Re: [time-nuts] GPSDO with all-digital phase/time measurement?
Bob wrote: You can achieve very good accuracy, but at the cost of waiting thousands of seconds between phase points; i.e. where your 1PPS coincides with the 10 millionth OCXO pulse. So, as your 1PPS pulse bobs back and forth, you will often encounter an OCXO pulse up to 10ns early, or up to 10ns late. So, might you count 9,999,999 pulses from the OCXO immediately followed by 10,000,001 pulses. Neither of those, by itself is a signal to change the EFC voltage to your OCXO. In fact, it is normal for your count to alternate between the two for long periods, if you are very very close to exactly 10MHz, just from the quantization error on the 1PPS. It is also normal for 1/T to control the time between phase crossings. So you have to wait for two miscounts in a row in the same direction to make a change. I have been puzzled more than once by your comments about only changing the DAC count every several minutes or more. I am not familiar with the circuit you are using, but in a digital PLL the errors (assessed every second) typically feed a digital filter that drives the DAC. So, there is generally a very small correction every second according to the long running average of the individual errors, rather than a large correction after hundreds or thousands of seconds. If you only adjust the DAC every two miscounts in one direction, you are guaranteed to get slipped cycles (which appeared to be one of the problems you were having when comparing oscillators). This is a reasonable way to get an oscillator roughly on frequency if it is substantially off to start with, but it is not a good way to hold an oscillator within ppb of the desired frequency, and no way at all to hold it in phase lock with the reference. If that is really the way the circuit you are using works, perhaps it would be better to implement a proper all-digital PLL with digital filter than trying to get better results out of the circuit you are using than it is capable of delivering. Or, perhaps I'm not understanding what you are doing? Best regards, Charles ___ 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] GPSDO with all-digital phase/time measurement?
Hi Charles, The problem is that the only information available is the fact that a phase crossing occurred and whether there were more than 10M counts (or less) since the last PPS. The phase error value is not available to me, nor is the sawtooth value; which would of course be of no value. So, if I have a + phase crossing and then a - phase crossing, what do I know? If they are closely spaced, I can guess that the reason for the bouncing is the jitter on the PPS. If they are not closely spaced, then I can't really conclude anything other than that there is a phase offset in one direction or the other.. I could count the number of crossings over time and estimate the angle of the phase crossing, but I can't really be sure of the direction. Also, since this is not a timing receiver, it tends to wander around about 10-20 ns. So, that wander might be the only reason for a +/- count. In the case where there are two + crossings, or two - crossings in a row, it is a 1/T question. And with 1/T, it may be a long time until the next crossing, depending on how close you are in frequency, and how much the receiver wanders around. I'm aware of the limitations of the hardware. That's why I'm working on a TIC daughterboard. I could have used someone else's board, and a different GPS receiver, and on and on, but what fun would that have been? My goal is to do as much as possible with as little extra as possible using this particular board, learn as much as possible, and enjoy myself. I hope that helps. It's entirely possible that I've made some newbie mistake and that there's a good answer available. But, in that case, I would think that someone else would have already applied it to this board. Oh, and my granddaughter has been pestering me the whole time I've been writing this, so I hope I haven't been a bit short. =) Bob From: Charles Steinmetz csteinm...@yandex.com To: Discussion of precise time and frequency measurement time-nuts@febo.com Sent: Friday, February 28, 2014 12:29 PM Subject: Re: [time-nuts] GPSDO with all-digital phase/time measurement? Bob wrote: You can achieve very good accuracy, but at the cost of waiting thousands of seconds between phase points; i.e. where your 1PPS coincides with the 10 millionth OCXO pulse. So, as your 1PPS pulse bobs back and forth, you will often encounter an OCXO pulse up to 10ns early, or up to 10ns late. So, might you count 9,999,999 pulses from the OCXO immediately followed by 10,000,001 pulses. Neither of those, by itself is a signal to change the EFC voltage to your OCXO. In fact, it is normal for your count to alternate between the two for long periods, if you are very very close to exactly 10MHz, just from the quantization error on the 1PPS. It is also normal for 1/T to control the time between phase crossings. So you have to wait for two miscounts in a row in the same direction to make a change. I have been puzzled more than once by your comments about only changing the DAC count every several minutes or more. I am not familiar with the circuit you are using, but in a digital PLL the errors (assessed every second) typically feed a digital filter that drives the DAC. So, there is generally a very small correction every second according to the long running average of the individual errors, rather than a large correction after hundreds or thousands of seconds. If you only adjust the DAC every two miscounts in one direction, you are guaranteed to get slipped cycles (which appeared to be one of the problems you were having when comparing oscillators). This is a reasonable way to get an oscillator roughly on frequency if it is substantially off to start with, but it is not a good way to hold an oscillator within ppb of the desired frequency, and no way at all to hold it in phase lock with the reference. If that is really the way the circuit you are using works, perhaps it would be better to implement a proper all-digital PLL with digital filter than trying to get better results out of the circuit you are using than it is capable of delivering. Or, perhaps I'm not understanding what you are doing? Best regards, Charles ___ 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 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] GPSDO with all-digital phase/time measurement?
