Matlab, How to get frequency spectrum?

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davyzhu

Joined: 23 May 2004
Posts: 521
Helped: 3
Location: oriental

24 Dec 2005 7:38 matlab spectrum

Hi all,

I am a Matlab newbie Smile

How to get frequency spectrum of a vector data? (something like [3 4 2 10 ...])

I heard FFT is the time-frequency 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

Circuit_seller

Joined: 23 Sep 2002
Posts: 415
Helped: 15

24 Dec 2005 10:42 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

No one

Joined: 11 May 2005
Posts: 352
Helped: 3

24 Dec 2005 11:57 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?

Circuit_seller

Joined: 23 Sep 2002
Posts: 415
Helped: 15

24 Dec 2005 12:18 spectrum matlab

Hi

If N is the FFT length and Fs is sampling frequency then

f = 0 : Fs / N : ( Fs - 1 ) / N;

Regards

No one

Joined: 11 May 2005
Posts: 352
Helped: 3

24 Dec 2005 13:25 matlab frequency plot

Circuit_seller wrote:

Quote:

If N is the FFT length and Fs is sampling frequency then

f = 0 : Fs / N : ( Fs - 1 ) / N;

I think that your expression isn't true.

Fs / N > ( Fs - 1 ) / N

Can you explain more? How did you take this result?

claudiocamera

Joined: 19 Aug 2005
Posts: 249
Helped: 22
Location: Salvador-BA-Brazil

24 Dec 2005 19:51 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 ( n-N 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: (n-1)/2]*Fs/n using this and the fftshift the values in the plot are directly given in Hertz.

I hope have helped

Circuit_seller

Joined: 23 Sep 2002
Posts: 415
Helped: 15

25 Dec 2005 8:44 frequency plot matlab

Hi

Sorry,
f = 0 : Fs / N : Fs - 1 / N;

Naveed Alam

Joined: 06 Jan 2006
Posts: 292
Helped: 16

06 Jan 2006 19:33 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..

No one

Joined: 11 May 2005
Posts: 352
Helped: 3

17 Jan 2006 16:21 plot frequency spectrum matlab

Hello;
Circuit_Seller wrote this relation:

Quote:

f = 0 : Fs / N : Fs - 1 / N

Can anyone explain more about this relation? How is the relation resulted?

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Circuit_seller

Joined: 23 Sep 2002
Posts: 415
Helped: 15

18 Jan 2006 10:45 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

starfish

Joined: 08 Jul 2004
Posts: 73

10 Jul 2006 17:39 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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tronix

Joined: 06 Jul 2006
Posts: 121
Helped: 6

10 Jul 2006 18:49 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

mario82

Joined: 18 Aug 2005
Posts: 15
Helped: 3

12 Jul 2006 15:01 frequency spectrum in matlab

Perhaps that tutorial about FFT can help you

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beargebinjie

Joined: 04 May 2005
Posts: 35

12 Sep 2006 10:11 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).
time-domain 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 t-domain and f-domain are N=T0/T.
so the t-domain data is x(T*n) ,n=[0:N0-1]. data point in t-domain is 0:T:T*(N0-1).
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).
time-domain 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 t-domain and f-domain are N=T0/T.
so the t-domain data is x(T*n) ,n=[0:N0-1]. data point in t-domain is 0:T:T*(N0-1).
and x(t)'s discrete fourier tranform is fft(T*x(T*n)). and the freq point is 0:f0:FS.

smith123

Joined: 06 Apr 2007
Posts: 105
Helped: 5

20 Aug 2007 11:08 matlab spectrum plot

a=abs(fft

b=angle(fft)

plot(a)

plot(angle(b))

zhenyabb

Joined: 28 Jun 2007
Posts: 2

20 Aug 2007 14:04 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 peak-to-peak that corresponds 10 dBm(at)50 Ohm for sine signal with amplitude 2Vp-p. MatLab shows power 0dB(at)fc

Can somebody say how take into account this 50 Ohm output impedance to get proper power?

cfreng2

Joined: 12 Jul 2006
Posts: 112
Helped: 9

30 Sep 2007 11:20 matlab plot frequency

if you want to plot the frequency and phase response of the vector you have, you can use the function freqz....

suru

Joined: 31 Aug 2007
Posts: 164
Helped: 4

10 Oct 2007 12:07 spectral plot matlab

select fft box from DSp tool box.

ddkrishna

Joined: 09 Aug 2005
Posts: 73
Helped: 4

31 Oct 2007 16:38 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 Sad

N/2)-1);
plot(f,p(1:N/2));

will give you the power spectrum

vlsi_iitkgp

Joined: 03 Nov 2007
Posts: 2

03 Nov 2007 18:27 spectrum in matlab

davyzhu wrote:

Hi all,

I am a Matlab newbie Smile

hi,
the solution of this problem is that first find the fft then try the command freqz()
that have the arguments fft result,1,1024.

pngzhi

Joined: 20 Jan 2008
Posts: 112
Helped: 7

26 Mar 2008 3:43 plot frequency matlab

Use psd(x) function.

shabzbd

Joined: 09 Jul 2008
Posts: 3

10 Jul 2008 4:29 plotting spectrum in matlab

claudiocamera wrote:

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 ( n-N is zero padding).

Why are we dividing by N??

If you could please explain.

xulfee

Joined: 27 May 2008
Posts: 434
Helped: 32
Location: Pakistan

10 Jul 2008 4:49 how to plot spectrum in matlab

fft is command to see spectrum plot it against frequency

shabzbd

Joined: 09 Jul 2008
Posts: 3

10 Jul 2008 8:41 fft matlab spectrum

I understand that FFT is for plotting in the Frequency domain.. but why are we "Dividing by N"??

Added after 1 minutes:

zhenyabb wrote:

Did anyone get an answer to this.. please??

Aya2002

Joined: 12 Dec 2006
Posts: 1409
Helped: 254
Location: Iraq

02 Feb 2009 23:42 signal spectrum matlab

Hi to every body

realy this forum is very good

such discussions are very useful

thanks to all the replyers

chanphat01001

Joined: 26 Oct 2009
Posts: 1
Location: VietNam

26 Oct 2009 11:30 Re: Matlab, How to get frequency spectrum?

i think, only need use plot(abs(fft(y)))!

tavakoliahmad

Joined: 14 Nov 2009
Posts: 2

14 Nov 2009 13:16 Re: fourier spectrum matlab

starfish wrote:

hi,
I want to get FFT from non-stationary wave. I know that I should use data window.
I've wrriten a code for it but I don't know my code is correct or not?
please help me

Aya2002

Joined: 12 Dec 2006
Posts: 1409
Helped: 254
Location: Iraq

15 Nov 2009 18:42 Re: fourier spectrum matlab

tavakoliahmad wrote:

hi,
I want to get FFT from non-stationary wave. I know that I should use data window.
I've wrriten a code for it but I don't know my code is correct or not?
please help me

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http://www.edaboard.com/ftopic343334.html

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