[time-nuts] Holdover recovery
Bob Camp
kb8tq at n1k.org
Mon Apr 4 17:50:44 EDT 2016
Hi
On a surplus cell phone GPSDO it’s going to be:
1) System has been down for a while (= something failed)
2) System is now working ( = something was fixed)
3) The primary system spec must now be met (+/- 100 ns time alignment).
4) The secondary system spec should be met (+/- 5x10^-8 frequency)
5) Get it done now.
Both the time alignment and frequency specs are highly system dependent. The time
offset is likely to be aligned in a modulo 100 ns step. It then is fine tuned over some
time period to take out the rest of the error. If it’s done over a 50 second period, they
can do it at a 1x10^-9 frequency offset (but probably don’t …. more likely it’s a higher
offset at the start of the period).
Bob
> On Apr 3, 2016, at 11:39 PM, Hal Murray <hmurray at megapathdsl.net> wrote:
>
>
> Has anybody studied what happens when a GPSDO comes out of holdover? Has
> anybody seen any specs? I don't think I have.
>
> I think you have a choice of quick recovery for time or frequency, but you
> can't get both.
>
> Suppose your setup has been in holdover for a while. The frequency is
> slightly off. The time offset of the PPS pulse will be the integral of the
> frequency offset.
>
> What happens when you come out of holdover? If you fix the frequency, the
> PPS will stay off.
>
> Suppose the PPS has drifted by 1 ns. If you correct that in 1 second, the
> frequency will need to be off by 1E9 during that second.
>
>
> --
> These are my opinions. I hate spam.
>
>
>
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