[time-nuts] 1PPS for the beginner

[Mike Cook]

> Le 14 août 2018 à 09:29, Mike Cook <michael.cook at sfr.fr> a écrit :
> 
> Sorry about the previous blank mail. Finger jitter.
> 
> 
> 
>> Le 14 août 2018 à 04:29, Chris Caudle <chris at chriscaudle.org> a écrit :
>> 
>> On Mon, August 13, 2018 9:16 pm, Chris Burford wrote:
>>> I have a (generic?) GPSDO which contains an Oscilloquartz STAR 4+ OCXO
>>> that I am using to steer a PRS10 RFS. I'm a little confused on where the
>>> 1PPS is coming from with respect to the GPSDO.
> 
> As Chris points out the 1PPS from a GPSDO will « generally » be derived from the primary frequency and can show better performance than directly from a GPS receiver.
> However this is becoming less and less true.
> If you look at the Oscilloquarz blurb for the Star 4+ ( I found some here <http://pdf.directindustry.com/pdf/oscilloquartz-sa/star3-4/62169-330779.html#search-en-oscilloquartz-star-4> ) , you will see that the phase stability (jitter) on the 1PPS output is +/- 30ns when locked to GPS, an it has a timing grade GPS receiver. This is not as good as other GPS modules now. 15ns is normal, with some less than half that.
> The PRS10 has outstanding PLL control already. The SRS product doc gives +/- 10ns accuracy with +/-1ns resolution.   
> I don’t think that you are buying much with disciplining the PRS10 with a GPSDO 1PPS. Do you have any TIC measurements in this config to compare with a direct GPS 1PPS feed? 
> 

I forgot to mention one other thing which may be of interest to some. The 1PPS wave form output from the PRS10 is pretty mediocre. I put the details in another post here sometime back.
The Star4 spec is +/- 10ns, something I can only get from my PRS10s with a 74HC7001 shaper. 


> 
>> 
>> A GPS disciplined oscillator contains a GPS receiver which outputs 1PPS
>> based on receiving the GPS signals and calculating the position  + time
>> equation. That PPS signal is noisy in time, it jitters around relative to
>> the ideal 1 second period.  The GPSDO implements a long time constant PLL
>> to synchronize the output of the OCXO to the long term average frequency
>> and phase of the GPS PPS, so what you see externally is 10MHz directly
>> from the OCXO, 1 Hz (PPS) which is divided down from the 10MHz OCXO, and
>> those are controlled by a PLL so that long term the phase of the PPS
>> divided down from the OCXO follows the PPS calculated by the GPS receiver,
>> but with lower jitter.
>> 
>> -- 
>> Chris Caudle
>> 
>> 
>> 
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> 
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