FFT output bias

Dear All,

I put the data from the output of FFT into a power spectrum, and there is a
bias in the data, changing sample speed and number or samples there always
seems to be a ratio between low and high frequencies in the data.

I am not sure if this is correct with FFT, I expected to see changes
independently on the low and high frequencies.

Should I try some sort of filter pre FFT?

The output is always biased to the left of the spectrum sloping down to the
right, most of the time the largest numbers is in the left most position

Thanks

>Dear All,
>
>I put the data from the output of FFT into a power spectrum, and there is
a
>bias in the data, changing sample speed and number or samples there
always
>seems to be a ratio between low and high frequencies in the data.
>
>I am not sure if this is correct with FFT, I expected to see changes
>independently on the low and high frequencies.
>
>Should I try some sort of filter pre FFT?
>
>The output is always biased to the left of the spectrum sloping down to
the
>right, most of the time the largest numbers is in the left most position
>
>Thanks
>
>
>

I am sampling at 100ms intervals and using 2048 samples

On Aug 27, 7:26&#4294967295;am, "tinkerz" <nwoodhamuk@n_o_s_p_a_m.googlemail.com>
wrote:
> Dear All,
>
> I put the data from the output of FFT into a power spectrum, and there is a
> bias in the data, changing sample speed and number or samples there always
> seems to be a ratio between low and high frequencies in the data.
>
> I am not sure if this is correct with FFT, I expected to see changes
> independently on the low and high frequencies.
>
> Should I try some sort of filter pre FFT?
>
> The output is always biased to the left of the spectrum sloping down to the
> right, most of the time the largest numbers is in the left most position
>
> Thanks

can you repeat please in English? Output of an FFT into a Power
spectrum? The periodogram from an FFT is the approximation to a
continuous time power spetrum.
Just take mag^2/N. Of course you will need to window the data and
average the FFTs to get an accurate result.


Hardy

I probably have not coded the output correctly

Power Spectral Density (PSD)

I have just added the two outputs and squared them for every frequency
output.

that maybe my error, what is the correct way to make FFT output into Power
Spectral Density (PSD)

Thanks

Still trying find someone who can consult me on this, ant suggestions of
where to go 

thanks

On Sat, 27 Aug 2011 05:45:57 -0500, "tinkerz"
<nwoodhamuk@n_o_s_p_a_m.googlemail.com> wrote:

>Still trying find someone who can consult me on this, ant suggestions of
>where to go 
>
>thanks

How did you test your FFT to assure that it is working correctly?
What happens when you provide a sine wave at the input at various
frequencies?


Eric Jacobsen
http://www.ericjacobsen.org
http://www.dsprelated.com/blogs-1//Eric_Jacobsen.php

C# code from the the internet, I assumed it works

My code for Power spectral density is from wiki

FLowpower+=(Math.Pow(data[index],2)*index)+(Math.Pow(data[index+1],2)*index);	

This may be the problem, I would like power at each frequency and the total
power.

to compare the power of the lower half to the upper half

On Sat, 27 Aug 2011 10:36:55 -0500, "tinkerz"
<nwoodhamuk@n_o_s_p_a_m.googlemail.com> wrote:

>C# code from the the internet, I assumed it works
>
>My code for Power spectral density is from wiki
>
>FLowpower+=(Math.Pow(data[index],2)*index)+(Math.Pow(data[index+1],2)*index);	
>
>This may be the problem, I would like power at each frequency and the total
>power.
>
>to compare the power of the lower half to the upper half

You can test the FFT and your integration of it by giving it signals
with known spectra and comparing the results.   e.g., sine waves at
various frequencies, combinations thereof, etc.   You should be able
to isolate whether the FFT is the problem or something else.


Eric Jacobsen
http://www.ericjacobsen.org
http://www.dsprelated.com/blogs-1//Eric_Jacobsen.php

Ok will try and look at the results

I dont have the knowledge to understand the result, I just have the concept