EXFO FTB-5230S Optical Spectrum Analyzer for FTB 200 v2 - Manual (Page 121)

Setting Up the Instrument in Drift Mode
OSA/OCA 115
Setting Up Drift Analysis Parameters
4. Under Global analysis parameters, define the following parameters
as needed:

Peak detection level (dBm): indicates the minimum power level
from where the peak can be considered as a signal.

RBW for OSNR (nm): indicates the resolution bandwidth selected
for the OSNR calculation. This parameter is generally set to 0.1 nm
to allow for a common basis of comparison between different
OSA/OCAs having different effective resolutions. The instrument’s
RBW value is written below the graph. This parameter does not
actually have an effect on the acquisition, but is only a
normalization factor used to provide the OSNR value in a
standardized manner.