how to get frequency of a discrete signal by using fft in matlab
6 visualizaciones (últimos 30 días)
Mostrar comentarios más antiguos
i want to find the frequency of a discrete signal using matlab and also want to apply a bandpass filter to that spectrum how can i do that pls help me out guys!!!
0 comentarios
Respuesta aceptada
Wayne King
el 18 de Mayo de 2012
You identify the peak in the Fourier transform, find the index in DFT vector corresponding to that peak and relate that to frequency.
For example:
% sampling frequency is 1 kHz
Fs = 1000;
% I'll create a signal consisting of a 100 Hz sine wave in noise
t = 0:0.001:1-0.001;
x = cos(2*pi*100*t)+randn(size(t));
xdft = fft(x);
% I only need to search 1/2 of xdft for the max because x is real-valued
xdft(1:length(x)/2+1);
[Y,I] = max(abs(xdft));
freq = 0:Fs/length(x):Fs/2;
fprintf('Maximum occurs at %3.2f Hz\n.',freq(I))
Note that the frequency bins in the DFT are spaced at Fs/N where Fs is the sampling frequency and N is the length of the signal. The first DFT "bin" corresponds to zero frequency.
Your next step is to construct a bandpass filter around 1 kHz, you can do that a number of ways in MATLAB. See fdesign.bandpass for example.
Más respuestas (1)
Wayne King
el 18 de Mayo de 2012
You don't have to do fftshift() to get the real spectrum. fftshift just puts 0 in the center with "negative" frequencies to the left and "positive" frequencies to the right. That is convenient in many applications, but not necessary.
If the signal is real-valued (not complex-valued), then a symmetry will exist between the two halfs of the Fourier transform. Therefore, in order to identify the peak frequency, you only need to work with 1/2 of the result.
0 comentarios
Ver también
Categorías
Más información sobre Spectral Measurements en Help Center y File Exchange.
Community Treasure Hunt
Find the treasures in MATLAB Central and discover how the community can help you!
Start Hunting!