Agilent Technologies 4000 X-Series User Manual page 103

4
Math Waveforms
• 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 Units
0 dBV is the amplitude of a 1 Vrms sinusoid. When the FFT source is
channel 1 or channel 2 (or channel 3 or 4 on 4- channel models), FFT
units will be displayed in dBV when channel units is set to Volts and
channel impedance is set to 1 M .
FFT units will be displayed in dBm when channel units is set to Volts and
channel impedance is set to 50 .
FFT units will be displayed as dB for all other FFT sources or when a
source channel's units has been set to Amps.
FFT DC Value
The FFT computation produces a DC value that is incorrect. It does not
take the offset at center screen into account. The DC value is not
corrected in order to accurately represent frequency components near DC.
FFT Aliasing
When using FFTs, it is important to be aware of frequency aliasing. This
requires that the operator have some knowledge as to what the frequency
domain should contain, and also consider the sampling rate, frequency
Agilent InfiniiVision 4000 X-Series Oscilloscopes User's Guide 103