define frequency of signal from fft output

2 次查看（过去 30 天）
Ngoc Lam
Ngoc Lam 2014-9-29
评论： YAMASANI MANOJ REDDY 2018-8-21
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?

请先登录，再回答此问题。

采纳的回答

Rick Rosson
Rick Rosson 2014-9-29
编辑：Rick Rosson 2014-9-29
dF = Fs/N;
f = dF*(0:N-1)';

更多回答（4 个）

Ngoc Lam
Ngoc Lam 2014-9-30
编辑：Ngoc Lam 2014-9-30
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 个评论
Rick Rosson
Rick Rosson 2014-9-30
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.

请先登录，再进行评论。

Rick Rosson
Rick Rosson 2014-9-30
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 个评论
Syed Masoud Shah kakakhel
Syed Masoud Shah kakakhel 2017-3-23
编辑：Syed Masoud Shah kakakhel 2017-3-23
kindly @Rick Rosson can u comment...variables like what is N?? kindly will be more helpful..m waiting for response

请先登录，再进行评论。

Ngoc Lam
Ngoc Lam 2014-10-1
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 2014-10-1
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?

类别

在 Help Center 和 File Exchange 中查找有关 Fourier Analysis and Filtering 的更多信息

标签

Community Treasure Hunt

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

Start Hunting!

Translated by