EXFO FTB-5230S Optical Spectrum Analyzer for FTB 200 v2 - Manual (Page 75)

Setting Up the Instrument in WDM Mode
OSA/OCA 69
Setting Up WDM Analysis Parameters
to define the fit and exclusion zones for your tests using the OSNR
distance and noise region fields. The exclusion zone is indirectly
obtained from the OSNR distance.

OSNR distance (GHz or nm): Except for the fifth order polyfit
selection, the OSNR distance is automatically set at the channel
edge, that is, at half of the channel width from the center
wavelength.
For the fifth order polyfit, the OSNR distance corresponds to the
distance from the channel peak to the center of the fit zone. It is
independent of the channel width.

Noise region: The noise region, or fit zone, defines the region
where the polynomial fit applies. Two identical regions are
centered at the OSNR distance.
Channel width
Noise
region
OSNR
distance
Exclusion
zone
OSNR
distance
Noise
region
Channel center