How to view the frequency contents of a signal in MATLAB?

• Nov 9, 2006

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usman

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Hi, if i have a signal or composite signal, how can i view its frequency contents in MATLAB, i used fft but i am not satisfied with the result.

 

• Nov 9, 2006

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plot frequency in matlab

hey fft gives peaks @ the frequencies which r present in the signal. wat didn't satisfy you.does the signal consists of so many frequencies tat u were not convenient visualizing it.

 

• Nov 9, 2006

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echo47

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windowed short time fouire transform

If you are seeing energy at frequencies that you didn't expect, try applying a "window" to the data before performing the FFT. If you don't know what that is, refer to MATLAB help, or any book on digital signal processing.

 

• Nov 9, 2006

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Re: frequency in MATLAB

Try this simple example

pulse in time --> sinc in freq

working fine for any pulse of width <100

function []=fourier(m)
x=[ones(1,m),zeros(1,100-m)];

X=fft(x);
f=-50:1:49;
figure;plot(f, fftshift(abs(X))), grid

Click to expand...

 

• Nov 13, 2006

• #5

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fzhang

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frequency in MATLAB

the output of fft is the "true" frequency of the signal, for there is a tradeoff between the time resolution and frequency resolution. If you want the frequency setting in the generator of the signal, you need to design a special way based on the properties of the signals.

 

• Nov 14, 2006

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deepabhargavi

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Re: frequency in MATLAB

if your sampling frequency is Fs and if u r taking an N pt FFT, then ur frequency resolution is Fs/N. so if u r not satisfied with ur resolution try Zero padding the signal to make it large in samples or simply collect more samples before taking FFT.

 

• Nov 25, 2006

• #7

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usman

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Re: frequency in MATLAB

AOA!
my question is very simple. let me reexplain agian.
i have 3 signals of frequencies: f1=100HZ, f2=150hz, f3=50 hz. and added all the three sinosoids to get a composite signal.
now i want to plot the composite signal in frequency domain to see the frequency contents in the it.

regards,

 

• Nov 25, 2006

• #8

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echo47

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frequency in MATLAB

That's a much clearer question.

Try this:

Code:

N  = 1000;      % number of points
fs = 1000;      % sample rate
f1 = 100;       % signal 1 frequency
f2 = 150;       % signal 2 frequency
f3 = 50;        % signal 3 frequency
a1 = 2.3;       % signal 1 amplitude
a2 = 1.5;       % signal 2 amplitude
a3 = 1.1;       % signal 3 amplitude
t = (0 : N-1) / fs;
y = a1 * cos(2*pi*f1*t) + a2 * cos(2*pi*f2*t) + a3 * cos(2*pi*f3*t);
h = fft(y);
% Discard duplicate upper half. Scale frequency and amplitude.
freq = fs * (0 : N/2) / N;
plot(freq, 2 / N * abs(h(1 : N/2+1))); xlabel('Hertz');

 

• Nov 26, 2006

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frequency in MATLAB

perhaps you could use short time fouire transform or wavelet transform

 

• Nov 27, 2006

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Re: frequency in MATLAB

When you try the above example you should know that N must be a power ot two for example 1024. Otherwise the fft function is computationally ineffective.
If you make N=1024 in the above example you probbbably will be unsatisfied . you will seee not just peakse but slopes around them also since the signal period is not multiple of the window length (N). Therefore you need a carefull selection of N according to the signal frequencies and the neccessary resolution

 

• Nov 28, 2006

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echo47

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frequency in MATLAB

Yes, a 1024 point FFT is about twice as fast as a 1000 point FFT.
You can change both N and fs to 1024. The plot will be similar, but with a max freq of 512 Hz.

 

• Nov 28, 2006

• #12

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Re: frequency in MATLAB

Hey,you must map of the samples on to the 0 2*pi interval then by using the sampling frequency you can fin the frequency content which has peak.Don't forget that sampling frequency is equal to pi in discrete case.