Hello; Can someone please help me matlab doesn't plot my excel data it show the axis only I am trying to make spectral analysis.i have attested the excel file and code as well.

filename = 'datacollect2.xlsx';
N = length(filename);
fs = 400; 
fnyquest = fs/2;
num = xlsread(filename);
 y = num(: ,2);
 mags = abs(fft(y));
bin_vals = [0 : N-1];
fax_Hz = bin_vals*fs/N;
N_2 = ceil(N/2);
plot(mags(1:N_2),fax_hz(1:N_2))
xlabel('Frequency (Hz)')
ylabel('Power (dB)');
title('Single-sided Power spectrum (Hertz)');
axis tight

[Jan]

What happens instead of the expected plotting? Do you get an error message? Does the axes appear but is empty? Please explain any details.

[dpb]

For one thing you've reversed the role of X and Y in

plot(mags(1:N_2), fax_hz(1:N_2));

The file isn't attached

[Image Analyst]

You might also use semilogy() instead of plot(),

semilogy(mags(1:N_2), fax_hz(1:N_2))

or take the log of the data before plotting, like

mags = log(abs(fft(y)));

[arif hussain]

yes the axis is appeared its empty but when I removed this line (fax_hz(1:N_2)) from the plot command its shows the complete spectral. No there is no any error.

[arif hussain]

here is the data file attached

Answer 1

[dpb]

filename = 'datacollect2.xlsx';
N = length(filename);
fs = 400; 
fnyquest = fs/2;
num = xlsread(filename);
y = num(: ,2);
mags = abs(fft(y));
bin_vals = [0 : N-1];

Probably biggest problem is you've used the length of the filename as the length of the data. The file name is just that; the string containing the name of the file on the disk, it has nothing whatsoever to do with the data contained in the file.

Here,

>> filename = 'datacollect2.xlsx';
N = length(filename)
N =
  17
>>

so all you've plotted are the first 9 points out of the arrays; not much at all... :)

N = length(y);

after you've defined y will undoubtedly help a lot, particularly when you get the roles of X,Y corrected in the plot statement.

You may want to also plot 2*abs(fft(y))/N to normalize the spectrum amplitude to input. See

doc fft % example there shows some other details as well

[arif hussain]

Thankyou very much it works

[arif hussain]

i dont think so this is accurate spectrum may be something wrong here.data file and code attested below.

filename = 'datacollect2.xlsx';
num = xlsread(filename);
fs = 400;
x = num(:,1);
N = length(x);
 y = num(: ,2);
 mags = abs(fft(y));
bin_vals = [0 : N-1];
fax_Hz = bin_vals*fs/N;
N_2 = ceil(N/2);
plot(mags(1:N_2),fax_Hz(1:N_2))

[dpb]

You're still using the spectrum magnitude as the independent variable in the PLOT() call. WHY???

Also, you've taken the amplitude normalization out. WHY?

I would again refer you back to the example at

doc fft