Stanford Research Systems SR780, Operating Manual And Programming Reference

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FFT Averaging 2-23
SR780 Network Signal Analyzer
FFT Averaging
Averaging successive measurements together improves accuracy and repeatability. For
measurements computed from multiple spectra, averaging is performed on measurement
results or individual spectra depending upon the measurement definition. Time records
are not averaged.
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 amplitudes, 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).
Vector Averaging
Vector averaging computes the average of the real part (X) and imaginary part (Y) of a
measurement according to
Xavg = mean (X) and Yavg = mean (Y)
Linear averaging computes the equally weighted mean of X and Y over N measurements.
Exponential averaging weights new data more than old data and yields a continuous
moving average.
Since signed values are combined in the mean, random signals tend to average to zero.
This reduces the noise floor since random signals are not phase coherent from
measurement to measurement. Only signals with a constant phase have real and
imaginary parts which repeat from time record to time record and are preserved. Vector
averaging can substantially improve the dynamic range of a measurement as long as the
signals of interest have stable phases.
For single channel measurements, vector averaging requires a trigger. The signal of
interest MUST be phase synchronous with the trigger to have a stable phase.
For a two channel measurement, the phase is relative between Channel 2 and Channel 1.
As long as the signals of interest have stable relative phases, triggering is not required for
vector averaging. Triggering is still required to isolate time records which contain the
signals of interest.