Re: [time-nuts] question about multi-way measurement
From: Hal Murray On the Raspberry Pi, the Ethernet is on USB so there is another source of timing error. It's full speed USB rather than low speed USB so the timing noise isn't as bad as it could be - 1/8 ms vs 1 ms. (I could be off on the numbers or names, but the general idea is correct.) If you think you can avoid that by averaging, be sure you understand hanging bridges. If you need good times on a Pi, consider a GPS hat. The PPS goes in via a GPIO pin with an interrupt so the timing can be pretty good. = I have some Raspberry Pi systems running with PPS sync from an indoor GPS receiver (on the upper floor of a two storey building) and in the table below RasPi-1 is in an unheated cupboard. It plots the offset as reported by NTP every five minutes. https://www.satsignal.eu/mrtg/performance_ntp.php Avoid quick temperature changes for best performance (e.g RasPi-12 is located far too near a central heating radiator. There are some notes here: https://ava.upuaut.net/?p=951 https://www.satsignal.eu/ntp/Raspberry-Pi-quickstart.html The Uputronics article says, incorrectly, that NTP won't work stand-alone, without the Internet. I'll write to him about that! Cheers, David -- SatSignal Software - Quality software for you Web: http://www.satsignal.eu Email: david-tay...@blueyonder.co.uk Twitter: @gm8arv ___ 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.
Re: [time-nuts] question about multi-way measurement
strom...@nexgo.de said: > Over wired Ethernet you can expect to synchronize a bunch of systems to > within a ~200µs envelope of absolute time and maybe a factor of 2x-3x lower > if you can control certain things more tightly than usual if you run those > system on a single hop switched LAN that have a GPS-based stratum-1 NTP > server. For anything better than that you'd need PTP, which unfortunately > the rasPi is incapable of. Or do you plan to use the rasPi itself as the RX/ > TX? The few papers I've just looked up all use Atheros 9k hardware. On the Raspberry Pi, the Ethernet is on USB so there is another source of timing error. It's full speed USB rather than low speed USB so the timing noise isn't as bad as it could be - 1/8 ms vs 1 ms. (I could be off on the numbers or names, but the general idea is correct.) If you think you can avoid that by averaging, be sure you understand hanging bridges. If you need good times on a Pi, consider a GPS hat. The PPS goes in via a GPIO pin with an interrupt so the timing can be pretty good. -- These are my opinions. I hate spam. ___ 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.
Re: [time-nuts] question about multi-way measurement
Charles Wyble wrote: I built a dedicated server room in my house, with it's own air conditioner. I've been working on overall instrumentation , especially temperature. If the rasPi is a dedicated system and does not serve extra tasks, just record its CPU temperature, no extra sensor needed. The absolute temperature will be an almost constant offset from the ambient, so the changes are well preserved (except for short periods, e.g. when a cron job runs). Any temperature transient will show up in the timing error, however small. The FLL/PLL code in NTP will need to chase the changing crystal frequency and then eliminate the already accrued timing error. The faster the transient, the larger the error. So on-off aircon with forced convection is pretty close to worst case. Could you share the snippets of the PPS logging? I'm not 100% sure what you mean by the PPS timestamps. Just reading from /dev/pps0 (system time and capture timestamp in my case), really; additionally system load and CPU temperature and logging the everything into a file. Interesting. My ultimate application of this high precision timing is driving TDMA wifi links as low cost as possible. I'm not familiar with the requirements for that. I suspect that the absolute timing between stations is actually pretty unimportant. So the only thing that matters is that the relative timing error between beacons or syncs must keep below some threshold, which means the frequency offset of the system must be kept within (probably not too tight) bounds. The latter is much easier to achieve and maintain. Over wired Ethernet you can expect to synchronize a bunch of systems to within a ~200µs envelope of absolute time and maybe a factor of 2x-3x lower if you can control certain things more tightly than usual if you run those system on a single hop switched LAN that have a GPS-based stratum-1 NTP server. For anything better than that you'd need PTP, which unfortunately the rasPi is incapable of. Or do you plan to use the rasPi itself as the RX/TX? The few papers I've just looked up all use Atheros 9k hardware. -- Achim. (on the road :-) ___ 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.
