Agilent Technologies 54657A, Руководство пользователя

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FFT Measurement Hints – Continued
While the FFT spectrum is displayed, use the and Cursor keys to switch between
measurement functions and frequency domain controls in FFT menu. See the
end of the manual for display menus.
Decreasing the effective sampling rate by selecting a slower sweep speed 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 one-half of
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 FFT’s 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 amplitude modulated sine
wave. For example, at 2 MSa/sec effective sampling rate, a 1 MHz AM signal
can be resolved to 2 kHz. Increasing the effective sampling rate to 4 MSa/sec
reduces the resolution to 5 kHz.
For the best vertical accuracy on peak measurements:
•
Make sure the source impedance and probe attenuation is set correctly.
The impedance and probe attenuation are set from the Channel menu if
the operand is a channel.
•
Set the source sensitivity so that the input signal is near full screen, but
not clipped.
•
Use the flattop 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 f1 cursor on the frequency of interest.
•
Press Move f1 to Center softkey.
•
Adjust frequency span for better cursor placement.
•
Return to the Cursors menu to fine tune the f1 cursor.
For more information on the use of window please refer to Agilent Application
Note 243," The Fundamentals of Signal Analysis" Chapter III, Section 5 (Agilent
part number 5952-8898.) Additional information can be obtained from "Spectrum
and Network Measurements" by Robert A Witte, in Chapter 4 (Agilent part
number 5960-5718.)
Operating the Measurement/Storage Module
FFT Measurement
2–13