Campbell Scientific ID-2000W Software - Manual (Page 50)

SECTION 4. FFT VIEW
4-4
FIGURE 4-3. Selecting FFT Data
There are several options that may be set via
the FFT Preferences dialog including plot type,
FFT size, data windowing type, use of decibels,
and plot style. For more information refer to the
section on ID-2000 Preferences. You may
modify the FFT plot without going back to the
Plot view and reselecting the FFT data by
simply changing these options in the FFT
Preferences. For example, if you currently have
a 1024 point Amplitude Spectrum plot you can
easily switch to a 512 point Power Spectrum
plot using decibels and any type of data
windowing by selecting the appropriate options
in the FFT Preferences dialog. The FFT view
will automatically redraw to accommodate the
new options.
If you change the FFT size in the FFT view ID-
2000 attempts to keep the data section used for
FFT calculation “centered” about the same point
as the current FFT. This is no problem if you
select a smaller FFT size. However, selecting a
larger FFT size could result in the center point
of the FFT data having to be shifted.
If the current center data point is less than one
half the FFT size from the beginning of the file it
will be shifted forward to the point
corresponding with half the FFT size. The
entire data section used for calculating the FFT
will be set as the first FFT size data points.
Consider as an example a 1024 point FFT
centered about the 1500th data point in the file.
Now assume the FFT size were changed to
4096. The 1500th point could no longer be the
center of a 4096 point FFT since it is too close
to the beginning of the file. Therefore it would
automatically be moved to the 2048th point
which would correspond to the FFT data
ranging from the first to the 4096th data point.
A similar condition exists if the current center
data point is less than one half the FFT size
from the end of the file. In this case it will be
shifted backward to the point corresponding to
file’s total number of data points minus half the
FFT size.