National Instruments NI Spectral Measurements Toolkit - Manual (Page 10)

NI Spectral Measurements Toolkit User Guide 10 ni.com
Cross Power Spectrum Measurement
The Cross Power Spectrum Measurement example is located at samples\
smt\simulated\smtcrspwr\smtcrspwr.prj .
This example demonstrates how to configure the zoom FFT, then calculates
the averaged power spectrum of a stimulus and response signal from
a device under test and the averaged cross spectrum between these
two signals. The example uses the continuous zoom FFT technique and
demonstrates how you must create a unique handle for the stimulus data
(handle1) and response data (handle2). Use handle1 when you calculate
the continuous zoom FFT and averaged power spectrum on the stimulus.
Use handle2 when you calculate the continuous zoom FFT and averaged
power spectrum on the response. You can use either handle1 or handle2
when you calculate the averaged cross power spectrum between the
stimulus and the response.
Using Spectral Measurements Toolkit Techniques
The Spectral Measurements Toolkit contains tools for block and
continuous zoom processing and spectrum averaging. You can specify the
zoom characteristics in terms of center frequency, span, and resolution
bandwidth. You can use spectrum averaging to reduce the effect of noise
on your measurement system.
Zoom FFT
The Spectral Measurements Toolkit uses the zoom FFT technique to
analyze the frequency spectra of stationary signals. This technique allows
you to zoom in on a small portion of the frequency spectrum with
high-frequency resolution by using fewer calculations than a standard FFT.
Figure 2 illustrates how a zoom FFT detects the presence of two tones of
closely spaced frequencies. The standard FFT, shown in the upper graph,
indicates a single peak, and the zoom FFT, shown in the lower graph,
indicates the presence of two separate tones in the signal.