Agilent Technologies 86038A User Manual page 45

Description of the 86038A Measurement Concepts
Relationship of setup parameters to phase wrapping
Equation 4 implies that the selection of modulation frequency and
wavelength increment play a part in the potential problem of phase
wrapping. The phase detectors used in the Agilent 86038A have a range of
±180-degrees. Exceeding the range of the phase detector causes a phase
wrapping error. The wrap-free range is expressed by:
∆φ ⋅ ⋅ ⋅ ∆λ
= 360° D f
m
∆φ < ±
180°
Equation 6
Consider this limitation in the context of the stepped mode of operation.
After each wavelength step, the phase detector is automatically zeroed to
make the entire ±180-degree range available for the next step. If the next
wavelength step produces a phase change larger than ±180 degrees, the
phase detector loses the history of the actual phase change and gives us
only an apparent phase change. For example, if a wavelength step
produces an actual phase change of positive 185 degrees, it will be
interpreted by the phase detector as a change of negative 175 degrees.
This is the phase wrapping error. The phase detector is zeroed
mathematically, not physically, so there is no noise contribution,
regardless of the wavelength step size.
The user can avoid phase wrapping errors in the step mode in either of two
ways. For fiber measurements, use the Automatic Modulation Frequency
Selection feature, which adjusts the modulation frequency as the
measurement proceeds to avoid phase wrapping (the wavelength step is
not changed from the user selected value). Second, use Equation 6 to
check your selections of wavelength increment step and modulation
frequency, to assure that the ±180 degree limit is not exceeded.
Phase wrapping errors can also occur in the swept mode, though they are
less likely to occur when using small wavelength increments (large
numbers of amplitude and phase samples across the measurement
wavelength range).
Diagnosing a phase wrapping error is not always easy, because the extent
to which phase wrapping affects the shape of the measured relative group
delay curve depends upon the details of both the test device and the
measurement setup parameters. Be conservative in the use of Equation 6,
as it is possible for phase wrapping errors to go entirely unnoticed. Agilent
provides an Excel spreadsheet calculator for estimating the maximum
allowable step size. It is called the Wavelength Step Calculator for
Modulation Phase Shift CD Measurements and can be found in the Library
section of the public Agilent product web page for the 86038A.
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Agilent 86038A Optical Dispersion Analyzer, Third Edition