In my pile of Trimble GPS receivers, I have a small handful of 4000SSi CORS receivers with a BNC clock reference input, and in the receiver firmware I can switch the receiver over to the external 10 MHz clock. It's all documented in the manual, but what I don't understand is... why? I'm trying to reconcile my mental model of the GPS receiver, and in my model, any critical timing in the receiver is all relative to the received GPS signals, what would I gain from feeding the 10 MHz from a GPSDO other than the more stable OCXO in the GPSDO? I imagine it might improve jitter on the 1PPS signal... Thanks, Keelan

Hi Simple answer: Feed the 10 MHz out of your Rb (or whatever). Send the data off to NRCan (or wherever). Get back a plot that lets you know how many parts in 10^-15 your standard is off frequency. There are other applications. The more general answer is: to remove local clock issues from the solution. Bob > On Jun 9, 2023, at 7:37 PM, Keelan Lightfoot via time-nuts <time-nuts@lists.febo.com> wrote: > > In my pile of Trimble GPS receivers, I have a small handful of 4000SSi CORS > receivers with a BNC clock reference input, and in the receiver firmware I > can switch the receiver over to the external 10 MHz clock. It's all > documented in the manual, but what I don't understand is... why? > > I'm trying to reconcile my mental model of the GPS receiver, and in my > model, any critical timing in the receiver is all relative to the received > GPS signals, what would I gain from feeding the 10 MHz from a GPSDO other > than the more stable OCXO in the GPSDO? I imagine it might improve jitter > on the 1PPS signal... > > Thanks, > > Keelan > _______________________________________________ > time-nuts mailing list -- time-nuts@lists.febo.com > To unsubscribe send an email to time-nuts-leave@lists.febo.com

Bob, I did that experiment a while back. Running a static survey in my back yard with the receiver exposed to the sun over the course of the day, the internal clock offset was quite a mess, with 4 milliseconds of drift over 9 hours: http://beefchicken.com/dump/clockoffsetbad.png Once I acquired the receiver with the reference input, I used a fairly recently calibrated HP 5316A with the OCXO option as a clock source, and kept the receiver indoors in my underground lair where the temperature is fairly constant. The results were much better, with -3 ns of drift over a 16 hour survey. http://beefchicken.com/dump/clockoffsetgood.png The results are interesting, but in terms of the static survey solution, (and to be painfully reductive), the terrible clock seems to have had little impact on the final survey, with both surveys having identical error ellipses. And the internally clocked survey had the odds stacked against it; it was a shorter survey, and there were a number of trees blocking a clear view of the sky. What are the other applications? These receivers were specifically marketed as CORS receivers, I imagine having a reference station running for years with a clock with milliseconds of drift per day might have some negative consequences... - Keelan On Fri, Jun 9, 2023 at 5:07 PM Bob Camp <kb8tq@n1k.org> wrote: > Hi > > Simple answer: > > Feed the 10 MHz out of your Rb (or whatever). Send the data off to NRCan > (or wherever). Get > back a plot that lets you know how many parts in 10^-15 your standard is > off frequency. > > There are other applications. The more general answer is: to remove local > clock issues from > the solution. > > Bob > > > On Jun 9, 2023, at 7:37 PM, Keelan Lightfoot via time-nuts < > time-nuts@lists.febo.com> wrote: > > > > In my pile of Trimble GPS receivers, I have a small handful of 4000SSi > CORS > > receivers with a BNC clock reference input, and in the receiver firmware > I > > can switch the receiver over to the external 10 MHz clock. It's all > > documented in the manual, but what I don't understand is... why? > > > > I'm trying to reconcile my mental model of the GPS receiver, and in my > > model, any critical timing in the receiver is all relative to the > received > > GPS signals, what would I gain from feeding the 10 MHz from a GPSDO other > > than the more stable OCXO in the GPSDO? I imagine it might improve jitter > > on the 1PPS signal... > > > > Thanks, > > > > Keelan > > _______________________________________________ > > time-nuts mailing list -- time-nuts@lists.febo.com > > To unsubscribe send an email to time-nuts-leave@lists.febo.com > >

