Tektronix OM4006D, User Manual

Page: 137 / 205

Appendix C: Calibration and adjustment (RT oscilloscope)
ChDelay(3): delay of OM4000 RX channel Y-I relative to X-I
ChDelay(4): delay of OM4000 RX channel Y-Q relative to X-I
This will also include connecting cable and digitizer delays if the digitizer/cable
combination has not been de-skewed separately.
Manual ChDelay determination with a 1-pol BPSK input signal.
Once you have used
the sliders to set the delays at least close to their actual values, you can use the
shape of the phase-diagram curves to
fi ne tune as described here. It is best to do
this one polarization at a time using the following steps:
1. Delete everything from the MATLAB Engine Window except for
CoreProcessingCommands or CoreProcessing.
2. Perform a DC calibration (See page 111, DC calibration (RT) .).
3. Adjust the input polarization so the signal is mostly on Vblock(1) and
Vblock(2) which correspond to receiver channels X-I and X-Q. Depending
on your receiver connections, this may correspond to different oscilloscope
channels. This can be done using a polarization adjustor, or mathematically
by putting the following statements before CoreProcessing:
Vblock(3).Values = Vblock(3).Values*0.001;
Vblock(4).Values = Vblock(4).Values*0.001;
In either case it is good to adjust the fi ber somewhat to maximize the signals
on RX channels X-I and X-Q.
Now only the top slider will make any difference.
4. Click Run to get a repeating constellation and eye-diagram update.
5. Click the + and – to make 0.1-ps adjustments to the top slider until the
BPSK constellation as perfectly straight lines connecting the two groups of
constellation points as shown in the
fi gure above.
6. Displaying the X-Q eye will show the BPSK signal going into the “wrong”
quadrature. When the delay is set properly the X-Q signal shown should
only be noise.
7. Shut down RX channels X-I and X-Q by moving the polarization state of the
signal or by zeroing it by deleting the lines from step 3 and replacing them
with:
Vblock(1).Values = Vblock(1).Values*0.001;
Vblock(2).Values = Vblock(2).Values*0.001;
now only the bottom slider will matter.
8. Move the last slider using the + and – buttons to get 0.1-ps changes until the
constellation looks as it did with channels 1 and 2. Once again use the X-Q
eye as a measure of residual error as well as constellation quality.
OM4000D Series Coherent Lightwave Signal Analyzer 113