golgarfrincham at yahoo.com
Tue Aug 28 17:01:39 UTC 2012
>I don't know that there is any data showing the TBolt adjusting for aging
>during holdover. In other words: showing the holdover DAC voltage changing
>at a completely constant temperature.
I haven't seen any data either but here is information from section 5.1.2
of the 2003 Thunderbolt manual:
"Kalman filtering is a technique that improves the performance of a GPS
disciplined clock when GPS drops out. This state is called holdover.
During holdover the clock relies solely on the oscillator. Oscillator
performance is subject to two basic effects. First, changes in environmental
temperature can cause the oscillator to speed up [or] slow down. Second, the
oscillator has a natural tendency to drift over time. This is called aging.
Both temperature and aging can be mathematically predicted. However, the
characteristics vary from crystal to crystal. The Kalman filtering monitors the
unique oscillator performance over time and temperature and records this
behavior. Then when the clock goes into holdover this filtering corrects for
these effects producing a more accurate clock. The longer a clock has
to ’train’ the better the Kalman filtering performance will be. 24 hours is
considered the minimum necessary for good performance"
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