EXFO FTB-5600 Distributed PMD Analyzer - Manual (Page 140)

Optimizing Measurements
134 FTB-5600
Use a Receive Fiber
If you use a filter value that is too high, in the case below, 8000 m, you can
notice that there are not enough values to make proper measurements. It
would still be possible, but placing the markers in the exact positions is
very difficult as the flat section in the middle is not very clear.
The anti-causal filter has an effect on the PMD curves; it will cause them to
end before the actual end of the fiber. With a correct dynamic range, the
cumulative PMD curve ends halfway through the actual fiber length.
Use a Receive Fiber
If you want to take measurements at the end of the link, you must add a
receive fiber of at least the length of the filter you are using. For example, if
you use a spatial filter of 4000 m, you will need a 2000-m receive fiber.
Note: The receive fiber should have a low PMD and long beating length to help
you with your measurements.