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FFT,
F&t
Fourier Transform
The
FLATTOP
window has the greatest frequency uncertainty of the windows, but it has
outstanding side lobe suppression and amplitude flatness. Use
FLATTOP
to transform periodic
signals.
The HANNING window is a traditional passband window found in most real-time spectrum
analyzers. The HANNING window offers a compromise between the
FLATTOP
and UNIFORM
windows. Use the HANNING window when transforming periodic or random data.
The values in the window trace range from -32,768 to 32,767 and are treated as fractional
numbers. No offset is used. When FFT is called, the average window value is computed and
used to correct the results in absolute units.
The Fourier transforms of the window functions (created with TWNDOW) are shown below.
Use the graphs to estimate resolution and amplitude uncertainty of a Fourier transform display.
Each horizontal division of the graphs equals l/sweep-time or
Fmax/BOO,
and represents two
trace array elements.
When the source is longer than the destination, the source is truncated to fit. When the source
is shorter than the destination, the last element is repeated to fill the destination.
Note
TWNDOW should always be set before
FFI’
is executed. Execute
MKREAD FFT
to
read the marker value as a frequency value instead of time value.
0
-10
-20
-30
-40
cn
-50
1
-60
ii
-70
-
-80
0
w
-90
n
-100
-110
-120
-130
-140
-150
-160"
-it--
cu115e
Figure 5-4. Hanning Filter Window
5-162 Programming Commands