Spectracom - Essential Ingenuity

About MaxTFOM

Question

What is MaxTFOM field and what is used for?

Answer

The “Maximum TFOM” (Time Figure of Merit) field defines the largest TFOM value (TFOM is the NTP server’s estimation of how accurately it's synchronized with its Time and 1PPS reference inputs, based on several factors - known as the estimated time error or “ETE”) that is allowed before disciplining is no longer performed on the oscillator. The larger the TFOM value, the less accurate the disciplining of the internal clock with its 1PPS reference input.  If this estimated error is too large, it could adversely affect the performance of oscillator disciplining.  The available Max TFOM range is 1 through 15.

 Notes about TFOM / Max TFOM
  • The Default MaxTFOM value is 15 (in which case TFOM never exceeds Max TFOM).
  • MaxTFOM is configured/viewed on the Disciplining page of the web browser interface. It is available via the Management Menu in application software version 5 and above or the Setup Menu in version 4.
  • MaxTFOM can also be viewed in various status pages of the browser.
  • When the current TFOM (Time Figure OF Merit) value exceeds the user-defined Max TFOM value the unit will be reported as being in “Holdover”  mode, until the TFOM value falls below  the Max TFOM value. Important note:The input reference is still used to discipline the oscillator while TFOM is exceeding MaxTFOM.  So its not truly in “Holdover” mode.  It's just reported as being in “Holdover”, until MaxTFom is no longer exceeded.
  • When TFOM exceeds MaxTFOM, “1PPS not in Specification” alarm is asserted. See note about this alarm further below.  The "In Holdover" alarm/trap is also asserted.
  • When MaxTFOM is no longer exceeded, “1PPS Restored to Specification” is asserted. See note about this entry further below
  • If MaxTFOM is set to less than 15, User/User mode will not allow the unit to go into Sync.
  • There is a value for MaxTFOM that will always generate an error depending on the type of internal 10 MHz oscillator. It is rare that the unit will consistently achieve a TFOM of 1 so setting MaxTFOM to 1 will generate errors even with a high quality Rubidium oscillator installed. Crystal oscillators never achieve a TFOM higher than 3 or 4.

"1PPS not in Specification"/"1PPS Restored to Specification" entries in the unit's logs
These are events that occur and alarm entries that are asserted when TFOM exceed's MaxTFOM.  “1PPS not in Specification” occurs if the current TFOM values exceed the user-configurable max TFOM value.   When the TFOM falls below Max TFOM, the Restored to Specification event/alarm entry is asserted.

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