[time-nuts] 1PPS for the beginner

Mike Cook michael.cook at sfr.fr
Tue Aug 14 03:29:33 EDT 2018

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? 

> 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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