EXFO FTB -5240S/5240BP Optical Spectrum Analyzer - Manual (Page 120)

Setting Up the Instrument in Drift Mode
114 FTB-5240S/S-P/BP
Setting Up Drift Analysis Parameters
Fifth order polynomial fit (Fit): The fifth order polyfit method
calculates the noise curve and thus the signal to noise ratio. The
OSA will approximate the noise curve using a fifth order
polynomial fit. This fit definition relies on fit and exclusion zones.
Only the points in the fit zones are used to calculate the fifth order
polynomial fit. If you select the fifth order polyfit method, you will
have 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