[time-nuts] Holdover

Magnus Danielson magnus at rubidium.se
Wed Aug 17 02:05:03 EDT 2016


That is what seem to work well in commercial products, including my designs.
Have you seen the RAI report on GPSDOs? I think we discussed it before, 
that will be a relevant reading.

Not all system implements 3), and it is a bit complex, so consider it an 
option to add, but not necessarily always used. Sometimes you don't want 
to do that.


On 08/17/2016 02:08 AM, Bob Stewart wrote:
> Thanks Magnus!
> These look like good guidelines.  I'll see what I can come up with.
> Bob
> -----------------------------------------------------------------
> AE6RV.com
> GFS GPSDO list:
> groups.yahoo.com/neo/groups/GFS-GPSDOs/info
> ------------------------------------------------------------------------
> *From:* Magnus Danielson <magnus at rubidium.dyndns.org>
> *To:* time-nuts at febo.com
> *Cc:* magnus at rubidium.se
> *Sent:* Tuesday, August 16, 2016 6:49 PM
> *Subject:* Re: [time-nuts] Holdover
> Bob,
> 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.
> Cheers,
> Magnus
>> Bob
>>  -----------------------------------------------------------------
>> AE6RV.com
>> GFS GPSDO list:
>> groups.yahoo.com/neo/groups/GFS-GPSDOs/info
>>      From: Attila Kinali <attila at kinali.ch <mailto:attila at kinali.ch>>
>>  To: Bob Stewart <bob at evoria.net <mailto:bob at evoria.net>>; Discussion
> of precise time and frequency measurement <time-nuts at febo.com
> <mailto: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 <mailto: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
>> alignment".
>>                Attila Kinali
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