[time-nuts] 1PPS for the beginner

Dana Whitlow k8yumdoober at gmail.com
Tue Aug 14 08:15:36 EDT 2018


Could someone please define and explain the term 'hanging bridge' in this
context?

Thanks,

Dana


On Tue, Aug 14, 2018 at 7:02 AM, Azelio Boriani <azelio.boriani at gmail.com>
wrote:

> Hanging bridges out of a GPSDO's PPS? Interesting... time to try to
> setup a measurement and see the relation between the GPS's PPS hanging
> bridges and the corresponding DO's ones.
> On Tue, Aug 14, 2018 at 9:55 AM Mike Cook <michael.cook at sfr.fr> wrote:
> >
> >
> > > 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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