72 Keysight InfiniiVision HD3-Series Oscilloscopes User's Guide
4 FFT Spectral Analysis
• 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.
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 (or RBW
which is the quotient of the sampling rate and the number of FFT points), sample
rate, or bin size can be displayed in the FFT waveform grid.
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...
