5 Measurements and Math Functions
202 InfiniiVision 7000B Series Oscilloscopes User’s Guide
FFT Measurement Hints
The number of points acquired for the FFT record is 1000 and when
frequency span is at maximum, all points are displayed. Once the FFT
spectrum is displayed, the frequency span and center frequency controls
are used much like the controls of a spectrum analyzer to examine the
frequency of interest in greater detail. Place the desired part of the
waveform at the center of the screen and decrease frequency span to
increase the display resolution. As frequency span is decreased, the
number of points shown is reduced, and the display is magnified.
While the FFT spectrum is displayed, use the [Math] and [Cursors] keys to
switch between measurement functions and frequency domain controls in
FFT Menu.
Decreasing the effective sampling rate by selecting a slower sweep speed
will increase the low frequency resolution of the FFT display and also
increase the chance that an alias will be displayed. The resolution of the
FFT is the effective sample rate divided by the number of points in the
FFT. The actual resolution of the display will not be this fine as the shape
of the window will be the actual limiting factor in the FFTs ability to
resolve two closely space frequencies. A good way to test the ability of the
FFT to resolve two closely spaced frequencies is to examine the sidebands
of an amplitude modulated sine wave.
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