NI Spectral Measurements Toolkit User Guide
22
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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.
