BSA Flow Software Installation and User's Guide - CSI

6.4.3 Correlation object
Conclusion
Properties of the Correlation object
The Advanced Spectrum Add-on to Dantec's BSA Flow Software is
relatively insensitive to data rate and produces correct spectral estimates up
to a frequency corresponding to the data rate. This is a dramatic
improvement over traditional sample and hold, which requires 10-20 times
higher data rates.
The variance of the spectrum from the Advanced Spectrum Add-on is higher
than the one obtained using sample and hold, but can be reduced by
increasing the sample size.
Using FFT-techniques to obtain results as quickly as possible the correlation
object will estimate the time autocorrelation function on the basis of raw
data-samples.
The FFT-approach require samples that are equally spaced in time, and to
achieve this, sample/hold and resampling of the raw data is done prior to the
actual FFT-analysis, with the resampling frequency determined from the
number of correlation samples requested, and from the desired maximum
lag-time.
The properties of the correlation object is shown in Figure .
Figure 6-62 Properties of the correlation object
Correlation samples The property Correlation samples determine the number of discrete lagtimes
at which the autocorrelation function should be estimated.
The FFT-technique used require this number to be an integer power of 2, and
consequently the user has to choose among a limited number of possible
values: 32, 64, 128, ... , 65536. If a different value is entered, the nearest
higher power of two will automatically be selected.
Maximum lag-time The property Maximum lag-time determines the largest lag-time at which
the autocorrelation function should be estimated. In the example in Figure
1024 correlation samples are calculated with a maximum lag-time of
10.24 ms, yielding correlation estimates at 0 µS, 10 µS, 20 µS, ... , 10.24 ms.
Blocking Using the FFT-approach to estimate correlations, the raw samples need only
cover a time-span corresponding to the desired maximum lag-time. If more
samples are available several “raw” estimates will be calculated, and the
final correlation estimate is the average of all these “raw” estimates.
Since the raw samples are divided in consecutive blocks of equal duration,
BSA Flow Software:Options and Add-ons 6-65