FFT Spectral Analysis 4
Keysight InfiniiVision 3000T X-Series Oscilloscopes User's Guide 95
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
FFT (Magnitude)
Units
FFT units are displayed as dB.
FFT (Phase) Units In this case, the vertical units are degrees or radians.
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.
Nyquist Frequency and Aliasing in the Frequency Domain
The Nyquist frequency is the highest frequency that any real-time digitizing oscilloscope can
acquire without aliasing. This frequency is half of the sample rate. Frequencies above the
Nyquist frequency will be under sampled, which causes aliasing. The Nyquist frequency is also
called the folding frequency because aliased frequency components fold back from that
frequency when viewing the frequency domain.
To Next Page
Loading...
Need help?
General
