EXFO FTB -5240S/5240BP Optical Spectrum Analyzer for FTB 500 (NS-1434) - Manual (Page 137)

Setting Up the Instrument in Drift Mode
Optical Spectrum Analyzer 129
Setting Up Drift Analysis Parameters
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Peak detection level (dBm): indicates the minimum power level
from where the peak can be considered as a signal.
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RBW for OSNR (nm): indicates the resolution bandwidth for the
selected OSNR value. The resolution bandwidth of an OSA
determines its ability to deal with close optical channel spacing. It
is measured as the width of the response curve at half peak power
(i.e., 3 dB down) of the instrument to a monochromatic test signal.
The instrument’s RBW value is written below the graph on the right
(below wavelength offset).
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Wavelength offset (nm): indicates the offset value applied on the
wavelength. You can use an offset to adjust your unit. This does not
replace a calibration performed at EXFO, but it can help you
achieve the specifications if you have determined that, for
example, your modules are used beyond the normal allowed use.
Entering a value in THz is not possible. The wavelength offset
appears below the graph, on the right, next to the
arrow.