Re: [time-nuts] GPS-disciplined oscillators available
); SAEximRunCond expanded to false Errors-To: [EMAIL PROTECTED] i'm interested. tell me more? CK On 7/18/07, George Dubovsky <[EMAIL PROTECTED]> wrote: > ); SAEximRunCond expanded to false > Errors-To: [EMAIL PROTECTED] > > Last fall, I casually mentioned (over on the HP board) that I had a few > surplusTrimble Thunderbolt GPS-disciplined oscillator assemblies available, > and I was buried by the response! I have obtained a few more and, if you are > interested, please contact me for more info at n4ua.va(at)gmail.com. > > 73, > > geo - n4ua > ___ > time-nuts mailing list -- time-nuts@febo.com > To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > and follow the instructions there. > -- GDB has a 'break' feature; why doesn't it have 'fix' too? ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
[time-nuts] GPS-disciplined oscillators available
); SAEximRunCond expanded to false Errors-To: [EMAIL PROTECTED] Last fall, I casually mentioned (over on the HP board) that I had a few surplusTrimble Thunderbolt GPS-disciplined oscillator assemblies available, and I was buried by the response! I have obtained a few more and, if you are interested, please contact me for more info at n4ua.va(at)gmail.com. 73, geo - n4ua ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
Re: [time-nuts] GPS Disciplined Oscillators
Mike, If you have three different oscillators that are locked to GPS, then any difference seen when you read your clock once a day will be caused by the short term noise. When you take the noise difference counts as a percentage of counts in a day, the result is a smaller number than if the oscillators were not locked to GPS. Some people on this list are worried down to the phase noise level because they are correlating events that have been timed by clocks whose only link is GPS or a portable secondary time standard. Primary standards tend not to be portable, but quantum physics could change that. Locking means that there is only a phase difference between GPS and the oscillator, not a frequency difference. For 10 MHz, that's 100 nanoseconds or (much) better while locked. Said another way, if GPS lock is maintained for some length of time, the maximum difference at the end of that length of time is 100 nanoseconds, plus the drift of GPS, if any. Ageing is compensated by a control servo while the frequencies are locked. The full name for "lock" is "phase lock" which means that any phase error is restored to nominal by the control servo. The servo is constrained by a long time constant filter so that it won't dither around zero error, trying to follow every noise spike and always being too late. Losing lock means that the control servo has saturated, and can no longer move in the direction required to maintain lock. At that point, there is no controlled relationship between GPS and the oscillator. The performance of the oscillator is then the performance with no GPS available. You can measure ageing by measuring the output of the controller. If the loss of lock was due to some hiccup, then lock could be restored but the clock counter will be wrong by the number of counts added or dropped while lock was lost. Hope that helps. Bill Hawkins -Original Message- From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED] On Behalf Of Mike Feher Sent: Wednesday, April 25, 2007 11:46 AM To: 'Discussion of precise time and frequency measurement' Subject: [time-nuts] GPS Disciplined Oscillators I have to show my ignorance here, because this has been bothering me for a while, and, I wonder if there is a relatively simple answer. This question has to do with frequency accuracy and stability only. Also, let's talk of long term like 24 hours or more, so let's ignore phase noise and just concern ourselves with long term accuracy/stability. If I have three separate oscillators, let's say a Rubidium, an OCXO and a TCXO all with EFC's capable of closing the loop to lock to GPS, what kind of absolute frequency difference should I see amongst the three at any given time, random times, or, over the entire test period. Let's also make it simple and say all three are at 10 MHz nominal, unlocked to GPS. When locked, and properly designed with a narrow loop filter, I would expect the long term accuracy to be very close amongst all 3 oscillators. Certainly better than a few parts in 10^-11. First, am I wrong in this assumption? In either case, crystals, and even Rubidium cells do age, while at different rates, so, it is possible, that if lock with GPS is lost for some reason, because the oscillator may have drifted/aged out of loop range, it cannot be disciplined again. I, for the time being, also assume that the EFC on all 3 oscillators has a range wide enough to keep the oscillator locked even as it ages. Are the narrow loop bandwidths and wide EFC ranges contradictory? So, to reiterate the question, if I was clear enough, what kind of frequency excursions should I anticipate to see amongst my three disciplined oscillators in lets say 24 hours, or in a month. Assume GPS disciplining was working all of that time (can I even assume that with aging?). BTW, how is my assumption regarding the oscillators aging? If the oscillator basic frequency determining element drifts out of lock range, during lock, will it stay in lock? - Thanks in advance for any enlightenment - Mike Mike B. Feher, N4FS 89 Arnold Blvd. Howell, NJ, 07731 732-886-5960 ___ time-nuts mailing list time-nuts@febo.com https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts ___ time-nuts mailing list time-nuts@febo.com https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
