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 VersaSync’s estimation of how accurately it is synchronized with its time and 1PPS reference inputs, based on several factors, known as the Estimated Time Error or ETE. The larger the TFOM value, the less accurate VersaSync 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 performance of oscillator disciplining. The available TFOM range is 1 through 15. You may refer to the following for the TFOM to ETE conversions:
|Reported TFOM Value||Estimated Time Error (ETE)|
|1||<= 1 nsec|
|2||1 nsec < ETE <= 10 nsec|
|3||10 nsec < ETE <= 100 nsec|
|4||100 nsec < ETE <= 1 μsec|
|5||1 μsec < ETE <= 10 μsec|
|6||10 μsec < ETE <= 100 μsec|
|7||100 μsec < ETE <= 1 msec|
|8||1 msec < ETE <= 10 msec|
|9||10 msec < ETE <= 100 msec|
|10||100 msec < ETE <= 1 sec|
|11||1 sec < ETE <= 10 sec|
|12||10 sec < ETE <= 100 sec|
|13||100 sec < ETE <= 1000 sec|
|14||1000 sec < ETE <= 10000 sec|
|15||ETE > 10000 sec|
TFOM is a value between 1 and 15. TFOM can never exceed the default MaxTFOM value of 15.
Typically the MaxTFOM requires no adjustment, but in some instances it may be advisable to decrease MaxTFOM so that TFOM can potentially exceed it: For example, by lowering the MaxTFOM to “5” it is now possible for TFOM to be always higher than the MaxTFOM value:
Assuming the MaxTFOM is set to 5 and the TFOM happens to go to a 6, i.e. TFOM is now exceeding MaxTFOM. This condition will cause a 1PPS out of specification alarm to be asserted and the oscillator disciplining will change in order to speed-up the alignment of the system 1PPS to the selected reference (causing it to take less time getting closer into alignment with the reference):
This will cause the TFOM to start to decrease faster. Once TFOM no longer exceeds MaxTFOM because the System 1PPS is now much closer to the reference 1PPS, the disciplining slows back down again as the system 1PPS continues to be brought into alignment with the selected 1PPS input.