FFT resolution

Hi,I did a strain test with one quarter gauge and stgm module,but i found there is always a spectrum line based on FFT resolution existed in 2D plot after FFT and the value is very closed to DC value. For example , when i set FFT resotion df=1Hz, then one value closed to DC value at 1Hz point, and then the value appeared in 5Hz if FFT resolution changed into df=5Hz. if the FFT resolution approached to 0Hz, then there is a value closed to DC value at the frequency point close to 0Hz,what do you think this happen and i d like to know how to see real DC value ?

my setting parameter and raw data as attached

What you are referring to is the frequency resolution. If the resolution is set to 5Hz, all the frequencies below 5Hz will be put into the same bin.

To get the exact amplitude I suggest using the lowest frequency resolution possible. With your data file, you can go as low as 0,00153 Hz.

For correct amplitude, you should also use the Rectangular window in FFT analysis.

Best regards,

Hi Sirk：

Thanks for your reply！

and another question for you is that when I set line resoluiton df=1Hz and sepatately apply different window for the datafile， I found different value at 1Hz in FFT plot ，especially the value at 1Hz are all bigger than DC value beside the data applied with Retangualr window and I think most enegy are concentrated on 0Hz. so I d like to know why?

Each FFT window has a different effect on the amplitude of the spectrum, therefore, different window functions produce different amplitudes in the spectrum.

https://training.dewesoft.com/online/course/fft-spectral-analysis#windowing

https://en.wikipedia.org/wiki/Window_function

Thanks for your reply。

But I still dont understand why the max value appeared on frequency resolution line with different windows . could you explain someting for me?

The amplitude of FFT spectrum changes, when different windowing is applied.

Filter that should be used in specific applications, can be found here:

I appreciate your effort very much. Thank you for your hard work.