CEMB USA N500 - Manual (Page 35)

FFT analyzer mode 6 - 3
5. N° of lines
Such parameter defines the number of lines used in the FFT algorithm, in practice
associated with the resolution in frequency in the spectrum. This determines how close can
be the frequency of two peaks so that they still remain distinct in the FFT graph. Such
resolution is equal to
therefore to maintain it constant, when the max. frequency is increased, likewise the
number of lines should be increased.
It is useful to remember that the time required for acquisition of the correct number of
samples is exactly equal to the inverse of the resolution; then the time required for data
processing should be added to this time. An example of the relation between resolution-
acquisition time may be derived from the following table:
Resolution [Hz] t
acquisition
[sec]
5 0.2
2.5 0.4
1.25 0.8
0.625 1.6
0.3125 3.2
N.B.
The use of an excessively high number of lines is not recommended unless in
situations where an extreme resolution is essential. In fact, such choice would lead
to an increase in calculation times and space required for data saving, often without
adding particular information.
A reasonable choice would be 200, 400 or max. 800 lines, being careful to set a
max. frequency coherent with the situation in question.
6. Average N°
This is the number of spectra which should be calculated and averaged between each other
to increase stability of the measurement. Four averages are more than sufficient for normal
vibration measurements on rotating machines.
Press to access the SPECTRUM ANALYSIS (FFT) measurement screen.
linee
max
N
f