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 <k8yumdoo...@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.bori...@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.c...@sfr.fr> wrote: >>> >>> >>>> Le 14 août 2018 à 09:29, Mike Cook <michael.c...@sfr.fr> a écrit : >>>> >>>> Sorry about the previous blank mail. Finger jitter. >>>> >>>> >>>> >>>>> Le 14 août 2018 à 04:29, Chris Caudle <ch...@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 >>>>> >>>>> >>>>> >>>>> _______________________________________________ >>>>> time-nuts mailing list -- time-nuts@lists.febo.com >>>>> To unsubscribe, go to http://lists.febo.com/mailman/ >> listinfo/time-nuts_lists.febo.com >>>>> and follow the instructions there. >>>> >>>> Don’t worry about how powerful the machines are. Worry about who the >> machines are giving power to. >>>> >>>> >>>> _______________________________________________ >>>> time-nuts mailing list -- time-nuts@lists.febo.com >>>> To unsubscribe, go to http://lists.febo.com/mailman/ >> listinfo/time-nuts_lists.febo.com >>>> and follow the instructions there. >>> >>> Don’t worry about how powerful the machines are. Worry about who the >> machines are giving power to. >>> >>> >>> _______________________________________________ >>> time-nuts mailing list -- time-nuts@lists.febo.com >>> To unsubscribe, go to http://lists.febo.com/mailman/ >> listinfo/time-nuts_lists.febo.com >>> and follow the instructions there. >> >> _______________________________________________ >> time-nuts mailing list -- time-nuts@lists.febo.com >> To unsubscribe, go to http://lists.febo.com/mailman/ >> listinfo/time-nuts_lists.febo.com >> and follow the instructions there. >> > _______________________________________________ > time-nuts mailing list -- time-nuts@lists.febo.com > To unsubscribe, go to > http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com > and follow the instructions there. _______________________________________________ time-nuts mailing list -- time-nuts@lists.febo.com To unsubscribe, go to http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com and follow the instructions there.