Agilent Technologies 86038A User Manual page 267

Solving Measurement Problems Troubleshooting and Upgrades
To achieve the most accurate GD, any DGD component may need to be
removed. To measure GD with the DGD component removed, normalize
and measure for DGD (GD/CD + DGD/PDL, 70Hz IFBW, 201
Samples/Point). The GD measured in this mode is more accurate because
it is given at the mean of the DGD, thus the DGD component is removed. To
check how significant the DGD component is, view the GD with 'show
DGD component'. If the DGD component is negligible, the measurement
can be made faster without measuring DGD (GD/CD, 30Hz IFBW, 21 or 51
Samples/Point).
To achieve the lowest noise GD, when measuring DGD is not a
consideration, use the largest modulation frequency possible (2.5GHz,
unless limited by other considerations) with 30Hz IFBW and 21 or 51
Samples/Point. Smoothing the GD trace will further lower the GD noise.
When using smoothing, a larger number of wavelength steps also lowers
the noise. The drift effects of the measurement setup may be more
significant than the GD noise, so a faster measurement time (fewer
Samples/Point, fewer wavelength steps) may give more accurate results.
If the Group Delay measurement shows a sudden large offset
Sudden offsets in group delay are typically the results of a phase wrap
refer to "Relationship of setup parameters to phase wrapping" on page 45.
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.
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Agilent 86038A Optical Dispersion Analyzer, Third Edition