Why the length of power spectrum is fixed at 4096 when the signal's length varies ?

I was using pspectrum function to obtain the power spectrum of time series signals. However, I find that when I set different length of time series signal, the power spectrum return by the pspectrum function has fixed length 4096. I want to konw what underlying mechanism is causing this result ？

diffSignalLengths = 50:50:5000; % different signal length for experiment
Fs = 3e3; % sampling frequency
lenPS = zeros(numel(diffSignalLengths),1);

for ii = 1:numel(diffSignalLengths)
    x = rand(diffSignalLengths(ii),1);
    P = pspectrum(x,Fs);
    lenPS(ii) = length(P);
end

disp(all(lenPS == 4096))

   1

Answer 1

[Mathieu NOE]

hello

in "auto" mode (i.e when you don't specify the frequency resoltion) , the fft is done with 4096

this is visible in the pspectrum code :

if isempty(opts.FrequencyResolution)
    % Auto mode
    Npoints = uint32(4096);
else
    % Set Npoints to achieve desired resolution
    fspan = 4*Fs;
    Npoints = uint32((1+fspan/opts.FrequencyResolution(1)));
end

[Chuguang Pan]

@Mathieu NOE. Thanks for your answer.

[Mathieu NOE]

my pleasure !