Tektronix SPECMON3 Manual page 130

General Signal Viewing
BW Tab
The BW (bandwidth) tab allows you to change Resolution Bandwidth and Video Bandwidth settings,
and set the windowing method used by the transform process by selecting a filter shape (not present
for all displays).
Setting
RBW
Auto
Span/RBW ratio
Filter Shape
VBW
xxx
Filter Shape Settings
In the analyzer, computationally efficient discrete Fourier transform algorithms such as FFT (Fast Fourier
Transform) or CZT (Chirp-Z Transform) are generally employed to transform time-domain signals into
frequency-domain spectra. There is an assumption inherent in the mathematics of Discrete Fourier
Transforms that the data to be processed is a single period of a periodically repeating signal. The
discontinuities between successive frames will generally occur when the periodic extension is made to the
signal. These artificial discontinuities generate spurious responses not present in the original signal, which
can make it impossible to detect small signals in the presence of nearby large ones. This phenomenon is
called spectral leakage.
Applying a filter, such as Kaiser, to the signal to be transformed is an effective method to combat the
spectral leakage problem. Generally the filter has a bell shape. Multiplying the transform frame by the
filter function eliminates or reduces the discontinuities at the ends of the frame, however, at the expense of
increased RBW.
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Description
Sets the Resolution Bandwidth value to be used in the spectrum analysis view. The
value is italicized when Auto is selected.
When Auto is checked, the RBW is calculated as a percentage of the Span. Kaiser is
selected as the windowing method. When Auto is unchecked, the RBW is set by the user.
Selecting any Window other than Kaiser changes the RBW setting to manual.
If Auto is checked, this value is used to calculate the RBW. If Auto is unchecked, this
setting is not selectable.
Specifies the windowing method used for the transform (when Auto is unchecked).
(Spectrum and Spectrogram displays only.)
Adjusts the VBW (Video Bandwidth) value. (Spectrum and Spectrogram displays only.)
SPECMON3 & SPECMON6 Printable Help
BW Tab