Thanks to everyone who responded. I had this bright idea that I would turn off mail delivery and just read the list through the pipermail interface. It works great, until you want to reply to a message. Sorry if this breaks the thread in two. Language is imprecise even when carefully thought out, and I can see I gave some erroneous impressions. What I am really trying to get at, is whether there is any advantage to using an outboard phase detector like the 4046-based circuit recently posted in the arduino GPSDO thread. I get that this is going to provide a much faster measurement of phase error than the digital counter method. (The RC time constant on that circuit looked pretty fast, ~ 1 second.) But if my loop filter has a time constant measured in the 10s or 100s of seconds, I'm not clear what difference this makes. That was the gist of my 100ns/T thought experiment. Ditto on the comment re: long averaging times != infrequent tuning updates. To clarify my requirements a bit more, I am mostly interested in the learning exercise, but it would be nice to end up with a frequency standard capable of supporting symbol periods (coherent) on the order of seconds for VLF and shortwave digital comms experiments. So, a pretty modest goal of, say, 1E-9. Of course, being an engineer I want to do the best job possible with the parts at hand. Toolkit: I have a handful of the 26-MHz Pletronics ebay OCXOs. The spec sheet says +/- 0.5 ppb over 30 seconds short-term stability. I will try to use a GPS module enabling SW sawtooth correction, an NV08C if I could get it. MCU of choice is STM32 (ARM Cortex-M4). I am still reading the fine print but I believe interrupt handling is strictly deterministic. So, based on the OCXO short-term spec and a 1E-9 performance requirement, could I not then estimate the loop time constant at ~100 seconds, and furthermore argue that my TIC has an effective measurement accuracy of 1/(26 MHz)/(100 s) = 0.4 ns ? Meaning, also, that SW sawtooth correction would be worthwhile? Mark KJ6PC ___ 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] GPSDO with all-digital phase/time measurement?
Bob, If your hardware can only measure if the phase leads or lags. I thnk you still want to change the DAC every second. But you don't do it directly. Used a PI controller. (PI is just PID with no D) You phase detector DOES measure the phase error but the resolution is poor, you only get one bit plus the sign. So you must assume a value for the one bit. Just call it 0.5 degrees or whatever. It may not matter much if you guess wrong. So you get either -0.5 or +0.5 for each phase measurement. Then the correction is to be applied each second is Kp * (phase measurement) + Ki (the sum of all phase measurements) Then you set the DAC to whatever it currently is plus the above correction. If your GPSDO is working correctly and in sync the sum of all phase measurements is be nearly zero but if at start up yu are way off the error will add up and you will apply more and more correction. Then it over shoots but the error integrated over time eventually goes to zero. The key d finding Kp and Ki This works if your local oscillation is good enough not to loose or gain a full cycle per second. Have some other way (based on counting cycles) to set it close then let the Pi controller take over. Once teetotal error gets to zero the PI controller should dither the DAC by one count up and down. On Fri, Feb 28, 2014 at 11:31 AM, Bob Stewart b...@evoria.net wrote: Hi Charles, The problem is that the only information available is the fact that a phase crossing occurred and whether there were more than 10M counts (or less) since the last PPS. The phase error value is not available to me, nor is the sawtooth value; which would of course be of no value. So, if I have a + phase crossing and then a - phase crossing, what do I know? If they are closely spaced, I can guess that the reason for the bouncing is the jitter on the PPS. If they are not closely spaced, then I can't really conclude anything other than that there is a phase offset in one direction or the other.. I could count the number of crossings over time and estimate the angle of the phase crossing, but I can't really be sure of the direction. Also, since this