Re: [time-nuts] question about multi-way measurement
Charles Wyble wrote: > I’m using a raspberry pi with gps hat for my master time source. > Shortly I’ll be having a total of three systems (two using the same > hat, one using the adafruit hat and being a pi2). I’ve got some > interest in multiple way comparison and will follow this thread > shortly. I'd say three doesn't really get you good enough visibility. It depends somewhat on how good your GPS reception is and how stable the environment, especially temperature. At around five NTP servers with suitable precision you start to see "interesting" things like asymmetric latency in your local network and can more easily throw out the inevitable spurs from degraded GPS reception (unless you have a really good antenna location). I'd suggest you also log at least the PPS timestamps to correlate to the NTP logging. NTP peer logging will be dominated by network latency and jitter, provided you took care to tune the residual loop error to below 1µs. I'm running a Perl script that also records the CPU temperature and system utilization synchronized with the PPS. All my logging is into files at the moment, which puts some extra stress on the SD card that several no-name cards have not survived for long. I've salvaged an SSD that I plan to connect via an USB to SATA converter and then set up a proper time series database on one the boxes to feed all data into. Alternatively you could log into a tmpfs and rotate onto SD card whenever you've collected a full Flash block. I currently have seven stratum-1 NTP servers (five different rasPi and two TinkerBoard) on my LAN. I've self-ovenized six of them (the exception is the rasPi 1B+, which simply isn't powerful enought to pull that off) to keep the crystal temperature very near the turnover point of the f vs. T curve, which leaves me with just the jitter and drift of the (apparent) system frequency most of the time. The rasPi crystals (or the interrupt system on the SoC) are a bit noisy with seemingly unprovoked frequency jumps on a not too-long timescale, so that keeps you to within a 5ppb window after removing the drift. The TinkerBoard doesn't have those jumps and I keep both routinely within 1ppb of the expected drift curve. I've experimented with both low and high thermal mass designs, but so far I don't see a difference in timing performance between the two. The high-thermal mass design does smooth out the external temperature swings more effectively, so with further refinements to the oven controller it might eventually provide a usable advantage. -- Achim. (on the road :-) ___ 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.
Re: [time-nuts] question about multi-way measurement
Hi There also are a lot of papers going back a ways by Jim Barnes and David Alan (sometimes together and sometimes separately) related to multiple clocks driving a single “estimate” of what time it actually is. Bob > On Dec 27, 2018, at 2:34 PM, Steve Allen wrote: > > On Wed 2018-12-26T10:30:24-0600 Chris Howard hath writ: >> I see the different forms of deviation measurements and they are all >> one-to-one comparisons. >> >> Is there anything to be learned from doing mass data gathering? > >> So, has this sort of thing been done? >> Why is everything one-to-one only? > > Doing this was the reason for the creation of the Bureau International > de l'Heure (BIH) a century ago. The initial announcement of their > work invited observatories around the world to participate via > correspondence sending the received times of radio time signals. > http://adsbit.harvard.edu/full/1922BuBIH...11. > > A few years later they presented to the 1928 General Assembly of the > IAU a complete history of timekeeping and an inventory of their > equipment including the clocks at l'Observatoire de Paris which were > located down in the catacombs to maintain stable conditions > http://adsbit.harvard.edu/full/1929BuBIH...3..255. > > The progression of issues of Bulletin Horaire shows the development of > technologies and techniques for intercomparing clocks from the age of > pendulum clocks with constant pressure cases into the age of atomic > chronometers. At the retirement of two long-time staffers they > published plots of the improvement of timekeeping from 1922 to 1964. > https://www.ucolick.org/~sla/leapsecs/annastoyko.html > > -- > Steve Allen WGS-84 (GPS) > UCO/Lick Observatory--ISB 260 Natural Sciences II, Room 165 Lat +36.99855 > 1156 High Street Voice: +1 831 459 3046 Lng -122.06015 > Santa Cruz, CA 95064 http://www.ucolick.org/~sla/ Hgt +250 m > > ___ > 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.
Re: [time-nuts] question about multi-way measurement
It looks like the image I sent got trimmed off. Did I do something wrong? Regards, Jerry > On Dec 26, 2018, at 4:21 PM, Jerry Hancock wrote: > > The other issue is application priority in windows. Here’s an example. I > have a timer running at .75 seconds in Visual Basic. To the left is normal > priority and to the right I set the application to real time priority in > device manager. > > I then calculated the time between timer interrupts. I was having a problem > reading my 3457a over GPIB using Visual basic at a consistent rate. I still > can’t figure out why every 4 or 5 reads, the time increases by about .05 > seconds. It’s not in the 3457a, could be in the Keysight VISA code. What I > need is a way to read GPIB with an STM32F7 board. > > After setting the priority to real time, the standard deviation improved by a > factor of 7. By the way, I thought it was interesting that my PC counts > ticks to .1us. > > > > Regards, > > Jerry > > >> On Dec 26, 2018, at 3:42 PM, Hal Murray wrote: >> >> >> t...@leapsecond.com said: >>> But most people have only one counter (one internal or external timebase >>> reference) and one clock to be measured. So the measurements are one-to-one. >>> If you have more references or more clocks, you're welcome to combine 2, or >>> 3, or as many as you want. It gets complicated but in some cases this >>> complexity is justified. >> >> Lots of people have several PCs, each with their own clock. If you have >> your >> time-nut hat on, they are crappy clocks with lots of common mode errors >> (temperature, network delays). I think the techniques should apply. >> >> >> -- >> These are my opinions. I hate spam. >> >> >> >> >> ___ >> 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.
