72 Agilent InfiniiVision 2000 X-Series Oscilloscopes User's Guide
4 Math Waveforms
• "FFT Units" on page 73
• "FFT DC Value" on page 73
• "FFT Aliasing" on page 73
• "FFT Spectral Leakage" on page 75
• "Units for Math Waveforms" on page 65
FFT Measurement Hints
The number of points acquired for the FFT record can be up to 65,536,
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 greater time/div
setting 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.
For the best vertical accuracy on peak measurements:
• Make sure the probe attenuation is set correctly. The probe attenuation
is set from the Channel Menu if the operand is a channel.
FFT Resolution
The FFT resolution is the quotient of the sampling rate and the number of FFT points (f
S
/N).
With a fixed number of FFT points (up to 65,536), the lower the sampling rate, the better the
resolution.
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