Keelan ciao, I have the same input (capability) on Symmetricom XLi, but the difference is the internal firmware can manage in a 3 different way two sources: primary (Gps) and secondary as external reference (bnc). Let's say at the end could be just a kind of backup. In Your case (Trimble) not really useful unless the internal clock is faulty.. Cheers Inviato da Outlook per Android<https://aka.ms/AAb9ysg> ________________________________ From: Keelan Lightfoot via time-nuts <time-nuts@lists.febo.com> Sent: Saturday, June 10, 2023 1:37:13 AM To: Discussion of precise time and frequency measurement <time-nuts@lists.febo.com> Cc: Keelan Lightfoot <keelanlightfoot@gmail.com> Subject: [time-nuts] GPS receiver with reference clock input In my pile of Trimble GPS receivers, I have a small handful of 4000SSi CORS receivers with a BNC clock reference input, and in the receiver firmware I can switch the receiver over to the external 10 MHz clock. It's all documented in the manual, but what I don't understand is... why? I'm trying to reconcile my mental model of the GPS receiver, and in my model, any critical timing in the receiver is all relative to the received GPS signals, what would I gain from feeding the 10 MHz from a GPSDO other than the more stable OCXO in the GPSDO? I imagine it might improve jitter on the 1PPS signal... Thanks, Keelan _______________________________________________ time-nuts mailing list -- time-nuts@lists.febo.com To unsubscribe send an email to time-nuts-leave@lists.febo.com

Dear Keelan, On Sat, 10 Jun 2023, Keelan Lightfoot via time-nuts wrote: > Bob, > > I did that experiment a while back. Running a static survey in my back yard > with the receiver exposed to the sun over the course of the day, the > internal clock offset was quite a mess, with 4 milliseconds of drift over 9 > hours: > > http://beefchicken.com/dump/clockoffsetbad.png I think what is plotted is microseconds, not nanoseconds. Since it looks like it wraps at 1000000. If so, the blue curve is just a free-drifting clock? (4 second total drift) Best regards, Håkan > > Once I acquired the receiver with the reference input, I used a fairly > recently calibrated HP 5316A with the OCXO option as a clock source, and > kept the receiver indoors in my underground lair where the temperature is > fairly constant. The results were much better, with -3 ns of drift over a > 16 hour survey. > > http://beefchicken.com/dump/clockoffsetgood.png > > The results are interesting, but in terms of the static survey solution, > (and to be painfully reductive), the terrible clock seems to have had > little impact on the final survey, with both surveys having identical error > ellipses. And the internally clocked survey had the odds stacked against > it; it was a shorter survey, and there were a number of trees blocking a > clear view of the sky. > > What are the other applications? These receivers were specifically marketed > as CORS receivers, I imagine having a reference station running for years > with a clock with milliseconds of drift per day might have some negative > consequences... > > - Keelan > > On Fri, Jun 9, 2023 at 5:07 PM Bob Camp <kb8tq@n1k.org> wrote: > >> Hi >> >> Simple answer: >> >> Feed the 10 MHz out of your Rb (or whatever). Send the data off to NRCan >> (or wherever). Get >> back a plot that lets you know how many parts in 10^-15 your standard is >> off frequency. >> >> There are other applications. The more general answer is: to remove local >> clock issues from >> the solution. >> >> Bob >> >>> On Jun 9, 2023, at 7:37 PM, Keelan Lightfoot via time-nuts < >> time-nuts@lists.febo.com> wrote: >>> >>> In my pile of Trimble GPS receivers, I have a small handful of 4000SSi >> CORS >>> receivers with a BNC clock reference input, and in the receiver firmware >> I >>> can switch the receiver over to the external 10 MHz clock. It's all >>> documented in the manual, but what I don't understand is... why? >>> >>> I'm trying to reconcile my mental model of the GPS receiver, and in my >>> model, any critical timing in the receiver is all relative to the >> received >>> GPS signals, what would I gain from feeding the 10 MHz from a GPSDO other >>> than the more stable OCXO in the GPSDO? I imagine it might improve jitter >>> on the 1PPS signal... >>> >>> Thanks, >>> >>> Keelan >>> _______________________________________________ >>> time-nuts mailing list -- time-nuts@lists.febo.com >>> To unsubscribe send an email to time-nuts-leave@lists.febo.com >> >> > _______________________________________________ > time-nuts mailing list -- time-nuts@lists.febo.com > To unsubscribe send an email to time-nuts-leave@lists.febo.com

