

You need to first load data into MATLAB and then perform FFT. Assume that each column is a different data set, you can do something like

load data.mat
xfft = fft(x)   % assume the data is in x

For the power spectrum, you can use spectrum object, e.g., to get the power spectrum for the first dataset,

h = spectrum.Periodogram
psd(h,x(:,1),'Fs',20)

HTH

[Erd Akt]

Hi Mr. Chen,

Your answer can help me for beginning. I'm new and I've project. My question;

• I've 36 months sales data and I have to use fft and obtain Fourier complex numbers.
• frequency, amplitude, A, B coefficient ets.
• Then I obtain Fourier formula to forecats next 12 months.

How can I progress?

Answer 3

[Arturo Moncada-Torres]

I recommend you to check this wonderful tutorial by Quan Quach. Just remember the power spectrum is the square of the magnitude component of the signal.