Math Waveforms 4
Agilent InfiniiVision 3000 X-Series Oscilloscopes User's Guide 87
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.
• Set the source sensitivity so that the input signal is near full screen,
but not clipped.
• Use the Flat Top window.
• Set the FFT sensitivity to a sensitive range, such as 2 dB/division.
For best frequency accuracy on peaks:
• Use the Hanning window.
• Use Cursors to place an X cursor on the frequency of interest.
• Adjust frequency span for better cursor placement.
• Return to the Cursors Menu to fine tune the X cursor.
For more information on the use of FFTs please refer to Agilent
Application Note 243, The Fundamentals of Signal Analysis at
"http://cp.literature.agilent.com/litweb/pdf/5952-8898E.pdf". Additional
information can be obtained from Chapter 4 of the book Spectrum and
Network Measurements by Robert A. Witte.
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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