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© National Instruments Corporation 21 NI Spectral Measurements Toolkit User Guide
Figure 8 shows the shape of the equivalent filter corresponding to a 7-Term
Blackman-Harris window. The cursors are placed at the 3 dB points of the
filter response, and the resolution bandwidth is the distance between the
cursors.
spectral lines controls how many frequency bins are present in the zoom
spectrum result that the VI displays. If you request more spectral lines
than resolution bandwidth requires, the parameter zero-pads the FFT to
interpolate the spectrum to the desired number of lines.
window controls the window applied to the time-domain signal. The
major benefit of windowing is to confine spectral leakage to the main lobe,
thereby reducing it in the side lobes, as shown in Figure 8. The window
determines the shape of the equivalent filter of an FFT bin; therefore,
the window choice influences any calculations involving the resolution
bandwidth.
The configuration VIs use resolution bandwidth, spectral lines, and
window to determine the acquisition size, which is the number of points
that you must acquire for a particular zoom operation. You must specify
a value in at least one of the two parameters: resolution bandwidth or
spectral lines. If you specify a value in only one parameter, the value
determines the acquisition size, and the acquisition size value determines
the value of the other parameter. If you specify both resolution bandwidth
and spectral lines, the VI compares the acquisition size that each parameter
requires and uses the smaller of the two as the actual acquisition size. For a
real input signal, the acquisition size that the spectral lines value determines
is calculated by the following formula:
acquisition size = 2 × spectral lines × zoom factor
where zoom factor relates the zoom span to the full spectrum as follows:
For a real input signal, the acquisition size that the resolution bandwidth
value determines is calculated by the following formula:
zoom factor
f
s
2⁄
span
-----------=
acquisition size
3 dB BW[]f
s
×
RBW
------------------------------------=