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What\'s the process to keep the SRO\'s holdover/free running stability at 1us/24hr?

A tracking (or learning) time of 24 hours is required to achieve a 1us/24hr performance, allowing the average frequency to be successfully saved in the EEPROM inside the SRO.

If the tracking is stopped after 12 hours, the SRO goes back to the factory setting frequency, affecting the holdover (or free running) performance. Ideally the SRO's holdover tests is as follows:

1) Power on the SRO and let it stabilize for 1 day.

2) Start the tracking process by HW (hardware) or SW (software). Sync or no sync mode has no influence.

3) Let it track for at least 24 hours.

4) Put the SRO in holdover mode by stopping the 1PPSREF or optionally removing the GPS antenna. Stopping the tracking by HW or SW will have a different holdover performance, as a sudden frequency jump to the last frequency value will be stored in the EEPROM. If the 1PPSREF is stopped during the tracking process, the SRO will go in holdover mode with a smooth (or negligible) frequency jump, which will be the last frequency value stored in the EEPROM. Below is the basic concept:

• Long period of tracking, many days, 24 hours frequency saving will be activated: thus no difference in holdover performance

• Short period of tracking, <1day, 24 hours frequency saving will not be activated: thus a large difference in holdover performance

Notes:

a) The frequency learning during the tracking process is related to the command FS (frequency save)

b) Check if the 24 hours frequency saving is activated with the "FS9" command. The answer should be 1.

c) If 24 hours are too long for the frequency learning, you may test with the FS2 or FS3 command during the tracking process. This command must be sent just before going back to the holdover mode by HW or SW.