NI Spectral Measurements Toolkit User Guide
18
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Configuring Zoom FFT VIs
When using Spectral Measurements Toolkit VIs, you must enter several
values to completely specify a zoom FFT. The Spectral Measurements
Toolkit provides two configuration VIs that select values for each setting
and that require you to enter a minimal number of values. The SMT Config
Zoom FFT VI configures the block zoom FFT. The SMT Config Cont
Zoom FFT VI configures the continuous zoom FFT. These configuration
VIs ensure that the input values are compatible and yield valid results.
Enter values for specific settings, and the configuration VIs calculate
the rest.
Center Frequency and Span
The two fundamental characteristics of a zoom FFT are
center frequency
,
which is the location of the zoom within the spectrum, and
span
, which
is the degree to which the FFT zooms in. For basic zoom applications,
center frequency and span are often the only values you must enter.
The following restrictions apply to the input values for
center frequency
and
span
:
•
center frequency
must fall within the
effective band
of the input
signal. The effective band is the frequency band in which the data from
the input signal is valid. You can use the effective band to exclude the
roll-off region of an analog antialiasing filter from consideration.
The effective band defaults to the full bandwidth of the input signal,
up to half the sample rate. If the specified frequency falls outside the
effective band, the configuration VI uses the center of the effective
band as the center frequency.
•
span
must be smaller than the effective band because you can only
zoom in on the spectrum. You cannot zoom out. If
span
is larger than
the effective band, the configuration VI sets the span to the largest
value that does not fall outside of the effective band.
•
When you combine
center frequency
and
span
, neither endpoint of
the desired frequency span can fall outside the frequency range of the
effective band. If both
center frequency
and
span
meet the previously
stated restrictions, but a portion of the zoom span region falls outside
the effective band, the configuration VI moves the center frequency far
enough to ensure that the entire span is inside the effective band.
