User`s guide

Measurements and Math Functions 6
InfiniiVision Oscilloscopes User’s Guide 241
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.