Re: [time-nuts] GPS Disciplined Oscillators
Hi Mike: Nice to see you here. This is my take on it, but there are others on this list that are much more knowledgeable. The key to the performance of a GPSDO has to do with the Allan plots for the oscillator, GPS receiver and it's antenna and . . . . Also the Time Interval counter used in the GPSDO that compares the oscillator and the GPS 1 PPS is important. The key idea is that when you overlay the GPS receiver's plot with the oscillator plot there will be only one point where they intersect. That's what the loop time constant should be for the PLL. That way for time intervals that are shorter you get the good performance of the xtal oscillator and for longer time intervals you get the long term stability of GPS. Inside the GPSDO it somehow compares the oscillator output with the 1 PPS from GPS. It's very important how that's done. Suppose that it's done by dividing the oscillator's output down to 1 PPS and then measuring the time interval between the two pulses. The one shot resolution of that TI counter is a point on the Allan plot at 1 second (because we're using 1 pulse/second). You can draw a line through this point slopping down at 45 degrees to indicate what stability you will get if you average for longer times or if you make measurements at a faster pace. If that line is below the GPS - Oscillator intersection point then all is well. But it the TI line is above the intersection point then the TI counter is limiting the performance of the GPSDO. Of all the GPSDOs kits out there It's my understanding that the Brooks Shera is the best. But it was designed back when the 8 channel Motorola GPS receivers were the standard. But now with the M12+T (or other newer 12 channel) receivers that have much better performance it's limited by the TI counter. It's difficult to answer your questions in a general way. One of the problems has to do with the sensitivity and range of the EFC input to the oscillator. On one hand you want to use a D/A converter that has very small steps in order to allow tuning the oscillator with steps of E-12 or smaller. And if you know the direction of drift and set the manual adjustment so the EFC is very near one end of it's range you then have almost the full range available until the next manual adjustment is needed. When you combine both of these it turns out that you can't buy a DAC with that many bits. So you can see that it's hard to generalize about what the stability will be at 24 hours for different designs of GPSDO. A well designed GPSDO would have long term performance that was about the same as the GPS receiver. Tom has a number of plots on his web page, but this one compares a number of GPS receivers. http://www.leapsecond.com/pages/3gps/gps-adev-mdev.gif The Datum2000 is a GPSDO, not just a receiver. It would be interesting to see what happens to these receivers if the test was extended. I expect that they all would bottom out and go horizontal in after a few days. But at different stability values. Seeing that would answer your question in a better way. I have some T&F info at: http://www.pacificsites.com/~brooke/timefreq.shtml Have Fun, Brooke Clarke, N6GCE http://www.PRC68.com http://www.pacificsites.com/~brooke/PRC68COM.shtml http://www.precisionclock.com Mike Feher wrote: > I have to show my ignorance here, because this has been bothering me for a > while, and, I wonder if there is a relatively simple answer. This question > has to do with frequency accuracy and stability only. Also, let's talk of > long term like 24 hours or more, so let's ignore phase noise and just > concern ourselves with long term accuracy/stability. If I have three > separate oscillators, let's say a Rubidium, an OCXO and a TCXO all with > EFC's capable of closing the loop to lock to GPS, what kind of absolute > frequency difference should I see amongst the three at any given time, > random times, or, over the entire test period. Let's also make it simple and > say all three are at 10 MHz nominal, unlocked to GPS. When locked, and > properly designed with a narrow loop filter, I would expect the long term > accuracy to be very close amongst all 3 oscillators. Certainly better than a > few parts in 10^-11. First, am I wrong in this assumption? In either case, > crystals, and even Rubidium cells do age, while at different rates, so, it > is possible, that if lock with GPS is lost for some reason, because the > oscillator may have drifted/aged out of loop range, it cannot be disciplined > again. I, for the time being, also assume that the EFC on all 3 oscillators > has a range wide enough to keep the oscillator locked even as it ages. Are > the narrow loop bandwidths and wide EFC ranges contradictory? So, to > reiterate the question, if I was clear enough, what kind of frequency > excursions should I anticipate to see amongst my three disciplined > oscillators in lets say 24 hours, or in a month. Assume GPS disciplining was >
