Tektronix OM4006D, User Manual

Page: 76 / 205

Operating basics
Bit-Error-Rate reporting Bit error rates are determined by examination of the data payload. You may
choose BER or Differential BER. Differential BER compares the output of a
simulated delay-line interferometer to a differential form of the data pattern
speci
fi ed in the Analysis Parameters. If you choose to pre-code your data signal
prior to the modulator as in a typical differential transmitter, you will need to enter
the patterns seen at the I and Q modulators into the respective pattern variables,
(for example, PattXRe.Values and PattXIm.Values). If no pre-coding is used,
then you may use the drop-down menus to specify standard PRBS codes. (See
page 38,
Front end fi ltering .)
For multilevel signal types such as QAM, the Gray code BER or the direct BER
may be reported. The check box BER: Apply gray coding for QAM under
Analysis Parameters selects which BER type is reported. More information
is given in a detailed application note on automated BER measurements at the
end of this manual.
PMD measurement Polarization mode dispersion (PMD) is an effect associated with propagation
through long distances of optical
fi ber that degrades the signal through
inter-symbol interference. It is described by several coef fi cients. Often the fi rst
order and second order coef fi cients are all that is needed. The OUI can estimate the
amount of PMD that a signal has experienced from the structure of the waveform.
The method used is described in the research paper by Taylor
1 . The PMD
measurement works with dual polarization signals. Two kinds of measurement are
possible, reference based and non-reference based, according to the check box in
PMD: Use PMD Reference under Analysis Parameters. If the non-reference based
measurement is chosen then the OUI estimates the PMD directly from the signal.
The
fi rst and second order PMD values are reported in the Measurements window.
The reference based measurement is sometimes more accurate than the
non-reference based measurement. If the signal itself has an offset in time
between the X and Y polarizations then with the non-reference measurement that
offset is effectively added to the reported PMD values. With the reference based
measurement, that offset between polarizations is taken into account, by
fi rst
acquiring a measurement (the reference) direct from the transmitter.
It is necessary to tell the OUI when the reference is being acquired, and that is done
by checking the PMD: Acquire PMD Reference check box. When the reference
measurement is complete this check box must be unchecked. The reference-based
measurement uses a linear impulse response, and the settings under Averaging
(See page 48,
Waveform averaging .), also apply to the reference-based PMD
measurement. The PMD: Measure PMD check box must be checked for the PMD
values to be reported in the Measurements window.
1 M.G. Taylor & R.M. Sova, “Accurate PMD Measurement by Observation of Data-Bearing Signals,” IEEE Photonics
Conf. 2012, paper ThS4, Burlingame CA, 2012.
52 OM4000D Series Coherent Lightwave Signal Analyzer