Stanford Research Systems SR780, Operating Manual And Programming Reference

Page: 275 / 497

FFT Average Menu 4-93
SR780 Network Signal Analyzer
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. 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.
The Time Record Increment is ignored whenever the measurement is vector averaged.
Overlap processing is not allowed in this case and 100% increment is used.
Peak Hold Averaging
Peak Hold is not really averaging, rather the magnitude of the new data is compared to
the magnitude of the averaged data, and if the new magnitude is larger, then the new data
becomes the averaged data. This is done on a frequency bin by bin basis. The result is
averaged data with the largest magnitudes which occurred over a number of
measurements. Peak Hold can compare a Fixed Number of measurements or run
Continuously.
Peak Hold detects the peaks in the magnitudes but stores the data points in their original
complex form.
Averaging Type
Select the Averaging Type (weighting) for the active display. Choose Linear or
Exponential if the Averaging Mode is RMS or Vector. Choose Fixed Length or
Continuous if the Averaging Mode is Peak Hold.
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.