Tektronix OM4006D, User Manual

Page: 143 / 205

Appendix C: Calibration and adjustment (RT oscilloscope)
Absolute power calibration
As of OM4000 version 1.2.0, OUI can plot signal data on an absolute scale
independent of the LO signal strength. This requires absolute scaling of the
pHybCalib.mat
fi le, which was not available on all earlier versions of OUI. The
following power calibration should be accurate to 15%:
1. Check the absolute scale by connecting a CW signal of known power (no
modulation) with suf
fi cient power to fi ll most of the oscilloscope display.
2. Be sure the OUI and LRCP are connected by checking for the green square
on the SetUp Tab.
3. Do a DC calibration. The OUI should display one group of symbols in the X
constellation. The distance from the center of that group to the origin is the
signal strength in root-Watts. Square this value and compare to the known
value in Watts.
4. To use the built in Magnitude measurement, choose BPSK signal type and
a clock frequency equal to twice the offset between the signal and LO
frequencies. This will display two clusters of points and the Magnitude
measurement will provide the average signal strength.
Laser linewidth factor
Refer to discussion on modifying the default value of Alpha. (See page 34.)
Receiver equalization
Receiver Equalization is a factory calibration. Digital equalization is applied
to the four channels of the OM4000 instrument to account for the non-ideal
frequency dependent response introduced by the coherent optical receiver and
the receiver radio frequency front end. Depending on the sampling rate of the
oscilloscope and the bandwidth of the OM4000 instrument, digital equalization
is applied so that the combined effect of the receiver and the digital equalization
fi lter meet a speci fi ed reference response:
Oscilloscope sampling rate
Reference magnitude
response
reference phase
≤ 2x the bandwidth of the
OM4000 (BW)
Flat Linear
> 2x the bandwidth of the
OM4000 (BW) 4th order digital (bilinear)
Bessel
fi lter with 3 dB cutoff
at BW + 2 GHz
Linear
OM4000D Series Coherent Lightwave Signal Analyzer 119