Re: [time-nuts] question about multi-way measurement
The other issue is application priority in windows. Here’s an example. I have a timer running at .75 seconds in Visual Basic. To the left is normal priority and to the right I set the application to real time priority in device manager. I then calculated the time between timer interrupts. I was having a problem reading my 3457a over GPIB using Visual basic at a consistent rate. I still can’t figure out why every 4 or 5 reads, the time increases by about .05 seconds. It’s not in the 3457a, could be in the Keysight VISA code. What I need is a way to read GPIB with an STM32F7 board. After setting the priority to real time, the standard deviation improved by a factor of 7. By the way, I thought it was interesting that my PC counts ticks to .1us. Regards, Jerry > On Dec 26, 2018, at 3:42 PM, Hal Murray wrote: > > > t...@leapsecond.com said: >> But most people have only one counter (one internal or external timebase >> reference) and one clock to be measured. So the measurements are one-to-one. >> If you have more references or more clocks, you're welcome to combine 2, or >> 3, or as many as you want. It gets complicated but in some cases this >> complexity is justified. > > Lots of people have several PCs, each with their own clock. If you have your > time-nut hat on, they are crappy clocks with lots of common mode errors > (temperature, network delays). I think the techniques should apply. > > > -- > These are my opinions. I hate spam. > > > > > ___ > 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.
Re: [time-nuts] question about multi-way measurement
t...@leapsecond.com said: > But most people have only one counter (one internal or external timebase > reference) and one clock to be measured. So the measurements are one-to-one. > If you have more references or more clocks, you're welcome to combine 2, or > 3, or as many as you want. It gets complicated but in some cases this > complexity is justified. Lots of people have several PCs, each with their own clock. If you have your time-nut hat on, they are crappy clocks with lots of common mode errors (temperature, network delays). I think the techniques should apply. -- These are my opinions. I hate spam. ___ 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.
Re: [time-nuts] question about multi-way measurement
Thanks! No, I don't really have a specific use case. And your reply is very helpful in my continuing education! I had wondered how to do something like this. I always got stuck on how to know which signal came from which clock. But it came up again in my mind when I was watching a waterfall display of the FT-8 herd on 80 meters the other day. FT-8 is an amateur digital mode which has synchronized transmit times. So I had a visual solution of how to resolve times for multiple signals. Is the statistical benefit of the paper-clock always sqrt(number of clocks) if the common error modes are accounted for? What kind of search should I do to find multi-way measurement strategies, is that the correct terminology? On 12/26/18 11:31 AM, Tom Van Baak wrote: I see the different forms of deviation measurements and they are all one-to-one comparisons. Not all. But, yes, often. UTC itself is a wonderful example of making mutual measurements of several hundred atomic clocks and establishing a superior "paper" clock out of them collectively. But most people have only one counter (one internal or external timebase reference) and one clock to be measured. So the measurements are one-to-one. If you have more references or more clocks, you're welcome to combine 2, or 3, or as many as you want. It gets complicated but in some cases this complexity is justified. For example, if I had a device of relatively good resolution that would let me timestamp the events from 100 different clocks, then questions about the change of the mean of the cloud of events, distance from the mean of individual events, etc. could be obtained. Right, in some cases this works. If you had 100 cesium clocks and combined them all, then the result would be a paper clock that's about sqrt(100) or 10x better than any one of them. But in other cases, the improvement is not so good. For example, if you combined 100 quartz wristwatches you may find that the improvement is only a tiny bit better. The reason: it's likely that most of your watches all gain or lose time in proportion to temperature, a common mode effect. So your cloud result is more likely to be a 10x better paper thermometer than a 10x better paper clock. It seems like, if there were a significant number of clocks involved, the mean of the cloud of events would help cancel out positive and negatives and particularly remove the short term randomness ? It's important to look at the statistics to make sure that negatives and positives do in fact cancel. Depending on the type of clock noise and over a finite time span, this assumption is not at all guaranteed. Also note, especially for short-term, that your success depends on how precisely can you compare the clocks and how long it takes to make an accurate comparison. If you measure too quickly or not accurately enough then the measurement noise will ruin your paper clock. If you have some specific use case in mind, let us know. It might be easier to talk real numbers and real clocks than to discuss this topic in generalities. /tvb ___ 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.
