Campbell Scientific ID-2000W Software - Manual (Page 49)

SECTION 4. FFT VIEW
4-3
There is another option available for the FFT
calculation called “Data Windowing”. This is a
technique of modifying the FFT data prior to
calculating the FFT in order to accent the
results. There are different types of data
windowing that may be selected from the FFT
Preferences dialog. The types of data
windowing available are:
•
Rectangle or None
•
Exact Blackman
•
Hamming
•
Hanning
•
Parzen
•
Welch
Now you know the very basics about frequency
analysis and how the FFT settings can be used
to affect the FFT plot. There are several steps
that you have to take in order to produce an
Amplitude or Power Spectrum plot of your data.
STEP 1
Make a normal plot (X-Y or Strip Chart) of the
parameter for which you wish to create an
Amplitude or Power Spectrum plot. There are a
couple of restrictions to consider.
•
The data must have been recorded at a
constant rate. If multiple rates or random
sampling was used in your data file there is
no way to calculate the frequency ranges.
•
The data file must contain at least as many
data points as the FFT size you are going to
use. If your data file only contains 500 data
points then you obviously cannot use an
FFT size of 512, 1024, 2048, or 4096 since
there are not enough data points.
•
The current plot must have at least as many
data points plotted as the FFT size you are
going to use.
STEP 2
Select the View - Switch to FFT menu item or
click the FFT toolbar button to enter the FFT
data selection mode.
The cursor will change to the FFT selection
cursor:
Now you must tell ID-2000 what data to use for
calculating the FFT. This is done by moving the
mouse into the graph and placing the mouse so
that the highlighted area covers the data you
wish to use as illustrated in Figure 4-3. When
the desired data is highlighted click the mouse
button to create the FFT plot.