Tektronix OM4006D, User Manual

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Core Processing function reference
Rail0 – structure describing straight line fi t to 0 rail:
.S – row vector of signal values used for fi t, at which decision threshold
was set
.Q – inverse error function of bit error rate at decision threshold values S
.Mean – mean signal of 0 rail, based on straight line fi t
.Sigma – standard deviation of noise on 0 rail, based on straight line fi t
Rail1 – structure describing straight line fi t to 1 rail:
.S – row vector of signal values used for fi t, at which decision threshold
was set
.Q – inverse error function of bit error rate at decision threshold values S
.Mean – mean signal of 1 rail, based on straight line fi t
.Sigma – standard deviation of noise on 1 rail, based on straight line fi t
BoundValsOut – structure of boundary values to be passed to next block
AlertsOut – structure of alerts including any alerts raised during execution of
function
This function uses the decision threshold method [1] to estimate the Q-factor of a
component of the optical signal. The method is useful because it quickly gives
an accurate estimate of Q-factor (the output signal-to-noise ratio) even if there
are no bit errors, or if it would take a long time to wait for a suf
fi cient number
of bit errors.
The actual signal vs. time, including noise and distortions, is provided as input S,
together with the true data sequence Seq it corresponds to. The function varies the
decision threshold and counts the number of bit errors at each decision threshold,
then
fi ts a straight line to the points of (decision threshold, inverse error function
of bit error rate). The maximum decision threshold used (farthest away from the
middle of the rail) is just before the number of errors counted is too small to be
statistically signi
fi cant.
The minimum decision threshold used in the straight line fi t (closest to the middle
of the rail) is given by MinStdDevFit. Negative values of MinStdDevFit are
acceptable. The value for MinStdDevFit is typically chosen by plotting the rails
from the middle of the rail
fi rst (MinStdDevFit = 0), and then clipping to the
region where there is no curvature in the (.S,.Q) points.
References N.S. Bergano, F.W. Kerfoot, C.R. Davidson, "Margin measurements in optical
ampli
fi er systems," IEEE Phot. Tech. Lett., vol. 5, no. 3, p. 304-306, 1993.
OM4000D Series Coherent Lightwave Signal Analyzer 105