Agilent Technologies 6000 Series User Manual page 175

6000 Series Oscilloscope User's Guide
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.
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.
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