is not a timing receiver, it tends to wander around about 10-20 ns. So, that wander might be the only reason for a +/- count. In the case where there are two + crossings, or two - crossings in a row, it is a 1/T question. And with 1/T, it may be a long time until the next crossing, depending on how close you are in frequency, and how much the receiver wanders around. I'm aware of the limitations of the hardware. That's why I'm working on a TIC daughterboard. I could have used someone else's board, and a different GPS receiver, and on and on, but what fun would that have been? My goal is to do as much as possible with as little extra as possible using this particular board, learn as much as possible, and enjoy myself. I hope that helps. It's entirely possible that I've made some newbie mistake and that there's a good answer available. But, in that case, I would think that someone else would have already applied it to this board. Oh, and my granddaughter has been pestering me the whole time I've been writing this, so I hope I haven't been a bit short. =) Bob From: Charles Steinmetz csteinm...@yandex.com To: Discussion of precise time and frequency measurement time-nuts@febo.com Sent: Friday, February 28, 2014 12:29 PM Subject: Re: [time-nuts] GPSDO with all-digital phase/time measurement? Bob wrote: You can achieve very good accuracy, but at the cost of waiting thousands of seconds between phase points; i.e. where your 1PPS coincides with the 10 millionth OCXO pulse. So, as your 1PPS pulse bobs back and forth, you will often encounter an OCXO pulse up to 10ns early, or up to 10ns late. So, might you count 9,999,999 pulses from the OCXO immediately followed by 10,000,001 pulses. Neither of those, by itself is a signal to change the EFC voltage to your OCXO. In fact, it is normal for your count to alternate between the two for long periods, if you are very very close to exactly 10MHz, just from the quantization error on the 1PPS. It is also normal for 1/T to control the time between phase crossings. So you have to wait for two miscounts in a row in the same direction to make a change. I have been puzzled more than once by your comments about only changing the DAC count every several minutes or more. I am not familiar with the circuit you are using, but in a digital PLL the errors (assessed every second) typically feed a digital filter that drives the DAC. So, there is generally a very small correction every second according to the long running average of the individual errors, rather than a large correction after hundreds or thousands of seconds. If you only adjust the DAC every two miscounts in one direction, you are guaranteed to get slipped cycles
Re: [time-nuts] GPSDO with all-digital phase/time measurement?
hau...@keteu.org said: That being said, I find myself wondering as follows: Suppose that we count OCXO cycles (at, say, 10 MHz) using one of the MCU's timer/counter peripherals, and periodically sample the counter value with an interrupt triggered on the rising edge of the GPS 1pps. Assume that this interrupt is the highest priority in the system, so that our measurement is fully deterministic, having only the +/- one cycle ambiguity inherent in the counting. Also assume that we keep the counter running continuously. In general, things like highest priority interrupt don't guarantee what you want. The fine print depends upon your MCU. Some instructions take more than 1 cycle. Consider what happens if your interrupt arrives just after another interrupt got started. For this problem, I'd check the fine print on the counter/timers. They often have a mode where it will copy the counter to a holding register (and generate an interrupt) on the rising edge of an input pin. If you also get an interrupt on the counter overflowing, you can work out the total number of cycles between rising edges. -- The other problem you need to be aware of if you want to build a GPSDO is hanging bridges. tvb: Motorola GPS M12+ Sawtooth http://www.leapsecond.com/pages/m12/sawtooth.htm Tom Clark and Rick Hambly: Timing for VLBI http://gpstime.com/files/tow-time2009.pdf -- These are my opinions. I hate spam. ___ 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] GPSDO with all-digital phase/time measurement?
