Signal processing for undersampled signal I have got a set of pressure data from a boiler which has been sampled at 45Hz and I need to extract a frequency component of 250Hz from the signal. Using a standard FFT method doesn't display anything apart from frequency components between 0 and 10Hz. 

I have read similar questions posted on the forum with no success in solving my problem. 

%Matlab code

filename = 'boilerp.xlsx';
columnA = xlsread(filename,'A:A');

Fs =45;

L=length(columnA);

nfft = 2^nextpow2(L);

XT = fft(columnA, nfft)/L;

f= Fs/2*linspace(0,1,nfft/2+1);

plot(f, 2*abs(XT(1:nfft/2+1)))


Many thanks,

Khel
Combining @JRE's comment and @Nikita's answer: if no anti-aliasing filter was applied to the signal, and no other frequencies alias to 20 Hz, then the 250 Hz component you're interested in will appear at 20 Hz in your sampled signal. See https://en.wikipedia.org/wiki/Aliasing#Sampling_sinusoidal_functions for more information on how this works.