[time-nuts] Setting up PRS10 with M12+T GPS

bg at lysator.liu.se bg at lysator.liu.se
Wed May 2 23:09:03 UTC 2012


> Björn,
>
> On 05/03/2012 12:30 AM, bg at lysator.liu.se wrote:
>> Magnus,
>>>>
>>>> Tbolt       PRS10A     PRS10B
>>>> 1pps  -->     PPSIN
>>>>               PPSOUT -->   PPS IN
>>>>
>>>> PRS10A slowly following the Tbolt (PRS10A PT8)
>>>> PRS10B "quickly" following the PRS10A (PRS10B PT0)
>>>>
>>>> Ok?
>>>
>>> Yes, but if this is a free-running thunderbolt (I think you said
>>> something about that) then PT8 will be a bit slow. For a tracking
>>> thunderbolt PT8 may work, even if it takes time for it to track in.
>>
>> No a GPS locked Tbolt.
>
> Thanks for that clarification, I am obviously a bit tired.
>
>> A free-running Tbolt was just an example of a very
>> low noise 1PPS source, where I would find it useful to have a "PT-1"
>> bias
>> calibration mode bypassing the PLL, and moving the PPSOUT directly with
>> the PPSIN.
>
> So this does not suffice?:
>
> Page 33:
> "When provided with an accurate and stable 1pps source, the unit will
> automatically align its 1pps output to the 1pps input and then adjust
> the frequency of the rubidium reference to maintain the alignment over
> time."
>
> Page 17:
> "After receiving 256 consecutive "good" 1pps inputs, the 1pps pulse
> delay is set to the last of the 256 time-tag values."
>
> I interpret this as this:
>
> When the 256s PPS has been approved, the last-time-tag is used to adjust
> the output delay such that the output signal is aligned, within the
> precision of the time-offset value.
>
> Then, the PLL is running. Either the output delay is not shifted and
> only the PPS alignment is done on approval, or it is updated in the
> background. The text only supports the former explicitly, but it would
> be nice to verify if it is either of these.
>
> Doing phase-jump of PPS like this on start-up is expected. I've even has
> a measurement on it somewhere.

It is surely working alright in the long run. I am not convinced it is
appropriate for applications where you fire up the PRS for a few hours and
then do a few hours of measurements.

Even if it jumps the 1pps, early on the PLL will still be 100s of ns away
from its reference. There should be a better way to make this, than how I
run the PRS...

--

   Björn




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