[time-nuts] 1PPS for the beginner

Dana Whitlow k8yumdoober at gmail.com
Tue Aug 14 09:34:06 EDT 2018


Thanks Bob and Azello.  Now I understand the terminology and also better the
mechanism for the sawtooth error.

I once read that the LO, at least, in some GPS receivers, was not even
crystal
controlled but was rather a ring oscillator based on a string of cascaded
logic inverters on the chip.  This always sounded improbable to me, and i've
long wondered it this claim was really true.  Can either of you shed light
on
this issue?

Thanks,

Dana


On Tue, Aug 14, 2018 at 8:04 AM, Bob kb8tq <kb8tq at n1k.org> wrote:

> Hi
>
> A normal GPS module generates it’s outputs off of a free running internal
> clock. Generally
> this is a TCXO in a “timing” GPS ( = one that puts out a rational PPS).
> This is a bit counter
> intuitive, since you would *think* they phase lock the local source in the
> module. They don’t
> mainly because it makes the math easier.
>
> The gotcha with a free running clock is that the device can only generate
> an edge (like a pps)
> when the clock edge(s) allow it to. For simplicity, lets just go with the
> rising edge and accept that
> there could be *double edge* designs as well.  Let’s also assume a 25 MHz
> clock. That’s in the
> general range of what shows up on the surplus GPSDO modules.
>
> The internal fix math in the module comes up with a solution for “when
> should I send the PPS”.
> The clock edges are 40 ns apart. The solution says that the “right time”
> is 10 ns after an edge.
> The module sends out a PPS that is 10 ns early. Next second the math says
> that the right time
> is 30 ns after an edge. The module sends out a PPS that is 10 ns late.
>
> As long as it keeps going early / late / early /late things will average
> out. What makes it do this
> is the local clock on the module being a bit off frequency (modulo 1 Hz).
> As the clock drifts around
> (and they do) you may hit a region where it is relatively stable. It will
> then send out early /early /early….
> ( or late / late /late…..)
>
> If the local clock is a TCXO, the “stable points” are likely to also be
> points of frequency reversal.
> The net result is that the early / early /early never gets a corresponding
> late /late / late to average
> against. It will pass right through a PLL and create an offset in the
> output.
>
> I believe that NIST was the first to spot this and document it with lots
> of plots. I could be wrong about
> that. It was pretty much ignored in the days before SA was turned off. The
> SA jitter masked out a lot
> of issues. Most modern GPSDO’s use sawtooth correction messages to get
> around the problem. There
> may be a few still in production that don’t.
>
> Without full doc’s on a GPSDO, you really don’t *know* where the PPS is
> set to originate. It may
> be coming from the disciplined clock on the board. It also *may* be coming
> straight from the GPS
> module. There are indeed units out there that will let you do it either
> way under software control.
>
> Bob
>
> > On Aug 14, 2018, at 8:15 AM, Dana Whitlow <k8yumdoober at gmail.com> wrote:
> >
> > 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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