EXFO FTB-5600 Distributed PMD Analyzer - Manual (Page 125)

Acquisition Data
Distributed PMD Analyzer 119
To measure the PMD, you need to take the frequency step according to the
PMD value. The reason is that the frequency sampling step must be less
than one half the period of DGD fluctuation to obtain the best
measurement.
A lower sampling rate will not allow a correct measurement, because of
the violation of the Nyquist sampling criteria. A sampling rate that is too
high will result in a measurement that is too noisy.
The figure below gives an example of a correct step and of a step which is
too large to measure the variation of the signal properly.
For example, if a PMD curve can span from 0 to 20 ps, one sampling step is
not enough to compute the curve over the entire PMD span. This is why
many scales are necessary to build the PMD curve.
One frequency step can measure from 20 % and saturates at 240 % of its
value. As an example, the 20 ps step can measure from 5 to 48 ps.