Stanford Research Systems SR785, Руководство по эксплуатации и программированию

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Display Setup Menu 4-31
SR785 Dynamic Signal Analyzer
Correlation is a real function and requires a baseband span (real time record). Non-
baseband time records do not preserve the original signal frequencies and thus do not
yield the correct correlation. The Start Frequency is always set to 0 in the Correlation
Measurement Group.
A window is applied to the time record of the first FFT in the computation. This is
because the FFT models the time domain as a single time record repeating itself over and
over. Computing the correlation over a
τ
greater than half of the time record length will
result in ‘wrap around’ errors where data starts to repeat itself. To avoid this, special
windows which zero half of the time record are used. The [0..T/2] window zeroes the
second half of the time record and the [-T/4..T/4] window zeroes the first and last quarter
of the time record. The [-T/2..T/2] is a uniform window which should only be used on
data which is self windowing (lasts less than half of the time record).
Cross Correlation
Cross correlation is a two channel measurement which is only available when the
[Input]<Analyzer Conig> softkey is set to 'Dual Channel.' In the time domain, it is a
comparison of a signal x(t) with a time shifted version of another signal y(t-
τ
) displayed
as a function of
τ
. This is useful for measuring time delays between two common signals.
The definition of Cross Correlation depends upon the Average Mode selected in the
[Average] menu.
Averaging
Cross Correlation = invFFT( FFTu2 • FFT1* )
Peak Hold or RMS Averaging On
Cross Correlation = invFFT(< FFTu2 • FFT1*> )
Vector Averaging On
Cross Correlation = invFFT( <FFTu2> • <FFT1*> )
where FFT1 is the windowed FFT of Channel 1 and FFTu2 is the un-windowed FFT
(uniform window) of Channel 2.
Correlation is a real function and requires a baseband span (real time record). Non-
baseband time records do not preserve the original signal frequencies and thus do not
yield the correct correlation. A display which is measuring a single channel correlation
will have its start frequency set to 0 Hz. A display which measures cross correlation will
set the start frequency of both displays to 0 Hz.
A correlation window is applied to the time record of the first FFT in the computation.
This is because the FFT models the time domain as a single time record repeating itself
over and over. Computing the correlation over a
τ
greater than half of the time record
length will result in ‘wrap around’ errors where data starts to repeat itself. To avoid this,
special windows which zero half of the time record are used. The [0..T/2] window zeroes
the second half of the time record and the [-T/4..T/4] window zeroes the first and last
quarter of the time record. The [-T/2..T/2] is a uniform window which should only be
used on data which is self windowing (lasts less than half of the time record).