Tektronix OM4006D, Руководство пользователя, Страница: 156 / 205

Appendix E: Equivalent-Time (ET) oscilloscope operation
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.
Figure 18: ChDelay(2) off by 2 ps causes curvature on constellation and signal on
Q-Eye for 28 Gbps BPSK
Note that the sliders control the 1:2 delay, the 1:3 delay and the 3:4 delay (or 5:6,
5:7, 7:8 if right-hand mainframe slots are chosen). These are the most convenient
groups:
The 1:2 delay error causes I-Q crosstalk on channels 1 and 2
The 3:4 delay error causes I-Q crosstalk on channels 3 and 4
The 1:3 delay error causes X-Y crosstalk
The proper slider values are found by zeroing out these crosstalks one at a time.
First by looking only at channels 1 and 2, then only 3 and 4, then
fi nally all
channels on to see the X-Y crosstalk. Do the following:
1. Delete everything from the MATLAB Engine Window except for
CoreProcessingCommands
2. Perform a DC calibration (See page 111, DC calibration (RT) .).
3. Adjust the input polarization so the signal is mostly on oscilloscope RX
X-polarization channels. This can be done mathematically by disabling the
RX Y-polarization channels:
132 OM4000D Series Coherent Lightwave Signal Analyzer