National Instruments NI Spectral Measurements Toolkit - Manual (Page 22)

NI Spectral Measurements Toolkit User Guide 22 ni.com
The acquisition size comes from the following basic relationship:
df = f
s
/N = 1/T
where N is equal acquisition size and RBW is the frequency resolution df
multiplied by the window spectral leakage correction factor of 3 dB
bandwidth.
If the spectral lines value requires a larger acquisition size than the
resolution bandwidth value requires, the VI uses zero-padding to
determine the number of FFT lines you need. If the resolution bandwidth
value requires a larger acquisition size than the spectral lines value
requires, the VI coerces resolution bandwidth to a value consistent with
the acquisition size you need and returns the value as actual resolution
bandwidth.
Note You might see actual values differ slightly from the values you need in two cases.
If the span and sampling frequency you need correspond to a zoom factor that is not an
integer, the VI coerces the zoom factor to an integer value, and the span varies accordingly.
The acquisition size also might vary slightly to ensure that you can use an efficient FFT
algorithm to optimize performance.
Choosing Useful Configuration Parameters
The choice of center frequency, span, and window type is application
dependent. For example, when testing CDMA signals, you might specify
a center frequency of 834 MHz and a span of 2 MHz. SMT supports nine
window types. 7-Term Blackman-Harris, which is the default window
type, has the highest dynamic range and is ideal for signal-to-noise ratio
type applications. The choice of spectral lines depends on the display
resolution you require on the plot.
The choice of RBW depends on a number of factors, such as the spacing
between the two tones that you want to identify and the amplitude of
these tones.