creating frequency vector

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ekong nathaniel - 31 Oct 2008 18:50 GMT

u=[5.65657
4.09474
2.98886
3.74147
1.39051
3.1138
6.2747
5.87201
3.29907
4.17219
4.44912
4.73424
3.78176
2.83575
3.34117
3.95901
1.97765
1.91652
2.00037
4.24108
4.9018
3.91039
-0.170792
0.725255
0.490314
4.09817
5.52631
5.87125
3.57782
1.78822
5.50149
6.86609
6.101
5.64888
5.42735
8.4681
5.88525
9.69079
6.82079
4.64868
1.27876];
I am not so familiar with Matlab. The examples I have looked at arevery easy to follow but the application has been so difficult .

These data samples were taken after 3 sec, so I considerl the time interval, h between each sample as 3 sec.

Therefore,
Fs=1/h=1/3=0.3333Hz % sampling frequency
N=1199 % length of signal (total length at 60minutes)
w=fft(u);
subplot(2,1,1)
plot(u);

f=Fs/2*(0:L-1)/L;
subplot(2,1,2)
plot(f,abs(w(0:0.5*L)))

The spectral analysis of this is not giving clear frequency of the signal and I worry if the frequency vector I created is wrong.

Can anyone look into this for me?

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Rune Allnor - 31 Oct 2008 19:57 GMT

> I am not so familiar with Matlab. The examples I have looked at arevery easy to follow but the application has been so difficult .
>
> These data samples were taken after 3 sec, so I considerl the time interval, h between each sample as 3 sec.

> The spectral analysis of this is not giving clear frequency of the signal and I worry if the frequency vector I created is wrong.
>
> Can anyone look into this for me?

First, never start with measured data. Start with simulated
data where you know the exactl parameters (sampling frequency,
signal frequency, signal amplitude). Then do the analyis with
the simulated data and see if you can extract the parameetrs
from the analysis results.

Once you are happy that you understand what is going on (get the
frequency axis right, understand how the amplitude axis behaves)
then try with the measured data.

Lots of work, but this is the only way that gets you to a result.
The code you already have is a long way in the right direction,
there might be one or two details missing. Just repeat the analysis
with signals of known parameters and try and understand what is
going on.

Rune

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ekong nathaniel - 31 Oct 2008 20:27 GMT

> > I am not so familiar with Matlab. The examples I have looked at arevery easy to follow but the application has been so difficult .
> >
[quoted text clipped - 21 lines]
>
> Rune

Thanks Rune. Though I have done what you have suggested, I will try again.

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Math Forum / Math Software / MATLAB / October 2008

creating frequency vector