Hi Just what the survey world “likes” about an external reference is not 100% clear. The only answer the folks at Trimble gave me was “there are customers who feel they need it for their applications”. The explanation stoped at that point. One very basic application would be the monitoring process that feeds the IGS analysis system. At some point, you need some way to track the clocks in the sats against an outside source. ( = there is a clock solution …..). Another application would be very limited satellite availability. The accurate clock should help come up with a solution in that case. Bob > On Jun 10, 2023, at 1:28 AM, Keelan Lightfoot <keelanlightfoot@gmail.com> wrote: > > Bob, > > I did that experiment a while back. Running a static survey in my back yard with the receiver exposed to the sun over the course of the day, the internal clock offset was quite a mess, with 4 milliseconds of drift over 9 hours: > > http://beefchicken.com/dump/clockoffsetbad.png > > Once I acquired the receiver with the reference input, I used a fairly recently calibrated HP 5316A with the OCXO option as a clock source, and kept the receiver indoors in my underground lair where the temperature is fairly constant. The results were much better, with -3 ns of drift over a 16 hour survey. > > http://beefchicken.com/dump/clockoffsetgood.png > > The results are interesting, but in terms of the static survey solution, (and to be painfully reductive), the terrible clock seems to have had little impact on the final survey, with both surveys having identical error ellipses. And the internally clocked survey had the odds stacked against it; it was a shorter survey, and there were a number of trees blocking a clear view of the sky. > > What are the other applications? These receivers were specifically marketed as CORS receivers, I imagine having a reference station running for years with a clock with milliseconds of drift per day might have some negative consequences... > > - Keelan > > On Fri, Jun 9, 2023 at 5:07 PM Bob Camp <kb8tq@n1k.org <mailto:kb8tq@n1k.org>> wrote: >> Hi >> >> Simple answer: >> >> Feed the 10 MHz out of your Rb (or whatever). Send the data off to NRCan (or wherever). Get >> back a plot that lets you know how many parts in 10^-15 your standard is off frequency. >> >> There are other applications. The more general answer is: to remove local clock issues from >> the solution. >> >> Bob >> >> > On Jun 9, 2023, at 7:37 PM, Keelan Lightfoot via time-nuts <time-nuts@lists.febo.com <mailto:time-nuts@lists.febo.com>> wrote: >> > >> > In my pile of Trimble GPS receivers, I have a small handful of 4000SSi CORS >> > receivers with a BNC clock reference input, and in the receiver firmware I >> > can switch the receiver over to the external 10 MHz clock. It's all >> > documented in the manual, but what I don't understand is... why? >> > >> > I'm trying to reconcile my mental model of the GPS receiver, and in my >> > model, any critical timing in the receiver is all relative to the received >> > GPS signals, what would I gain from feeding the 10 MHz from a GPSDO other >> > than the more stable OCXO in the GPSDO? I imagine it might improve jitter >> > on the 1PPS signal... >> > >> > Thanks, >> > >> > Keelan >> > _______________________________________________ >> > time-nuts mailing list -- time-nuts@lists.febo.com <mailto:time-nuts@lists.febo.com> >> > To unsubscribe send an email to time-nuts-leave@lists.febo.com <mailto:time-nuts-leave@lists.febo.com> >>

Håkan, I think what is plotted is microseconds, not nanoseconds. Since it > looks like it wraps at 1000000. > > If so, the blue curve is just a free-drifting clock? > (4 second total drift) > The axes are labeled on either side, 'nanoseconds' and 'Sigma (nanoseconds)'. The blue curve is a free drifting clock.... Ohhh, I just realized that a scale value is also printed on the graph, so my "good" clock is actually terrible because the scale is 1e8. Digging through the data a bit more, the summary file lists the drift... "good": -464001302.4506 ns/day (not good at all, looks like the calibration on my counter is terrible) "bad": 234484.6468 ns/day I have a third: http://beefchicken.com/dump/clockoffsetugly.png for that dataset, the drift is 1661643.4279 ns/day - Keelan

