EXFO FTB -5240S/5240BP Optical Spectrum Analyzer for FTB 500 (NS-1434) - Manual (Page 303)

Managing Results
Optical Spectrum Analyzer 295
Managing WDM Test Results
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Noise for OSNR: indicates which calculation method to use for OSNR
value.
Fixed range IEC based (IEC): The IEC method uses the interpolation
of noise measured on both sides of the signal to estimate the noise
level. The position at which the noise is estimated from the center
wavelength is given by the OSNR distance.
In-Band (InB): The In-Band method uses a series of scans having
different polarization states to calculate the noise level in band.
In-Band narrow filter (InB nf): The In-Band narrow filter method
uses the In-Band method with additional processing to provide an
accurate OSNR value for the narrow carved noise. This is because with
narrow filters, the noise level under the peak is not uniform and the
OSNR value depends on the processing width selected.