Re: [time-nuts] question about multi-way measurement
> I see the different forms of deviation measurements and they are all > one-to-one comparisons. Not all. But, yes, often. UTC itself is a wonderful example of making mutual measurements of several hundred atomic clocks and establishing a superior "paper" clock out of them collectively. But most people have only one counter (one internal or external timebase reference) and one clock to be measured. So the measurements are one-to-one. If you have more references or more clocks, you're welcome to combine 2, or 3, or as many as you want. It gets complicated but in some cases this complexity is justified. > For example, if I had a device of relatively good resolution that would let me > timestamp the events from 100 different clocks, then questions about the > change of the mean of the cloud of events, distance from the mean of > individual events, etc. could be obtained. Right, in some cases this works. If you had 100 cesium clocks and combined them all, then the result would be a paper clock that's about sqrt(100) or 10x better than any one of them. But in other cases, the improvement is not so good. For example, if you combined 100 quartz wristwatches you may find that the improvement is only a tiny bit better. The reason: it's likely that most of your watches all gain or lose time in proportion to temperature, a common mode effect. So your cloud result is more likely to be a 10x better paper thermometer than a 10x better paper clock. > It seems like, if there were a significant number of clocks involved, > the mean of the cloud of events would help cancel out positive and negatives > and > particularly remove the short term randomness ? It's important to look at the statistics to make sure that negatives and positives do in fact cancel. Depending on the type of clock noise and over a finite time span, this assumption is not at all guaranteed. Also note, especially for short-term, that your success depends on how precisely can you compare the clocks and how long it takes to make an accurate comparison. If you measure too quickly or not accurately enough then the measurement noise will ruin your paper clock. If you have some specific use case in mind, let us know. It might be easier to talk real numbers and real clocks than to discuss this topic in generalities. /tvb ___ 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.
Re: [time-nuts] question about multi-way measurement
Hi One simple answer is that it’s not always 1:1 comparisons. The “three corner hat” approach is indeed used to compare three devices at one time. There are some issues with doing that. There are *lots* of papers on where the limits come from and what you need to do ( = pick fairly similar clocks) to eliminate most of them. Bob > On Dec 26, 2018, at 11:30 AM, Chris Howard wrote: > > > > I see the different forms of deviation measurements and they are all > one-to-one comparisons. > > Is there anything to be learned from doing mass data gathering? > > For example, if I had a device of relatively good resolution that would let me > timestamp the events from 100 different clocks, then questions about the > change of the mean of the cloud of events, distance from the mean of > individual > events, etc. could be obtained. > > One of many things I have learned hanging around here is that some > very very smart people have already thought of anything that > might come to me. > > It seems like, if there were a significant number of clocks involved, the mean > of the cloud of events would help cancel out positive and negatives and > particularly > remove the short term randomness ? > > So, has this sort of thing been done? > Why is everything one-to-one only? > > > > ___ > 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.
Re: [time-nuts] question about multi-way measurement
Relatively good resolution. Relative to what? :) Deviation requires you have something to measure against. A “source of truth”. So what are you measuring deviation from? From a system administration perspective , I want all my systems to be consistent. I’ll say that right == consistently wrong For many purposes. The inconsistency is what causes all manner of hair pulling for me as a system administrator across any meaningful fleet size. Very quickly you run into SSL and LDAP authentication issues. I do frequently run date/time checks to generate the cloud of data points , and I do log NTPD client and server via snmp using librenms . I’m about to dive into netdata graphing as well. I’m using a raspberry pi with gps hat for my master time source. Shortly I’ll be having a total of three systems (two using the same hat , one using the adafruit hat and being a pi2). I’ve got some interest in multiple way comparison and will follow this thread shortly. I’ll blog my setup and post a link. > On Dec 26, 2018, at 10:31, Chris Howard wrote: > > > > I see the different forms of deviation measurements and they are all > one-to-one comparisons. > > Is there anything to be learned from doing mass data gathering? > > For example, if I had a device of relatively good resolution that would let me > timestamp the events from 100 different clocks, then questions about the > change of the mean of the cloud of events, distance from the mean of > individual > events, etc. could be obtained. > > One of many things I have learned hanging around here is that some > very very smart people have already thought of anything that > might come to me. > > It seems like, if there were a significant number of clocks involved, the mean > of the cloud of events would help cancel out positive and negatives and > particularly > remove the short term randomness ? > > So, has this sort of thing been done? > Why is everything one-to-one only? > > > > ___ > 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] question about multi-way measurement
I see the different forms of deviation measurements and they are all one-to-one comparisons. Is there anything to be learned from doing mass data gathering? For example, if I had a device of relatively good resolution that would let me timestamp the events from 100 different clocks, then questions about the change of the mean of the cloud of events, distance from the mean of individual events, etc. could be obtained. One of many things I have learned hanging around here is that some very very smart people have already thought of anything that might come to me. It seems like, if there were a significant number of clocks involved, the mean of the cloud of events would help cancel out positive and negatives and particularly remove the short term randomness ? So, has this sort of thing been done? Why is everything one-to-one only? ___ 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.
