Tektronix OM4006D, User Manual

Page: 110 / 205

Core processing software guide
Count bit errors A decision is made on each component of the QPSK signal by testing whether
zXSym > 0. The decided values are compared with the known true data values,
via the xor function, to locate bit errors. The number of bit errors is stored in
variable Errs.
The decision threshold Q-factor of each component is calculated using the
QDecTh function. The outputs of QDecTh are the Q-factor and also the points of
inverse error function vs. decision threshold that are seen in the Q plot in the OUI.
The mean and standard deviation of the constellation points are calculated later
in EngineCommandBlock. (See page 87,
Block processing .) These quantities
are based on the sum and sum-of-squares of the constellation points, which are
calculated as
fi elds of zXSym.
Calculate signal vs. time
on fi ne time grid
All of the manipulations of the signal so far have been on a representation of the
signal at one sample per symbol, timed at the center of the symbol. The various
eye diagrams and constellation diagrams in the OUI contain traces that appear
as smooth lines over time. The
fi nal task in CoreProcessing is to calculate these
fi ne traces.
The fi rst call to function ClockRetime (See page 83, Clock recovery .) used
SymClock as the input variable de fi ning the clock frequency and phase.
ClockRetime is called again, this time with TraceClock as the clock input variable.
TraceClock has
fi eld .dt which is TracePtsPerSym time smaller than SymClock.dt.
The output of this call to ClockRetime is assigned to variable p.
The polarization and phase adjustment operations are repeated with fi ne trace
variable p. It is multiplied by the polarization rotation Jones matrix, and the X and
Y polarizations are separated (into variables zX and zY). The phase adjustment is
applied to zX and zY via the ApplyPhase function. zX and zY contain the
fi ne
traces that are displayed as eye diagrams. For single polarization QPSK format
signals, zX contains a modulated waveform, while zY contains only a low level
of noise.
86 OM4000D Series Coherent Lightwave Signal Analyzer