Graph error with Periodogram, incorrect plotting

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Mini Me 2014-5-20

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https://ww2.mathworks.cn/matlabcentral/answers/130406-graph-error-with-periodogram-incorrect-plotting

评论： dpb 2014-5-21

I have this portion of code, my goal is to have my data go from 1200 x 1 to 300 by 4 and then reshape back into a []x 1 matrix. my problem is when i change the matrix to 300 x 4 then take the periodogram...since my Fs is 5, w for each column is now 0 to 2.5 hz which is fine for a 300 x4 matrix. when i make everything back to [] x 1 matrix, since size of w is 300 x4 it keeps repeating 0 to 2.5 4 times. i want the value for w to be continuous since my FS is 5 for the whole data. the periodogram give me four diff graphs instead of one. here's my code. Help

   New_Pxx=[];
    New_w=[];
  test=Downsampled_data(:);
New_array=reshape(test,(length(test)/length(timerArray)),length(timerArray));
   for i=1:4
    Fs=5;
    [Pxx w]=periodogram(test,[],'onesided',[],Fs);
New_Pxx=[New_Pxx;Pxx];
        New_w=[New_w;w]
 end
loglog(New_w,New_Pxx);

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dpb 2014-5-21

编辑：dpb 2014-5-21

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That's still then same vector four times as test is a column vector of size(numel(test),1). It would, in fact, error on the second pass thru the loop on an indexing error.

Slow down, write a script or enter some commands at the command line interactively that actually work to demonstrate precisely what you're trying to do.

Then explain what it is that you think it means to do so and what's different than what you are getting.

ADDENDUM:

OK, on reflection with the first posting and the follow-on, maybe I'm able to fill in the blanks...

Fs=5;
test=reshape(Downsampled_data(:),length(test)/length(timerArray),[]);
for i=1:4
  [Pxx w]=periodogram(test(:,i),[],'onesided',[],Fs);
  New_Pxx=[New_Pxx;Pxx];
  New_w=[New_w;w];
end

That will at least run; it'll give four separate periodograms over the sections of the whole time series that will be similar or not depending on the stationarity of the signal with time.

Now I still don't understand what you expect the frequency associated with those to be other than that from 0 to half-Nyquist for each--that's what you've computed.