EXFO PSO-200 Optical Modulation Analyzer - Manual (Page 22)

Setting Up the Optical Modulation Analyzer
14 PSO-200
Configuring the Input Signal

Bit format (RZ or NRZ): this should be used according to the pulse
carving status of your signal under test (RZ for pulse-carved signals,
NRZ otherwise, as explained in Modulation Schemes on page 247).
This setting affects measurements associated with the waveform.

Symbol rate (in GBd): symbol rate of the input signal. An accurate rate
(±0.05 GBd or better) is needed to recover the correct waveform (via
pattern synchronization) and to get valid time-domain measurements
from the time reconstruction algorithms (see Sampling Methods on
page 240).

Linewidth: controls the speed of the IF tracking algorithm and should
correspond to the signal source’s linewidth (approximately
±0.25 MHz).
If it is set too low, there will be remaining phase noise on the captured
waveform (constellation symbols spread in the phase direction) or
even the phase will not be accurately tracked (cycle slips will occur).
If it is set too high, the algorithm will remove phase noise not
originating from the signal source (constellation symbols will be
compressed in the phase direction).
A proper setting balances these two conditions (equal noise in both
amplitude and phase, or a constellation with symmetric points).