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Making Measurements Using Spectrum Analyzer Mode
Measuring Amplitude Modulation with the Fast Fourier Transform Function
Figure 5-1. FFT Annotation
Some of the screen annotation is altered when the FFT function is active.
The left edge of the graticule is relabeled FFT START and represents 0 Hz
relative to the carrier. The right edge of the graticule is relabeled FFT
STOP and is the maximum FFT frequency used in the transformation. The
annotation LIN in the upper left corner refers to the scale of the incoming
data being transformed. The FFT results, which are being displayed, are
always in LOG scale. The carrier appears at the left edge of the graticule
with the modulation sidebands and any distortion appearing along the
horizontal axis. The amplitude relationships of all the signals are the same
as they would be if the components were displayed with normal swept-tuned
operation in log mode, 10
dB
per division.
5-4