Stanford Research Systems SR780, Руководство по эксплуатации и программированию

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4-92 FFT Average Menu
SR780 Network Signal Analyzer
When Averaging is turned On, averaging starts immediately without pressing the
[Start/Reset] key. When Averaging is turned Off (after a completed average), non-
averaged measurements start immediately.
The Averaging of each display can be turned On and Off separately. This entry field can
be linked to both displays by using the [Link] key.
Command: FAVG (?) d {, i}
Averaging Mode
Select the Averaging Mode for the active display [RMS, Vector, Peak Hold].
For a Coherence measurement, the Averaging Mode is pre-defined and this selection is
ignored. For Auto or Cross Correlation, the Averaging Mode changes the definition of
the measurement.
The [Start/Reset] key resets the current average and starts a new average. The
[Pause/Cont] key pauses the average in progress. Pressing [Pause/Cont] again will
continue the average from where it was paused.
If Averaging is On, changing the Averaging Mode resets the average and starts the new
average immediately without pressing the [Start/Reset] key.
Each display has its own Averaging Mode. This entry field can be linked to both displays
by using the [Link] key.
Command: FAVM (?) d {, i}
RMS Averaging
RMS averaging computes the average of the real part (X) and imaginary part (Y) of a
measurement according to
Xavg = sqrt ( mean (X
2
) ) and Yavg = sqrt ( mean (Y
2
) )
Linear averaging computes the equally weighted mean of X
2
and Y
2
over N
measurements. Exponential averaging weights new data more than old data and yields a
continuous moving average.
RMS averaging reduces fluctuations in the data but does not reduce the actual noise floor
(squared values never cancel). With a sufficient number of averages, a very good
approximation of the actual noise floor can be measured.
Log Magnitude View is normally used with rms averaged measurements. Since rms
averaging involves squared values, the real and imaginary parts of the average are always
positive. The phase of an rms averaged measurement is limited to the first quadrant (0-90
deg).