National Instruments Toolkit - page 13

© National Instruments Corporation 13 NI Spectral Measurements Toolkit User Guide
The continuous zoom FFT technique is sometimes called the real-time
zoom FFT because it continuously performs the frequency shifting,
decimation, and filtering processes on the arriving data. The FFT operation
itself cannot proceed until you acquire all the data. The FFT operation then
occurs in parallel with the next data acquisition.
You can use the SMT Cont Zoom FFT VI to perform the continuous zoom
FFT technique. This VI is located on the Zoom FFT palette, which is a
subpalette of the SMT Advanced palette. This VI also passes the complex
modulated and filtered time-domain data corresponding to the spectrum.
Figure 4 shows the in-phase (I) and quadrature-phase (Q) components
oft his complex time-domain data plotted on an I versus Q plot. The input
signal is a phase-modulation (PM) wave. The PM signal is modulated with
a square wave, with a carrier at 70 MHz. The RF signal analyzer is tuned to
the carrier frequency.
Figure 4. I/Q Plot
15.0m
12.0m
14.0m
10.0m
8.0m
6.0m
4.0m
2.0m
0.0
–2.0m
–4.0m
–6.0m
–8.0m
–10.0m
–12.0m
–14.0m
–15.0m
–15.0m –10.0m –5.0m 1.7a 5.0m 10.0m 15.0m
I (Volts)
Q (Volts)