Stanford Research Systems SR785, Руководство по эксплуатации и программированию

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4-110 Average Menus
SR785 Dynamic Signal Analyzer
Where X and Y are the real and imaginary parts of the instantaneous FFT measurements
and the <> angle brackets indicate either linearly weighted or exponentially weighted
averaging over N FFT records. For other measurements, the definition of the RMS
averaged measurement depends upon the measurement. In the above example, the RMS
averaged FFT is real, and has zero phase. In general however, rms averaged
measurements have real and imaginary parts and have non-zero phase.
Linear averaging computes the equally weighted mean 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.
Vector Averaging
For a simple FFT measurement, the definition of the Vector average measurement is:
Vector Avg(FFT) = <X> + j<Y>
Where X and Y are the real and imaginary parts of the instantaneous FFT measurements
and the <> angle brackets indicate either linearly weighted or exponentially weighted
averaging over N FFT records. For other measurements, the definition of the vector
averaged measurement depends upon the measurement.
Vector averaging is often called time averaging, since vector averaging in the frequency
domain yields the same value as taking the FFT of the averaged input time record.
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 should be set to 100% when vector averaged measurements
are being used.