Tektronix OM4006D, User Manual

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Appendix C: Calibration and adjustment (RT oscilloscope)
Each slider indicates the relative delay for the given channel pair. The
corresponding waveforms will be shifted to account for this skew. Delay (1:2)
means the delay of RX channel 2 (the X-Q channel) relative to RX channel 1
(the X-I channel).
Use the sliders to get the best possible eye diagram and constellation diagram.
Improper skew will cause horizontal eye closure and
fi lling in of the constellation
diagram. If the I and Q channels have unequal delay, there will be a phase offset
proportional to the difference frequency between the reference and signal laser
oscillation frequencies. This phase offset will turn a straight line on the phase
diagram into a circle or a portion of a circle.
So, for veri fi cation, it is best to use a known Mach-Zehnder modulator to generate
a BPSK optical input. As the input signal polarization is adjusted, the I-Q diagram
should always be a straight line.
Programmers note: The ChDelay variable in the MATLAB workspace is de fi ned
as follows:
ChDelay(1): zero.
ChDelay(2): delay of OM4000 RX channel X-Q relative to X-I
112 OM4000D Series Coherent Lightwave Signal Analyzer