magnus at rubidium.dyndns.org
Tue Aug 16 19:49:01 EDT 2016
On 08/16/2016 11:31 PM, Bob Stewart wrote:
> Hi Attila,
> In my unit, which is a frequency standard, I chose to tell the receiver to stop sending 1PPS pulses when it loses sync to the sats. And since the 1PPS is no longer coming, the PLL does nothing and the DAC doesn't change. (Let's avoid the question of aging correction for now.) So, I'm wondering where to go and what to do if I want to get time from my unit. Clearly I could just tell the receiver to continue to send 1PPS pulses and sync to those - marking the time as unreliable. When the receiver synced back up, then it would warp the time output, the 1PPS would warp in phase, and the PLL would correct the phase error.
> So, that's one way, but probably not a desirable way. My interest was in the option of using the OCXO to create the time, which clearly gives a better option when the receiver syncs back up to the sats. Is there a published standard for this, or is this something that everyone (except the newbie) knows so well that it's not worth discussing?
There is no standard, but a few basic ways to go about which seems
reasonable and used by most is:
1) As you go into hold-over, keep producing PPS etc
2) As you leave hold-over, attempt to adjust the phase back.
3) If your system been in hold-over for a longer time, say that it
reasonably deviates outside of +/- 10 us (or some other limit), alarm
and turn output off
I have selected a somewhat more intricate setup in which you can set a
re-assignment limit, so when the phase error is outside of that limit,
you turn the output off, jumps the phase difference, and then starts to
track in from there. The reason being that at some time deviation, the
time it takes to track in the phase error is too large to be practical
so turning of and jump has less impact.
> GFS GPSDO list:
> From: Attila Kinali <attila at kinali.ch>
> To: Bob Stewart <bob at evoria.net>; Discussion of precise time and frequency measurement <time-nuts at febo.com>
> Sent: Tuesday, August 16, 2016 3:46 PM
> Subject: Re: [time-nuts] Holdover
> On Tue, 16 Aug 2016 04:35:40 +0000 (UTC)
> Bob Stewart <bob at evoria.net> wrote:
>> It's been pointed out to me that I didn't understand the function of
>> the 1PPS of a time standard. I confess that somehow I had confused the
>> term to be timing standard; which would be an entirely different thing.
>> But, this is time-nuts, so I should have realized...
>> Anyway, is there a standard, or at least an accepted practice, for how
>> holdover is handled in a time standard?
> There are many ways how to do that and which one you choose depends
> on the application and its requirements. You can find everything between
> "jump imediatly" and "just keep the frequency stable and don't care about
> Attila Kinali
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