Agilent Technologies 86038B User Manual page 298

If the Group Delay measurement appears to be noisy
If the Group Delay measurement shows a sudden large offset
If GD ripple is being observed
298
Make sure that the environment in which the system is placed is
stable. Also keep in mind that the instrument requires a 90
minute warm-up. During that time, measurements will show
drift. The instrument stability can be observed by attaching a
patch fiber and running the Real-Time Port Monitor to check the
power and stability. To run the Real-Time Port Monitor, attach a
mouse and click on System->Real Time Port Monitor (F12).
Sudden offsets in group delay are typically the results of a phase
wrap; refer to "Relationship of setup parameters to phase
wrapping" on page 63. Phase wraps occur when the phase
change from one step to the next exceeds 360 degrees. To avoid
phase wrapping, try decreasing the Modulation Frequency or
Step Size. Remember that if the Modulation Frequency is
changed, a new normalization needs to be performed.
The Auto Modulation frequency option will aid in avoiding
phase wraps when measuring fiber spools in stepped mode. To
enable Auto Modulation frequency, press the Sens/Res front
panel key and put a check in the Auto Modulation Frequency
box.
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 cavity that could be present in
the device under test.
A common problem that leads to GD ripple is dirty fiber
connections..
Refer to "Relationship of setup parameters to ripple amplitude
accuracy" on page 68
Agilent 86038B Photonic Dispersion and Loss Analyzer, Second Edition
for more information.