EXFO FTB -5240S/5240BP Optical Spectrum Analyzer for FTB 500 (NS-1434) - Manual (Page 138)

Setting Up the Instrument in Drift Mode
130 FTB-5240S/5240BP
Setting Up Drift Analysis Parameters
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Power offset (dB): indicates the offset value applied on the power.
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. The power
offset appears in the graph, on the left, above the
arrow.
To edit the power offset value in percentage, press Edit % button.
The percentage value entered in Edit percentage will be
converted to a corresponding equivalent value in dB.
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Bandwidth at (dB): Set the power level used, relative to the
channel peak power, to compute the bandwidth.
5. Press OK to save the changes and close the window, or press Cancel to
exit without saving.
Press Restore Defaults to remove all the changes and apply the default
values.