VeEX TX300s, Руководство пользователя

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Not so good phase alignment
The chip scale atomic clock oscillator uses its 10 MHz frequency source for the disciplining process. Its 1PPS phase is initially
aligned to the 10 MHz phase, so it should be within ±100 ns (one 10 MHz cycle). Then the CSAC would start steering its frequency
to finely align its 1PPS output within a few nanoseconds to the “average” 1PPS input coming from the GPS receiver.
In the context of this document the term “GPS Receiver” is not considered a synonym of “GPS Clock” or “GPS-disciplined Clock”.
A GPS Clock is considered a combination of a GPS receiver and a highly stable precision oscillator.
Example of initial phase alignment
Although the relative phase alignment may converge rather fast in many occasions, users must still observe the minimum
recommended disciplining time.
If the disciplining time constant (TC) is changed in the middle of the process, from one long value to another, the phase may take
long time to converge to zero or could display a somewhat erratic behavior for a while. In this scenario, if users need to change the
TC, it may be worth temporarily changing it to a short TC (e.g. 60s) for faster steering and then change it to the desired value. (Note
that although the Sync 1PPS button could also be used to force alignment of the Atomic 1PPS output, it does not adjust the required
disciplining or steering parameters.)
Using short TC to force quicker phase convergence to zero
Phase Alignment and Holdover
Knowing whether the oscillator is still steering (changing) its frequency to correct the 1PPS output’s phase has a big impact in
deciding when to force the test set into holdover for indoors testing. The Phase Graph can help in identifying when the disciplining
process has stabilized.
A disciplined oscillator will continuously adjust its frequency to keep the 1PPS aligned to the standard second, but those offset
adjustments are usually small fractions of ppb when proper disciplining has been achieved.
Upon the loss of the GPS 1PPS reference, the oscillator enters holdover mode. This means that the precision oscillator will hold its
last frequency and the phase error will continue its trend. That means, you want the instantaneous frequency to be as accurate as
possible at the moment when the GPS receiver is turned off. Keep in mind that any ±X.XXX ppb frequency offset would result in a
cumulative time error of ±X.XXX ns per second and that would impact the resulting usable holdover time, by reaching the defined
error tolerance faster or slower.
TX300s_Platform_Manual_RevB00 Page 49 of 104