TFOM and TSync Board Monitoring
Technical & Application Notes
Introduction
The purpose of this tech brief is to offer an overview of the Time Figure of Merit (TFOM) values obtainable from the TSync board and to explain how these values can be interpreted for status monitoring along with how it can be utilized to configure the state of the board. Additionally, this brief will cover common commands used to query the TSync board’s status, aiding in a better understanding of its operation and performance.
Time Figure of Merit (TFOM)
Time figure of merit has been an integral part of characterizing accuracy of the time feed and is utilized in a quite widespread manner in the industry. TFOM shows the estimated error range between a reference 1PPS (e.g., GPS 1PPS) and the System 1PPS it’s aligned to, known as the 1PPS Phase Error. TFOM values indicate these error ranges, with larger errors leading to higher TFOM values. For example, TFOM 3 indicates an error between 10 ns to less than 100 ns. TFOM reflects how accurately Tsync is synchronized with its time and 1PPS reference inputs, based on factors like the Estimated Time Error (ETE). A higher TFOM value suggests less accurate alignment with the 1PPS input, potentially impacting oscillator disciplining. TFOM ranges from 1 to 15. The table below shows the value of TFOM with respect to the estimated time error.
| TFOM Value | Estimated Time Error (ETE) |
| 1 | ETE <= 1ns |
| 2 | 1ns < ETE <= 10ns |
| 3 | 10ns < ETE <= 100ns |
| 4 | 100ns <ETE <= 1µs |
| 5 | 1 µs < ETE <= 10 µs |
| 6 | 10 µs < ETE <= 100 µs |
| 7 | 100 µs < ETE <= 1ms |
| 8 | 1ms < ETE <= 10ms |
| 9 | 10ms < ETE <= 100ms |
| 10 | 100ms < ETE <= 1s |
| 11 | 1s < ETE <= 10s |
| 12 | 10s < ETE <= 100s |
| 13 | 100s < ETE <=1000s |
| 14 | 1000s < ETE <= 10000s |
| 15 | ETE > 10000s |
Monitoring the TSync board using TFOM values is a superior method, as it abstracts the details and gives you an idea of the sync quality.
Note: The phase error mentioned above is a filtered phase error that is used by the disciplining algorithm and not the raw phase error.
A max TFOM value can be set by the user using SS_SetMaxTfom, which will control the sync state of the TSync board. If the current TFOM is less than the max TFOM the TSync will be in sync, if the current TFOM is greater than the max TFOM, then the TSync will exit the Sync state and enter Holdover mode.
Note: Upon the loss/degradation of reference signals, the TSync board synchronization will go into holdover mode for set period of time (default 2 hours or 7200 seconds) instead of declaring loss of time, in which all the timing outputs will be derived from internal 10 MHz oscillator incrementing the system time, but it will not be disciplined by the external reference. The holdover mode has a timeout period which can be changed using SS_SetHoldoverTO (Current Timeout can be viewed using SS_GetHoldoverTO). On expiration of Timeout period the Unit will declare loss of synchronization.
Monitoring
There are few parameters that a user can monitor to get the idea of the current state of the Tsync board,
- Sync State: SS_GetSync can be used to identify whether the TSync board is in sync to the reference or not.
- Holdover: SS_GetHoldover can be used to see if the board is in Holdover mode or not.
- Oscillator State: XO_GetState can be used to see the disciplining algorithm state.
a. State 3: Oscillator is in Calibration and Quick Phase Mode (disciplining)
b. State 4: Oscillator is in Lock state (disciplining)
c. State 5: No reference available to lock on to - Phase Error: As mentioned above, the user can use TFOM to track the phase error in an abstract way and use it to set a threshold for the board to be in sync with the reference, but to monitor the phase error directly, there are two methods:
a. Filtered Phase Error: XO_GetPhaseError, to get the phase error used by the disciplining algorithm.
b. Raw Phase Error: To get the raw phase error, the user will have to register a new meter and then use that meter to get the unfiltered phase error. To do that follow the instructions below:
- XS_Register (This will output meter number of newly registered meter)
- XS_SetWindowSize (If you want to measure the latest phase error, we suggest to keep the window size to 1).
- XS_SetMeterCmd (Here you can set meter cmd to be 0)
- Once all of this is set up you can run the following command to get the unfiltered phase error alongside some other data. (The field that gives you the phase error is called Starting Phase Error)
- XS_GetMeterData
There are few other functions that can be used to monitor the TSync board such as TSYNC_ LS_GetAlarm that returns the alarm flag identified by the index given to the call or XO_GetAlarm to see status of oscillator alarms.
Out of the above-mentioned ways and others, a user can monitor the quality of the output using the raw phase error data, sync status and holdover status of the board in a minimal way, along with any TFOM data if desired.