BSA Flow Software Installation and User's Guide - CSI

Figure 6-51 Properties of the spectrum object
Spectral samples The property Spectral samples determines the number of discrete
frequencies at which the power spectral density should be estimated.
The FFT-technique used require this number to be an integer power of 2, and
consequently the user has to choose among a limited number of possible
values: 32, 64, 128, ... , 65536. If a different value is entered, the nearest
higher power of two will automatically be selected.
Maximum frequency The property Maximum frequency determines the highest frequency at
which the power spectral density should be estimated. In the example in
Figure 6-51 4096 spectral samples are calculated with a maximum frequency
of 40960 Hz. This yields spectral samples at 10 Hz, 20 Hz, 30 Hz, ... ,
40960 Hz.
Note: Please be aware that energy contained at frequencies above the selected
maximum may distort results due to aliasing. To avoid this make sure to
select a maximum frequency high enough that no significant energy is
contained at higher frequencies.
On the other hand a maximum frequency significantly above the mean
datarate of the raw samples will not yield reliable results either.
Particle rate filter In fact the random seeding particle arrivals combined with the sample-andhold
technique used act as a first order low-pass filter with cut-off frequency
at &n 2π , where is the mean datarate (See Figure 6-52). This so-called
particle rate filter will attenuate the high-frequency components of the signal
(i.e. the measured velocity) with f -1 above the cut-off frequency, and
consequently the power spectrum estimate will be attenuated with f -2 &n
.
BSA Flow Software:Options and Add-ons 6-53