FFT Measurement Hints
Up to 65,536 points may be acquired for the FFT record, and when frequency span is at
maximum, all points are displayed. Once the FFT spectrum is displayed, the frequency span and
center frequency controls are used much like the controls of a spectrum analyzer to examine the
frequency of interest in greater detail. Place the desired part of the waveform at the center of the
screen and decrease frequency span to increase the display resolution. As frequency span is
decreased, the number of points shown is reduced, and the display is magnified.
While the FFT spectrum is displayed, use the
[Math]
and
[Cursors]
keys to switch between
measurement functions and frequency domain controls in FFT Menu.
Decreasing the effective sampling rate by selecting a greater time/div setting will increase the
low frequency resolution of the FFT display and also increase the chance that an alias will be
displayed. The resolution of the FFT is the effective sample rate divided by the 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 AM sine wave.
For the best vertical accuracy on peak measurements, use the Flat Top window and:
– Configure the input signal to be near full screen, but not clipped.
– Set the FFT vertical resolution to a sensitive range, such as 2 dB/division.
For best frequency accuracy on peaks, use the Hanning window, and:
– 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 DC Value
The FFT computation produces an incorrect DC value. It does not take the offset at center
screen into account. The DC value is not corrected 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 user to
have some knowledge as to what the frequency domain should contain, and also consider the
sampling rate, frequency span, and instrument 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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Keysight IntegraVision PA2200 Series Power Analyzers Operating and Service Guide