Re: [time-nuts] GPS Disciplined Oscillators
In a message dated 4/25/2007 09:50:38 Pacific Daylight Time, [EMAIL PROTECTED] writes: >So, to >reiterate the question, if I was clear enough, what kind of frequency >excursions should I anticipate to see amongst my three disciplined >oscillators in lets say 24 hours, or in a month. Assume GPS disciplining was >working all of that time (can I even assume that with aging?). BTW, how is >my assumption regarding the oscillators aging? Hello Mike, from my experience, good oscillators will age parts in 1E-08 per day or less. Bad ones can age much faster, some parts in 1E-06 in the first days even. That's a pretty significant variation in EFC voltage required to compensate that. We have seen units that age significantly in the first 2 - 3 days, then slow down, and after 6 months or so almost have no aging. We have seen units where the aging actually accelerates over time. Aging is a very slow changing process after the crystal has run some days (second derivative is small), so GPS locking will usually easily compensate this error. There are oscillators that have popcorn noise (frequency jumps) that can be really annoying, and this effect is similar to rapid aging (from one second to another): almost sudden the phase/frequency of the OCXO changes radically from one second to another. This is usually caused by either dust particles leaving, or landing on the crystal, the crystal "cracking" in it's holders, or radioactive particles hitting the crystal. Also, thermal effects will usually require much larger shifts in EFC voltage than aging, unless you are using a double-oven high quality OCXO. Thermal effects on the required EFC voltage usually swamp aging effects and are harder to deal with because they are more or less random, and not a nice (almost) straight line on the EFC voltage. bye, Said ** See what's free at http://www.aol.com. ___ time-nuts mailing list time-nuts@febo.com https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
Re: [time-nuts] GPS Disciplined Oscillators
Mike, I just so happened to be reading through my TS2100 manual and came across a bit of info that might be applicable to your question: The info below is for the various oscillator options you can get, and while the number can vary based on the exact oscillator used (and its age), it should give you a good ballpark idea of the differences. VCXO: 48 milliseconds/day long term "flywheeling" OCXO: 5 milliseconds/day long term "flywheeling" Rubidium: 6.5 microseconds/month long term "flywheeling" Jason ___ time-nuts mailing list time-nuts@febo.com https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
[time-nuts] GPS Disciplined Oscillators
I have to show my ignorance here, because this has been bothering me for a while, and, I wonder if there is a relatively simple answer. This question has to do with frequency accuracy and stability only. Also, let's talk of long term like 24 hours or more, so let's ignore phase noise and just concern ourselves with long term accuracy/stability. If I have three separate oscillators, let's say a Rubidium, an OCXO and a TCXO all with EFC's capable of closing the loop to lock to GPS, what kind of absolute frequency difference should I see amongst the three at any given time, random times, or, over the entire test period. Let's also make it simple and say all three are at 10 MHz nominal, unlocked to GPS. When locked, and properly designed with a narrow loop filter, I would expect the long term accuracy to be very close amongst all 3 oscillators. Certainly better than a few parts in 10^-11. First, am I wrong in this assumption? In either case, crystals, and even Rubidium cells do age, while at different rates, so, it is possible, that if lock with GPS is lost for some reason, because the oscillator may have drifted/aged out of loop range, it cannot be disciplined again. I, for the time being, also assume that the EFC on all 3 oscillators has a range wide enough to keep the oscillator locked even as it ages. Are the narrow loop bandwidths and wide EFC ranges contradictory? So, to reiterate the question, if I was clear enough, what kind of frequency excursions should I anticipate to see amongst my three disciplined oscillators in lets say 24 hours, or in a month. Assume GPS disciplining was working all of that time (can I even assume that with aging?). BTW, how is my assumption regarding the oscillators aging? If the oscillator basic frequency determining element drifts out of lock range, during lock, will it stay in lock? - Thanks in advance for any enlightenment - Mike Mike B. Feher, N4FS 89 Arnold Blvd. Howell, NJ, 07731 732-886-5960 ___ time-nuts mailing list time-nuts@febo.com https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
Re: [time-nuts] GPS disciplined oscillators - how not to do it.
