Rigol DS1000C User Manual page 44

RIGOL
Key points for FFT
1 Signals that have a DC component or offset can cause incorrect FFT
waveform component magnitude values. To minimize the DC component,
choose AC Coupling on the source signal.
2 To reduce random noise and aliases components in repetitive or
single-shot events, set the oscilloscope acquisition mode to average.
3 To display FFT waveforms with a large dynamic range, use the dBVrms
scale. The dBVrms scale displays component magnitudes using a log
scale.
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© Copyright RIGOL Technologies, Inc. 2007.
User's Guide for DS1000 Series