Time Figure Of Merit (Tfom) - Spectracom PRISMA VelaSync User Manual

3.4 Managing the Oscillator
4.
Click Submit.
3.4.1.1 Time Figure of Merit (TFOM)
The TFOM reflects the estimated error range values between the reference 1PPS (such as GPS
1PPS) and the System 1PPS which is being aligned to the 1PPS. The estimated error is referred
to as the 1PPS Phase error. TFOM values are ranges of these phase errors. The larger the
phase error estimate, the larger the TFOM value will be. For example, TFOM 3 is reported
when the estimated phase error is any value between 10 ns to less than 100 ns of the offset
between the selected 1PPS reference and the system's 1PPS.
TFOM is VelaSync's estimation of how accurately it is synchronized with its time and 1PPS ref-
erence inputs, based on several factors, known as the Estimated Time Error or ETE. The larger the
TFOM value, the less accurate VelaSync believes it is aligned with its 1PPS input that is used to
perform disciplining. If this estimated error is too large, it could adversely affect the per-
formance of oscillator disciplining. The available TFOM range is 1 through 15. You may refer
to the following for the TFOM to ETE conversions:
Table 3-5:
Reported TFOM Value
1
2
3
144
cause the disciplining tracking to restart automatically (after a few minutes delay)
if that limit is exceeded. This will help to quickly re-align the System 1PPS with a
reference.
When using a Host Reference as a primary or backup reference, for improved
performance it is recommended to set the phase error limit for NTP to a suggested
value of 100000 ns (= 1 second). Adjust this value as needed, based on your
accuracy requirements.
Restart Tracking : Check this box, and click Submit if you want to manually restart
disciplining tracking.
This option causes the disciplining algorithm to stop tracking the input reference
and start over (as if it was just acquired). This can be useful if there is a large
phase offset between reference 1PPS and system 1PPS, as it may occur when
going back into sync to the external reference after a long holdover. A Restart
Tracking will re-align the system 1PPS with the reference 1PPS very quickly, but
may cause the 1PPS output to jump.
Recalibrate : In rare cases, existing calibration data may no longer be suitable to
calibrate the oscillator. This function will delete the existing calibration data, and
begin a new calibration process (not applicable for low phase-noise Rubidium
oscillators).
TFOM to ETE conversion
Estimated Time Error (ETE)
<= 1 nsec
1 nsec < ETE <= 10 nsec
10 nsec < ETE <= 100 nsec
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3 VelaSync User's Manual Rev. 1