Plot power spectrum from FFT

Hi,

I want to plot power spectrum using FFT. I use the below code but I didn't get the spectrum and instead I got the below graph.

Please advise where my code is wrong. Thanks

x = xlsread('testcode.xlsx');  fs = 100; x = x';
y = fft(x); 
n = length(x);          % number of samples
f = (0:n-1)*(fs/n);     % frequency range
power = abs(y).^2/n;    % power of the DFT
plot(f,power)
xlabel('Frequency')
ylabel('Power')

Answer 1

[Samhitha]

Hello,

To plot the power spectrum correctly, you need to make sure you are plotting the correct half of the FFT result.

For real-valued signals, the FFT output is symmetric. This means you only need the first half of the FFT result to analyze the positive frequencies.

To get the correct power spectrum, you should take the absolute value of the FFT, square it, and then scale it by the number of samples.

Here's how you can fix your code:

x = xlsread('testcode.xlsx');  
fs = 100;  
x = x'; 
n = length(x);             % number of samples 
y = fft(x);                % compute the FFT 
f = (0:n/2-1)*(fs/n);      % frequency range for half the spectrum 
power = abs(y(1:n/2)).^2/n; % power of the DFT for half the spectrum 

plot(f, power) 
xlabel('Frequency') 
ylabel('Power') 

For more details, please refer to the following MathWorks Documentation:

https://www.mathworks.com/help/signal/ug/power-spectral-density-estimates-using-fft.html

Hope this heps!