Peter Vince wrote: > It's a shame the magazine article got it so wrong, but I can well > imagine how an enthusiastic amateur, without the collective knowledge > of our little group, might, with all good intentions, make such a > mess of the design. > > I wonder if Bruce, or one of the other experts in our group, should > write to the magazine pointing out the major flaws, hence spreading > our knowledge for the greater good? (Hmmm, that sounds pompous - > sorry!) > > Peter Vince > Peter I've already sent an email pointing out the circuit errors (complete with the fixes - which only require a few jumpers eliminate use of a few of the inverters) and the flawed concept. This was about a month ago, either the email went astray or they've chosen to ignore it. Maybe I'll send another email. Bruce ___ time-nuts mailing list time-nuts@febo.com https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
Re: [time-nuts] GPS disciplined oscillators - how not to do it.
It's a shame the magazine article got it so wrong, but I can well imagine how an enthusiastic amateur, without the collective knowledge of our little group, might, with all good intentions, make such a mess of the design. I wonder if Bruce, or one of the other experts in our group, should write to the magazine pointing out the major flaws, hence spreading our knowledge for the greater good? (Hmmm, that sounds pompous - sorry!) Peter Vince > >The Post was intended as a warning don't be tempted to build the circuit >as you will be disappointed. >... >Its a shame that an opportunity for demonstrating the performance that >is achievable with a well designed crystal oscillator without an oven >can be. >... > >Bruce >>> An Australian Electronics magazine recently published a circuit for a >>> GPS disciplined crystal oscillator. >>> This particular implementation is the worst I've ever seen. ___ time-nuts mailing list time-nuts@febo.com https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
Re: [time-nuts] GPS disciplined oscillators - how not to do it.
Bob Paddock wrote: > On Tuesday 03 April 2007 21:48, Dr Bruce Griffiths wrote: > >> An Australian Electronics magazine recently published a circuit for a >> GPS disciplined crystal oscillator. >> This particular implementation is the worst I've ever seen. >> > > What would you consider the best you have ever seen? > > Bob Probably Quartzlock's carrier phase disciplined OCXOs. Although a few improvements spring to mind. If we are only considering phase locking to a GPS timing receiver's PPS output the HP58503 and its ilk weren't too bad although the double oven OCXO implementation used is a bit of a kludge. The Post was intended as a warning don't be tempted to build the circuit as you will be disappointed. The logic circuitry appears to have been designed by 2 people, one who knew how to cascade counters correctly and one who didn't. Whoever designed the PLL and its filters seems to be particularly clueless. Using parts of a hex inverter for an oscillator and its buffers as well as using the rest as buffers for other frequencies is also asking for trouble. Its a shame that an opportunity for demonstrating the performance that is achievable with a well designed crystal oscillator without an oven can be. The choice of a GPS receiver with a PPS timing accuracy more than 30 times worse than that possible with most good GPS timing receivers didn't help. Bruce ___ time-nuts mailing list time-nuts@febo.com https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
Re: [time-nuts] GPS disciplined oscillators - how not to do it.
On Tuesday 03 April 2007 21:48, Dr Bruce Griffiths wrote: > An Australian Electronics magazine recently published a circuit for a > GPS disciplined crystal oscillator. > This particular implementation is the worst I've ever seen. What would you consider the best you have ever seen? -- http://www.softwaresafety.net/ http://www.designer-iii.com/ http://www.unusualresearch.com/ ___ time-nuts mailing list time-nuts@febo.com https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
