Hi I can think of no way to get the close in phase noise of a ring oscillator down far enough to use it as the main clock for a GPS module. It *is* reasonable that the L band LO in the module is a phase locked ring oscillator.,
Bob > On Aug 14, 2018, at 9:34 AM, Dana Whitlow <[email protected]> wrote: > > 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 <[email protected]> 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 <[email protected]> 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 < >> [email protected]> >>> 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 <[email protected]> wrote: >>>>> >>>>> >>>>>> Le 14 août 2018 à 09:29, Mike Cook <[email protected]> a écrit : >>>>>> >>>>>> Sorry about the previous blank mail. Finger jitter. >>>>>> >>>>>> >>>>>> >>>>>>> Le 14 août 2018 à 04:29, Chris Caudle <[email protected]> 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 -- [email protected] >>>>>>> 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 -- [email protected] >>>>>> 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 -- [email protected] >>>>> To unsubscribe, go to http://lists.febo.com/mailman/ >>>> listinfo/time-nuts_lists.febo.com >>>>> and follow the instructions there. >>>> >>>> _______________________________________________ >>>> time-nuts mailing list -- [email protected] >>>> To unsubscribe, go to http://lists.febo.com/mailman/ >>>> listinfo/time-nuts_lists.febo.com >>>> and follow the instructions there. >>>> >>> _______________________________________________ >>> time-nuts mailing list -- [email protected] >>> To unsubscribe, go to http://lists.febo.com/mailman/ >> listinfo/time-nuts_lists.febo.com >>> and follow the instructions there. >> >> >> _______________________________________________ >> time-nuts mailing list -- [email protected] >> To unsubscribe, go to http://lists.febo.com/mailman/ >> listinfo/time-nuts_lists.febo.com >> and follow the instructions there. >> > _______________________________________________ > time-nuts mailing list -- [email protected] > To unsubscribe, go to > http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com > and follow the instructions there. _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com and follow the instructions there.
