Tektronix OM4006D, Руководство пользователя

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Operating basics
Table 11: OUI: Analysis Parameters window (cont.)
Parameter Description
The selection of the optimum value of Alpha is discussed later
in the CoreProcessing guide. (See page 99,
EstimatePhase .)
This optimum value depends on the laser linewidth and
level of additive noise moving from a value near 1 when the
additive noise is vastly greater than the phase noise to a
value near zero when phase noise is the only consideration
(e.g. no
fi ltering needed). In practice, a value of 0.8 is fi ne
for most lasers. If Alpha is too small for a given laser there
will be insuf
fi cient fi ltering which is evidenced by an elliptical
constellation group with its long axis pointed toward the origin
(along the symbol vector). When Alpha is too large then there
is excessive
fi ltering for the given laser linewidth. Excessive
phase fi ltering is evidenced by the constellation group stretching
out perpendicular to the symbol vector and may also lead to
non-ideal rotation of the entire constellation.
As is often the case, when laser frequency wander is greater
than the linewidth, very long record lengths will lead to larger
variance in laser phase. This means that an Alpha that worked
well with 5000 sample points might not work well with 500,000
points. Longer record lengths will not be a problem if you choose
a block size small enough that peak-to-peak frequency wander
is on the order of the laser linewidth. For the lasers supplied with
the OM4000 instruments, a block size of 50,000 points is a good
choice. Refer to Block Processing for more information. (See
page 30,
OUI Controls panel .)
Signal center freq Sets the approximate center frequency of the signal. If the signal
optical frequency is signi
fi cantly different from the local oscillator
frequency, then this control tells Core Processing where it is.
If entered incorrectly then frequency aliasing occurs, and the
constellation appears to rotate from one symbol to the next.
Balanced Differential
Detection (BDD) (RT
oscilloscopes only) Sets the differential-detection emulator to emulate balanced
instead of single-ended detection.
Continuous Traces Enables drawing fi ne trace lines that connect the constellation
points. If unchecked, the traces will be suppressed for
calculation speed if the calculations are not needed for other
plots such as eye diagrams.
Mask Threshold Sets the ratio of radius to symbol spacing used for the circular
constellation masks.
Symbol Center Width (ET
oscilloscopes only) Sets the fraction of the eye-center that should be considered
“symbol center” for the purpose of certain calculations such as
which symbols to color blue. The sample closest to symbol
center is used to represent that symbol for calculations such as
BER and Q-factor.
OM4000D Series Coherent Lightwave Signal Analyzer 35