Agilent Technologies 86038A User Manual page 269

Solving Measurement Problems Troubleshooting and Upgrades
If GD ripple is being observed
If unexpected group delay ripple is observed, first consider the amplitude
of the ripple. The scale might be zoomed-in to reveal features that have not
been visible with other instruments due to trace noise.
Next, note the period of the ripple. If the wavelength increment can be
reduced and the period of the ripple remains the same, then the ripple is
not aliased. If the ripple period does change, then keep reducing the
wavelength increment to reveal the true period. Once the true period is
found, the cause of the ripple may be evident, such as an etalon the makes
sense for the device under test.
Another factor to consider is the step size used for normalization and the
step size used in the measurement may need to be the same. Also, make
sure that you have a valid normalization.
A common problem that leads to GD ripple is dirty fiber connections. See
"Example of a Dirty Connector'.
Refer to "Relationship of setup parameters to ripple amplitude accuracy"
on page 48 for more information.
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Agilent 86038A Optical Dispersion Analyzer, Third Edition