VMI VIBER X5 MKIII, Инструкция по эксплуатации инструмента

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5.1.6. No of lines
(800, 1600, 3200, 6400, 12800 or 25600)
In this setting number of lines is set and displayed in spectra.
The resolution increases when the number of lines increases, but also the time necessary
for data
acquisition .
Example:
If you have a Max frequency of 400 Hz and your No of lines are 6400
Then total collection time is:
6400 𝑙𝑖𝑛𝑒𝑠
400 𝐻𝑧
= 16 𝑠𝑒𝑐𝑜𝑛𝑑𝑠
From this it can be seen that high resolution spectrum, requires long time to collect the
data. This has nothing to do with the speed of the calculations in the instrument; it is
simply a natural law of frequency analysis.
During measurement the dynamic parameters of the machine (one example is Speed)
should not change. For this reason it can be better to choose a low number of lines in
situations where the parameters of the machine is not steady, or you would like to study
“real time” transients.
5.1.7. Window(Windowing)
(Hanning, Hamming, Blackman, Kaiser-Bessel, Rectangular)
Windowing is used to shape the time portion of your measurement data, to minimize
edge effects that can result in spectral leakage in the FFT spectrum. By using Window
correctly the spectral resolution of your frequency result will increase.
Each window function has its own advantage and suitability for different situations. To
choose a window function, you should estimate the frequency content of the signal.

If the signal contains strong interfering frequency components distant from the
frequency of interest, choose a smoothing window with a high side lobe roll-off
rate.

If the signal contains strong interfering signals near the frequency of interest,
choose a window function with a low maximum side lobe level.

If the frequency of interest contains two or more signals very near to each other,
spectral resolution is important. In this case, it is best to choose a smoothing
window with a very narrow main lobe.

If the amplitude accuracy of a single frequency component is more important than
the exact location of the component in a given frequency bin, choose a window
with a wide main lobe.

If the signal spectrum is rather flat or broadband in frequency content, use the
rectangular window.
In general, the Hanning window is satisfactory in 90% of all cases. It has a good
frequency resolution and reduced spectral leakage.