matlab, obtaining frequency spectrum for uneven time series

[menik]

Hi all,
I am using fft to draw a spectrum. I already got results for fft(). But my problem is the data recorded time series is uneven.

like,
6.00E-05
7.06E-05
8.18E-05
9.37E-05
1.06E-04
1.20E-04
1.34E-04
1.49E-04

So, could you pls help me to get frequency?

Sampling frequency: Fs=1366
Used fft length: nfft=2048

 

[hassan789]

this means you are using a complex fft.... if you want an even fft output, you need to make sure that you have real inputs.
can you post:
1. your complete input
2. matlab commands you are using
3. output data

or a screenshot

 

[menik]

Hi Hassan, Thanks a lot for the reply.
I have done fft using integration of data to even time slots. I want to know whether it is correct or not. I have seen nonuniform DFT can be used, but don't know how to write codes. pls consider I am not familiar with matlab.

01. I'll post part of it.

format long
A=[0.000010 0.115949
0.000020 0.115362
0.000030 0.115218
0.000040 0.115354
0.000050 0.115492
0.000060 0.115604
0.000071 0.115685
0.000082 0.115742
0.000094 0.115778
0.000106 0.115797
0.000120 0.115803
0.000134 0.115801
0.000149 0.115793
0.000165 0.115777
0.000182 0.115759];

02. Fs=(A,1)); %size=1385
n=0:0.227493;
nn=0:1E-5:0.227493;
XX=interp1(A,1),A,2),nn);

N=2048;
R2=A,2);
X=fft(R2,N);
mX=abs(X);
fc=[0:FS/N:FS-1/N];
loglog(fc,mX),grid

 

[hassan789]

First of all your data needs to have 2^n samples for FFT... you have provided 15, so i have added 1 more to make it 16.
The main thing you were missing was "FFTSHIFT()"

try the following code and see the attached pic for results.

clear all; close all; clc;
A=[0.000010 0.115949
0.000020 0.115362
0.000030 0.115218
0.000040 0.115354
0.000050 0.115492
0.000060 0.115604
0.000071 0.115685
0.000082 0.115742
0.000094 0.115778
0.000106 0.115797
0.000120 0.115803
0.000134 0.115801
0.000149 0.115793
0.000165 0.115777
0.000182 0.115759
.000182+(.000182-.000165), .115759-(.115777-.115759)];

FS=(A(:,1)); %size=1385
n=0:0.227493;
nn=0:1E-5:0.227493;
XX=interp1(A(:,1),A(:,2),nn);

N=16;
R2=A(:,2);
X=fft(R2,N);
mX=abs(fftshift(X));
%fc=[0:FS/N:(FS-1)/N];  <--- what is this line??? Not using this, see next line
fc=FS/N; 
loglog(fc,mX),grid

 

[menik]

Dear hassan,
Thanks a lot.

 

[menik]

Can you please explain me the meaning of FS/N??