Tektronix OM4006D, Руководство пользователя

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Operating basics
Table 11: OUI: Analysis Parameters window (cont.)
Parameter Description
2nd Phase Estimate Checking this box forces Core Processing to do a second
estimate of the laser phase after the data is recovered. This
second estimate can catch cycle slips, that is, an error in
phase recovery that results in the entire constellation rotating
by a multiple of 90 degrees. Once the desired data pattern is
synchronized with the incoming data stream, these slips can be
removed using the known data sequence.
Homodyne (RT
oscilloscopes only) The fi rst step in phase estimation is to remove the residual IF
frequency that is the difference between the LO and Signal laser
frequencies. The function EstimatePhase will fail if it there is no
difference frequency. This case occurs when the Signal laser
is split to drive both the modulator and the Reference Input of
the receiver (ie. only one laser). Checking the Homodyne box
will prevent EstimatePhase from failing by adding an arti
fi cial
frequency shift, which is removed by EstimatePhase.
Phase estimation time
constant parameter
(Alpha) After removing the optical modulation from the measured optical
fi eld information, what remains is the instantaneous laser
phase fl uctuations plus additive noise. Filtering the sample
values improves the accuracy of the laser phase estimation by
averaging the additive noise.
The optimum digital
fi lter has been shown to be of the form:
1/(1+ α z
-1
)
where α is related to the time constant, τ , of the fi lter by the
relation
τ = –T/ln( α )
where T is the time between symbols.
So, an
α = 0.8 when the baud rate is 10 Gbaud gives a time
constant, τ = 450 ps, or a low-pass fi lter bandwidth of 350 MHz.
The value of α also gives an indication of how many samples
are needed to provide a good implementation of the fi lter since
the fi lter delay is approximately equal to the time constant.
Continuing with the above example, approximately 5 samples
(~
τ /T) are needed for the fi lter delay. This of course is not a
problem, but an α =0.999 would require 1000 samples and put a
practical lower limit on the record length and block size chosen
for the acquisition. As a simple rule, the record or block size
should be
≥ 10/(1- α ).
34 OM4000D Series Coherent Lightwave Signal Analyzer