define frequency of signal from fft output

2 vues (au cours des 30 derniers jours)
Ngoc Lam
Ngoc Lam le 29 Sep 2014
what is the relationship between the sampling frequency Fs and frequency of signal of FFT function in matlab? assUme that, we have a signal with spectrum as follow picture. with Fs= 44.1Khz, I know that X-axis is not the frequecy of signal, right? How to define the frequency of this signal with followed spectrum? for example, at the point X-axis = 10000, with Fs= 44.1Khz, how to define frequency of signal at that point?

Réponse acceptée

Rick Rosson
Rick Rosson le 29 Sep 2014
Modifié(e) : Rick Rosson le 29 Sep 2014
dF = Fs/N;
f = dF*(0:N-1)';

Plus de réponses (4)

Ngoc Lam
Ngoc Lam le 30 Sep 2014
Modifié(e) : Ngoc Lam le 30 Sep 2014
thanks for your opinions! it's helpful for me! But I have a trouble with using fft for analysing spectrum of signals. for example we have a code segment:
%%%% code segment %%%%
t= linspace (0,4*pi,500);
x= sin(100*pi*t)+2*cos(200*pi*t);
X= abs(fft(x));
%%%%%%%%%
with followed picture representing the spectrum of that picture.
  • *
how we can define frequency of any point in X-axis?
at point (125) in X-axis, how can we determine the frequency of that signal at that point?
thanks!* *
  1 commentaire
Rick Rosson
Rick Rosson le 30 Sep 2014
The sampling rate in your code is
Fs = 500/(4*pi);
which is approximately 40 samples per second. By the Nyquist Sampling Theorem, the maximum frequency that you can represent without aliasing is Fs/2 or approx 20 Hertz. Your signal, however, contains components at 50 Hertz and 100 Hertz, which are well above the maximum.

Connectez-vous pour commenter.


Rick Rosson
Rick Rosson le 30 Sep 2014
Please try:
Fs = 1000;
dt = 1/Fs;
N = 12000;
t = dt*(0:N-1)';
x = sin(100*pi*t) + 2*cos(200*pi*t);
X = fftshift(fft(x))/N;
dF = Fs/N;
f = -Fs/2:dF:Fs/2-dF;
plot(f,abs(X));
  1 commentaire
Syed Masoud Shah kakakhel
Syed Masoud Shah kakakhel le 23 Mar 2017
Modifié(e) : Syed Masoud Shah kakakhel le 23 Mar 2017
kindly @Rick Rosson can u comment...variables like what is N?? kindly will be more helpful..m waiting for response

Connectez-vous pour commenter.


Ngoc Lam
Ngoc Lam le 1 Oct 2014
Ah, I have no pay attention to Nyquyst sampling theorem aplied here! I have solved my problems, thanks for your help!

Ngoc Lam
Ngoc Lam le 1 Oct 2014
Do you know the meaning of the symmetry of spectrum when we plot spectrum of signal?
i thought that It's the negative spectrum of signal? Is it right?

Catégories

En savoir plus sur Fourier Analysis and Filtering dans Help Center et File Exchange

Tags

Community Treasure Hunt

Find the treasures in MATLAB Central and discover how the community can help you!

Start Hunting!

Translated by