Math Waveforms 4
Keysight InfiniiVision 2000 X-Series Oscilloscopes User's Guide 81
• 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 Keysight Application Note
243, The Fundamentals of Signal Analysis at
http://literature.cdn.keysight.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 MΩ.
FFT units will be displayed in dBm when channel units is set to Volts and channel
impedance is set to 50Ω.
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 span, and oscilloscope
vertical bandwidth when making FFT measurements. The FFT resolution (the
quotient of the sampling rate and the number of FFT points) is displayed directly
above the softkeys when the FFT Menu is displayed.
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