[time-nuts] 1PPS for the beginner

Azelio Boriani azelio.boriani at gmail.com
Tue Aug 14 09:03:42 EDT 2018


For a definition of hanging bridge in the GPS receiver's context see:

<http://www.leapsecond.com/pages/m12/sawtooth.htm>

On Tue, Aug 14, 2018 at 2:58 PM jimlux <jimlux at earthlink.net> wrote:
>
> On 8/14/18 12:29 AM, Mike Cook wrote:
>
> >> 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 think the 1pps uncertainty spec for a receiver is more a function of
> how they generate the 1pps - particularly if it's basically the period
> of the internal clock.  A tight spec might just mean they've got a high
> frequency clock.
>
> The spec is probably also for a test condition where the GPS signals
> into the receiver are perfect.  In real life, there's the ever changing
> multipath, ionospheric scintillation, etc..
>
> So I can imagine a receiver with very good performance on perfect
> signals, but with poor "averaging", so that in a real signal
> environment, the 1pps varies quite a lot on a pulse by pulse basis.
> Likewise, I can imagine a receiver with a fairly large 1pps uncertainty
> spec, but since the oscillator it is derived from is fairly stable and
> high quality, over the long term, it tracks a 1pps more precisely  and
> predictably (e.g. the receiver puts out offset/sawtooth correction data)
>
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