Issue with FFT

[StathPol]

Hi all,

I have used a signal to carry out an FFT analysis using DIAdem. I have used the following options available in DIAdem:

FFT (One Time Signal)

Windows type: Rectangle

FFT functions: Amplitude

Type of amplitude: Power spectrum

Third/Octave = No

Average amplitudes: No averaging.

I have attached the raw data in an Excel file. In addition, I have attached an image indicating the amplitude versus time plotted in DIAdem on the top. On the left hand side the data plotted using another software, whereas on the right hand side the data are plotted to DIADEM. The frequency on the x-axis is ok, but the amplitude on the y-axis is not the same. I would like to ask if there is any way that I can get the raw signal of the amplitude using DIAdem, as I think this is the normalisation ?

Thanks,

Pepis21

[StathPol]

Any help will be much appreciated. 

[Artemis125]

Which amplitude are you trying to match? the original data or the graph on the left? Rember that an FFT takes the data into the frequency domain. The amplitudes represent the sine waves that make up your original data, so I'm not sure what you're looking for.

[StathPol]

Hello 🙂 , 

The amplitude I got from the test is located in the Excel file and the top of the graph. I am trying to convert the data from the time domain to frequency domain. I have carried out FFT using DIAdem to check the frequencies. The amplitude should match the left graph on the bottom, so the amplitude to be 3.5 x 10^7. Please see attached image. 
Thank you.  

[StathPol]

Hello 🙂 , 

The amplitude I got from the test is located in the Excel file and the top of the graph. I am trying to convert the data from the time domain to frequency domain. I have carried out FFT using DIAdem to check the frequencies. The amplitude should match the left graph on the bottom, so the amplitude to be 3.5 x 10^7. Please see attached image. 
Thank you.  

[Artemis125]

Hi, 

Maybe it's just me but i feel that the DIAdem FFT frequencies make more sense. Most of the time FFTs make the amplitude smaller, not larger, and certainly not by that much. I'd take a look at your first method of calculated FFT to double check.

[AndresID · Accepted solution]

Hello Pepis,

Looking at your signal, the amplitude you want to reach is not possible (unless you are changing units or you are multiplying it).

Imagine that, if you had a perfect sine wave with an amplitude of 1 you should have an amplitude of 1 when you perform the FFT.

From your signal the maximum amplitude you could get would be less than 100 (very far from 3.5E7)

Best regards,

Andrés

[StathPol]

Thanks Artemis125 🙂

[StathPol]

Hello Andres, 

I have double checked it with others and it looks the same. 😕

[StathPol]

Hey Artemis and Andres, 

Just confirmed the FFT calculated using DIAdem with Origin. Thanks.