Tektronix OM4006D, Руководство пользователя

Страница: 135 / 205

Appendix C: Calibration and adjustment (RT oscilloscope)
Calibration and adjustment (RT)
The OM4000 receiver requires the following calibration before taking
measurements:
DC calibration (to compensate for any offsets in the photodiode outputs)
Delay adjustment (channel to channel skew in the scope connections)
Hybrid calibration (correction for cross-talk and phase error in the hybrid)
Laser linewidth factor (choosing the correct fi lter for phase recovery) (See
page 34.)
Receiver equalization
DC calibration (RT)
Although the OM4000D Series units use balanced detection, there is usually some
small offset voltage present at the signal outputs. This offset voltage depends
on the total optical power input to the system and so can change. The offset is
small enough that only large changes to the reference or optical input power
substantially affect the computation.
DC calibration determines the DC levels in the receiver’s photodiodes, and
subtracts this off within the analysis. This can be done as often as needed or
desired. If there is uncompensated dc offset in the system, this will be evident by
a smearing out of the constellation point groups. If the offset is large enough, the
point groups will begin to look like donuts. Perform a dc calibration whenever
there is any question.
To perform a DC calibration, click the DC Calibration button in the Calibrate tab.
Delay adjustment (system
deskew) RT
NOTE. Initial delay adjustment should be done by trained personnel. This section
is provided for experienced users. Delay adjustment should be done during
installation and should not require attention unless the scope is removed and/or
reconnected.
Delay adjustment among the four electrical channels of the system, including
the OM4000 receiver and oscilloscope, is done through the Channel Delay
Calibration section of the Calibrate ribbon. Once this is accomplished for a
speci
fi c oscilloscope installation, it should not need further attention, and should
be left untouched. If the receiver/oscilloscope interface is altered (for example,
installing new cables or shifting of the instrument), delay calibration may need
to be performed again.
Access to the delay sliders with the Enable Slider control in the Calibrate tab.
OM4000D Series Coherent Lightwave Signal Analyzer 111