XLi Time & Frequency System 193
XLi-man, Issue 8, 6/17/2008, Rev. H
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Additionally, by continual integration of the measured frequency error, time information is derived from
the frequency measurement. From this integration, accumulated Time Deviation from nominal and
System Time are calculated.
Accumulated Time Deviation represents the amount of time a clock would gain or lose if it used the
measured line frequency as its timekeeping reference.
System Time represents the time that would be displayed by a clock using the line frequency as its
timekeeping reference. System Time is calculated by adding Time Deviation plus a user-entered
constant, Time Deviation Preset, to the XLi's local time.
The beginning point of the Time Deviation calculation is when the Time Deviation Preset is initialized.
Whenever a new Time Deviation Preset value is entered, the current accumulated value of Time
Deviation is overwritten by the Time Deviation Preset value and the Time Deviation calculation is
continued from this new value. Entering a value of zero (0) effectively resets the accumulated Time
Deviation.
The FTM processes the incoming line frequency on a cycle-by-cycle basis, performing frequency
comparison and calculations over single second periods continuously with no dead time between
measurements.
FTM frequency and time measurements are referenced to UTC when XLi is locked to GPS.
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