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24th December 2005, 07:38 #1
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matlab spectrum
Hi all,
I am a Matlab newbie :)
How to get frequency spectrum of a vector data? (something like [3 4 2 10 ...])
I heard FFT is the timefrequency fransform. And I tried fft() function in Matlab.
But it return a complex number. Is it stand for both frequency and phase?
Any suggestions will be appreciated!
Best regards,
Davy

24th December 2005, 10:42 #2
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matlab frequency spectrum
Hi
Yes, the fft results contain both amplitude and phase information.
To plot only the amplitude try abs( fft( ... ) );
Another useful MATLAB function for psd plotting is pwelch.
Regards
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24th December 2005, 11:57 #3
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frequency spectrum matlab
Hello Circuit_Seller;
In fft() you get fft. But if you want to plot it, you need frequency, that is you must write:
plot( f , fft )
How you can get frequency vector?
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24th December 2005, 12:18 #4
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spectrum matlab
Hi
If N is the FFT length and Fs is sampling frequency then
f = 0 : Fs / N : ( Fs  1 ) / N;
Regards
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24th December 2005, 13:25 #5
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matlab frequency plot
Circuit_seller wrote:
If N is the FFT length and Fs is sampling frequency then
f = 0 : Fs / N : ( Fs  1 ) / N;
Fs / N > ( Fs  1 ) / N
Can you explain more? How did you take this result?

24th December 2005, 13:25

24th December 2005, 19:51 #6
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plot spectrum matlab
Lets try to help:
In order to plot a simple estimation of the spectrum using fft you must pay attention to some details.
Try to increase resolution using zero padding.
Try to avoid spectral leakage, it is easy when you know the spetrum and are just confirming it using fft.
use the command fftshift it takes the center of the signal to origin. eg Yn=abs(fftshift(fft(y,n)))/N where N is the number of points of the signal and n >N is the number of points of the DFT ( nN is zero padding).
In order to plot the frequency you do:
Having a DFT with n points the vector of frequency is f=[n/2: (n1)/2]*Fs/n using this and the fftshift the values in the plot are directly given in Hertz.
I hope have helped
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25th December 2005, 08:44 #7
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frequency plot matlab
Hi
Sorry,
f = 0 : Fs / N : Fs  1 / N;

6th January 2006, 19:33 #8
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matlab plot spectrum
type in matlab command window, the following one by one
help plot
help stem
help polar
furthermore, suppose that Z=complex matrix, then
real(Z) , GIVES U ONLY REAL VALVUES,,,
complex values can nicely be plot in polar graph, by plotting:
abs(Z) against angle(Z)...
i think it will help u..

17th January 2006, 16:21 #9
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plot frequency spectrum matlab
Hello;
Circuit_Seller wrote this relation:
f = 0 : Fs / N : Fs  1 / N

17th January 2006, 16:21

18th January 2006, 10:45 #10
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matlab fft spectrum
Hi
When you perform a FFT with length N, [0 Fs] is divided to N frequency bins.
So to produce frequency bins i wrote that expression.
Regards

18th January 2006, 10:45

10th July 2006, 17:39 #11
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fourier spectrum matlab
I am uploading 2 PDF files on Fourier Transform / Spectrum calculation and display in Matlab. I used it for the lab of DSP.
You can also use the below function for displaying the frequency spectrum in MATLAB. As "N" the number of points for fft increases the spectrum becomes more accurate to the theoratical values.
Here 'signal' is the data vector for which u want to see the frequency spectrum. 't' is the time vector for which the signal exists , 'ts' is the sampling time , 'N' is the number of fourier transform points.
function[freq,mag]=fouriertransform(signal,t,ts,N);
S=fft(signal,N);
CS=[S(N/2+1:N) S(1:N/2)];
freq=[N/2+1:N/2]/(N*ts);
mag=abs(CS);
figure;
plot(freq,mag);
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10th July 2006, 18:49 #12
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fft spectrum matlab
Hi
Well fft gives a complex function.
To plot absolute value and d phase type following
plot(abs(fft))
plot(angle(fft))
u can also refer to help files

12th July 2006, 15:01 #13
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frequency spectrum in matlab
Perhaps that tutorial about FFT can help you
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12th September 2006, 10:11 #14
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phase spectrum matlab
This is my understanding about DFT. DFT builds the relationship between the samples of signal x(t) and the samples of its FT X(f).
timedomain sampling interval is T, the sample freq is FS=1/T. and freq domain sample interval is f0, so time domain truncation duration is 1/f0=T0, and both sampling number in the tdomain and fdomain are N=T0/T.
so the tdomain data is x(T*n) ,n=[0:N01]. data point in tdomain is 0:T:T*(N01).
and x(t)'s discrete fourier tranform is fft(T*x(T*n)). and the freq point is 0:f0:FS.
Added after 1 minutes:
This is my understanding about DFT. DFT builds the relationship between the samples of signal x(t) and the samples of its FT X(f).
timedomain sampling interval is T, the sample freq is FS=1/T. and freq domain sample interval is f0, so time domain truncation duration is 1/f0=T0, and both sampling number in the tdomain and fdomain are N=T0/T.
so the tdomain data is x(T*n) ,n=[0:N01]. data point in tdomain is 0:T:T*(N01).
and x(t)'s discrete fourier tranform is fft(T*x(T*n)). and the freq point is 0:f0:FS.

20th August 2007, 11:08 #15
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matlab spectrum plot
a=abs(fft
b=angle(fft)
plot(a)
plot(angle(b))

20th August 2007, 14:04 #16
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matlab fourier spectrum
in MatLab help is good example for fft,
roughly output spectrum in dB is:
20log10(abs(fft(sin(2*pi*fc*tt))))
in this case the amlitude is 2 peaktopeak that corresponds 10 dBm@50 Ohm for sine signal with amplitude 2Vpp. MatLab shows power 0dB@fc
Can somebody say how take into account this 50 Ohm output impedance to get proper power?

30th September 2007, 11:20 #17
matlab plot frequency
if you want to plot the frequency and phase response of the vector you have, you can use the function freqz....

10th October 2007, 12:07 #18
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spectral plot matlab
select fft box from DSp tool box.

31st October 2007, 16:38 #19
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matlab plot frequency spectrum
let s be your signal;
N = NFFT
Fs = sampling freq
y = fft(s,N);
p = abs(y);
f=Fs/N.*(0:(N/2)1);
plot(f,p(1:N/2));
will give you the power spectrum
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31st October 2007, 16:38

3rd November 2007, 18:27 #20
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spectrum in matlab
Originally Posted by davyzhu
the solution of this problem is that first find the fft then try the command freqz()
that have the arguments fft result,1,1024.
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