10–21
FFT Setup
The constant K in the illustration includes the decimation factor
described above, as well as automatic display scaling factors.
This scaling is required to ensure that the FFT’s horizontal
display scale falls into a 1,2, or 5 factor.
In essence, the oscilloscope automatically adjusts the span (and
hence the FFT transform size) to account for the user-entered
“max points for math” and the display scaling. In some
combinations of these factors it also adjusts the length of the
displayed trace.
The user can determine the span from the Displayed Trace
Label for the math trace containing the FFT. This will show the
horizontal calibration in MHz/div. The span is also explicitly
shown as the Nyquist frequency in the information field that
appears at the bottom of the screen when the “MATH SETUP”
menus are displayed, as shown here.
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