-1

what is the reason for that? The gain should increase ideally I guess.

This is the code I tried. As I increase the length of F by zero padding,the gain of PSD reduces.

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B=A';
C1 = reshape(A,2,[])';
C=[C1(:,2) C1(:,1)];
D=join(C);   %512*1
E=regexprep(D,'[^\w'']','');  %remove spaces
F=hex2dec(E);   %hex2dec

N = length(F);
xdft = fft(F);
xdft = xdft(1:N/2+1);
psdx = (1/(Fs*N)) * abs(xdft).^2;
psdx(2:end-1) = 2*psdx(2:end-1);
freq = 0:Fs/length(F):Fs/2;
figure
plot(freq,20*log10(psdx))
grid on
title('Periodogram Using FFT')
xlabel('Frequency (Hz)')
ylabel('Power/Frequency (dB/Hz)')
Akshay Rathod
  • 13
  • 3
  • Possible duplicate of MATLAB: $\tt fft$ and $\tt ifft$ scaling – Marcus Müller Jul 18 '17 at 09:07
  • i read the info from the link,i am unable to link to my question,when i double the no of points in fft there shud be a 3db gain in psd,but in matlab it shows an exact 3db loss – Akshay Rathod Jul 18 '17 at 10:37
  • Can you please show us how you are forming F ? – Peter K. Jul 18 '17 at 11:17
  • @AkshayRathod no, there's nothing that says you should have a "gain". Mathematically justify that claim, and then check against the documentation of the fft function you're using. – Marcus Müller Jul 18 '17 at 16:00
  • @MarcusMüller the formula for fft gain is 10log(M/2)..where M is the no of points..for details pls visit this page,it has somewhat given the answer. https://www.designnews.com/aerospace/where-does-fft-process-gain-come/100022666833951 – Akshay Rathod Jul 19 '17 at 05:44
  • That article is about the processing gain for a very specific class of signals. Which is why I explicitly asked for you to mathematically show why there should be gain, since only you know your signal model. – Marcus Müller Jul 19 '17 at 06:51
  • I have edited the qs by giving my input signal..its an adc data of altimeter..the data is 10241 hex..needs to be converted to 5121 hex and then join and whitespace removed and decimal conversion which gives F @PeterK. – Akshay Rathod Jul 19 '17 at 08:41
  • u need matlab version 2016a or onwards version for to access this data.. – Akshay Rathod Jul 19 '17 at 08:43
  • @MarcusMüller the formula demands number of fft points as input parameter and the formula works irrespective of the type of signal i guess. – Akshay Rathod Jul 19 '17 at 09:00

1 Answers1

2

There is gain, and there is level, assuming you are asking about the bin levels, consider Parseval's theorem,

https://en.wikipedia.org/wiki/Parseval%27s_theorem

for the DFT,

$$ \sum_{n=0}^{N-1} | x[n] |^2 = \frac{1}{N} \sum_{k=0}^{N-1} | X[k] |^2 $$

when you increase $N$ by zero padding, you increase the denominator on the right, which reduces the levels.