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.

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