EXFO FTB -5240S/5240BP Optical Spectrum Analyzer for FTB 500 (NS-1434) - Manual (Page 171)

Setting Up the Instrument in Drift Mode
Optical Spectrum Analyzer 163
Setting Up Acquisition Parameters
3. Select a sampling unit and enter a sampling count to configure a time
that should be elapsed between the start of each acquisition during a
drift measurement.
4. Select a duration unit and enter a duration count to configure the total
duration of a drift measurement.
5. Enter a drift file name that should be used to save a drift file.
6. Select a location where the drift file should be saved.