Tektronix OM4006D, User Manual

Page: 107 / 205

Core processing software guide
Clock recovery The variable Vblock is a 1x4 array of data structures, and contains the four
oscilloscope channel voltages vs. time. Vblock(1) is a time series (having .t0,
.dt and .Values
fi elds) of scope channel 1 voltage, and similarly for Vblock(2)
... Vblock(4).
The fi rst step of clock recovery is to fi nd the symbol clock frequency and phase,
via a call to the function EstimateClock. (See page 97, EstimateClock .) In
addition to Vblock, EstimateClock takes as inputs ChDelay and pHyb. ChDelay
contains the relative delays between the four scope channels, due to cable length
mismatches for example. It is obtained from the delay calibration routine.
(See page 131,
Delay adjustment (system deskew) (ET) .) (See page 111, Delay
adjustment (system deskew) RT .)
pHyb is a 2x8 array of complex values that describes any non-idealities in the
optical hybrid within the OM4000. If the phase angle deviates from 90° or if the
polarizations are not perfectly orthogonal, that is included in pHyb and corrected
within CoreProcessing.
The main output of EstimateClock is SymClock, which has fi elds .t0 and .dt.
SymClock contains the recovered symbol clock and phase.
Next, ClockRetime uses the newly recovered symbol clock to retime the raw scope
records Vblock to a time series of Jones vectors, output pSym. Each element of
pSym is timed at the center of the symbol. ClockRetime also uses calibration
parameters ChDelay and pHyb, so that it corrects for non-idealities in the receiver.
Initial polarization estimate
The optical signal typically has a random state of polarization compared to the
axes of the hybrid within the OM4000. This means that each polarization of
pSym (each of the two rows of pSym.Values) contains some of the signal content.
The next step is to apply a polarization rotation so that one polarization (the X
polarization) contains the signal, and the other has only a low level of noise.
OM4000D Series Coherent Lightwave Signal Analyzer 83