Hi The first plot “wraps” at 1ms. That’s a pretty normal thing for one of these gizmos. It re-syncs to the nearest GPS code frame tick ( likely the wrong term ….). Total drift is about 4.6 ms. Over a 9 hour period, that puts the internal clock at about 0.17 ppm off frequency. It’s a reasonable outcome for the TCXO in these devices. On the second plot the key is the hard to spot “1e8” in the upper left corner of that plot. I’ve never seen it on my plots so I can only guess that it’s a scale factor. There are numeric data files in the package they send. You can confirm what it means from them. If the data is in nanoseconds x 1x10^8. The clock moves 0.33 seconds over the ~15 hour period. This puts you around 5.7 ppm off frequency. If the data is truly in nanoseconds, then the clock is off by 5.7 x 10^-14. At that level, the blue line is nowhere near that smooth. Bob > On Jun 10, 2023, at 5:21 AM, Håkan T Johansson via time-nuts <time-nuts@lists.febo.com> wrote: > > > Dear Keelan, > > On Sat, 10 Jun 2023, Keelan Lightfoot via time-nuts wrote: > >> Bob, >> >> I did that experiment a while back. Running a static survey in my back yard >> with the receiver exposed to the sun over the course of the day, the >> internal clock offset was quite a mess, with 4 milliseconds of drift over 9 >> hours: >> >> http://beefchicken.com/dump/clockoffsetbad.png > > I think what is plotted is microseconds, not nanoseconds. Since it looks like it wraps at 1000000. > > If so, the blue curve is just a free-drifting clock? > (4 second total drift) > > Best regards, > Håkan > > > >> >> Once I acquired the receiver with the reference input, I used a fairly >> recently calibrated HP 5316A with the OCXO option as a clock source, and >> kept the receiver indoors in my underground lair where the temperature is >> fairly constant. The results were much better, with -3 ns of drift over a >> 16 hour survey. >> >> http://beefchicken.com/dump/clockoffsetgood.png >> >> The results are interesting, but in terms of the static survey solution, >> (and to be painfully reductive), the terrible clock seems to have had >> little impact on the final survey, with both surveys having identical error >> ellipses. And the internally clocked survey had the odds stacked against >> it; it was a shorter survey, and there were a number of trees blocking a >> clear view of the sky. >> >> What are the other applications? These receivers were specifically marketed >> as CORS receivers, I imagine having a reference station running for years >> with a clock with milliseconds of drift per day might have some negative >> consequences... >> >> - Keelan >> >> On Fri, Jun 9, 2023 at 5:07 PM Bob Camp <kb8tq@n1k.org> wrote: >> >>> Hi >>> >>> Simple answer: >>> >>> Feed the 10 MHz out of your Rb (or whatever). Send the data off to NRCan >>> (or wherever). Get >>> back a plot that lets you know how many parts in 10^-15 your standard is >>> off frequency. >>> >>> There are other applications. The more general answer is: to remove local >>> clock issues from >>> the solution. >>> >>> Bob >>> >>>> On Jun 9, 2023, at 7:37 PM, Keelan Lightfoot via time-nuts < >>> time-nuts@lists.febo.com> wrote: >>>> >>>> In my pile of Trimble GPS receivers, I have a small handful of 4000SSi >>> CORS >>>> receivers with a BNC clock reference input, and in the receiver firmware >>> I >>>> can switch the receiver over to the external 10 MHz clock. It's all >>>> documented in the manual, but what I don't understand is... why? >>>> >>>> I'm trying to reconcile my mental model of the GPS receiver, and in my >>>> model, any critical timing in the receiver is all relative to the >>> received >>>> GPS signals, what would I gain from feeding the 10 MHz from a GPSDO other >>>> than the more stable OCXO in the GPSDO? I imagine it might improve jitter >>>> on the 1PPS signal... >>>> >>>> Thanks, >>>> >>>> Keelan >>>> _______________________________________________ >>>> time-nuts mailing list -- time-nuts@lists.febo.com >>>> To unsubscribe send an email to time-nuts-leave@lists.febo.com >>> >>> >> _______________________________________________ >> time-nuts mailing list -- time-nuts@lists.febo.com >> To unsubscribe send an email to time-nuts-leave@lists.febo.com > _______________________________________________ > time-nuts mailing list -- time-nuts@lists.febo.com > To unsubscribe send an email to time-nuts-leave@lists.febo.com

Hi Here’s a plot of NRCan data that *is* in the nanosecond range: \ Hopefully it comes through. There’s a couple of things on this plot. The first is that you can see the noise. It’s not a super smooth plot. The maser is running at < 1x10^-14, so the plot is pretty flat. The other interesting tidbit is the gap in the data right about 16:30. This happens to all The Trimble’s of this same type. It does not show up on other GPS devices. The device is a NetRS so it’s not exactly brand new. These gaps hit every day at about the same time. They have been happening for a month or two at this point. Since the dropouts don’t seem to impact the timing, I haven’t really dug into what’s going on. Bob