Hi Hal, I've reviewed this a bit and I used the wrong terminology. Either Timer1 or Timer3 can use the T1CKI input as a clock. Along with that, the CCP1 or CCP2 pin can be used to trigger a capture of the timer in use into the CCPR1 or CCPR2 register pair. I had considered this as an interrupt function, but after looking at the manual, it's a matter of switchable dedicated hardware. Sorry for the bad info. Bob From: Hal Murray hmur...@megapathdsl.net To: Bob Stewart b...@evoria.net; Discussion of precise time and frequency measurement time-nuts@febo.com Cc: hmur...@megapathdsl.net Sent: Thursday, February 27, 2014 1:13 AM Subject: Re: [time-nuts] GPSDO with all-digital phase/time measurement? b...@evoria.net said: At least on the PIC I'm using, the CCP and timer interrupts don't seem to be synchronous with the PIC clock. I could be mistaken. Unless you have a very strange architecture, it doesn't make sense for an interrupt to not be synchronous with the CPU clock. You are in the middle of an add instruction, and now you want to start an interrupt. What does that mean? I expect there is the standard 2 FF synchronizer on all the input pins. Things like the counter/timers run on the CPU clock, taking their input after the synchronizer. I don't remember seeing a data sheet that comes out and says that, but sometimes you can get some (strong?) hints with things like minimum pulse widths or max clock rate. -- These are my opinions. I hate spam. ___ 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] GPSDO with all-digital phase/time measurement?
Hi Alex, As these guys have told me, and as I have learned, Rb standards are very regular, but that doesn't mean they are very accurate. So, unless your Rb standard was being disciplined to the right frequency you'd probably have to do some sort of analysis to see the quantizing errors on your 1PPS pulse. Bob From: Alexander Pummer alex...@ieee.org To: Bob Stewart b...@evoria.net; Discussion of precise time and frequency measurement time-nuts@febo.com Sent: Thursday, February 27, 2014 7:42 AM Subject: Re: [time-nuts] GPSDO with all-digital phase/time measurement? it would be interesting to see the accuracy of the 1pps pulses by comparing them with a second 1pps pulse, which is derived from a rubidium standard, which on his own does not have quantizing errors, 73 KJ6UHN Alex ___ 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] GPSDO with all-digital phase/time measurement?
it would be interesting to see the accuracy of the 1pps pulses by comparing them with a second 1pps pulse, which is derived from a rubidium standard, which on his own does not have quantizing errors, 73 KJ6UHN Alex On 2/26/2014 6:03 PM, Bob Stewart wrote: Hi Mark, I'm neither an engineer, nor an expert, but here are my comments. I think that the idea of 100ns/T is wrong. There are several variables that control accuracy, but the time between pulses from your OCXO (assuming no phase or frequency drift) isn't one of them. So, that gives 1/T. Here the problem is that T must get large before your accuracy can be good. You can achieve very good accuracy, but at the cost of waiting thousands of seconds between phase points; i.e. where your 1PPS coincides with the 10 millionth OCXO pulse. The theoretical maximum would be infinity, of course, but your oscillator won't be that stable. Another big problem is the accuracy of the 1PPS pulse. I'm using an Adafruit GPS receiver, and it's listed as accurate to within 10ns. And it is, but you have to be wary of exactly what that means. It doesn't mean +/- 5ns. So, as your 1PPS pulse bobs back and forth, you will often encounter an OCXO pulse up to 10ns early, or up to 10ns late. So, might you count 9,999,999 pulses from the OCXO immediately followed by 10,000,001 pulses. Neither of those, by itself is a signal to change the EFC voltage to your OCXO. In fact, it is normal for your count to alternate between the two for long periods, if you are very very close to exactly 10MHz, just from the quantization error on the 1PPS. It is also normal for 1/T to control the time between phase crossings. So you have to wait for two miscounts in a row in the same direction to make a change. And even then, you can't be 100% sure that it's not due to the quantization errors in your 1PPS signal. The better GPS receivers will output a quantization error value every second. But if you're using the 1/T method, there's nothing you can do with it, so you have to live with whatever quantization errors you get. Anyway, those are my experiences. Bob - AE6RV From: Mark Haun hau...@keteu.org To: time-nuts@febo.com Sent: Wednesday, February 26, 2014 3:51 PM Subject: [time-nuts] GPSDO with all-digital phase/time measurement? Hi everyone, I'm new to the list, and have been reading the recent threads on Arduino-based GPSDOs and the pros/cons of 10-kHz vs 1-Hz time pulses with interest. As I understand it, there are a couple of reasons why one needs a time-interval / phase measurement implemented outside the MCU: 1) Time resolution inside the MCU is limited by its clock period, which is much too coarse. The GPSDO would ping-pong within a huge dead zone. 2) Software tends to inject non-determinism into the timing. snip Thanks, Mark KJ6PC ___ 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 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] GPSDO with all-digital phase/time measurement?
Hi everyone, I'm new to the list, and have been reading the recent threads on Arduino-based GPSDOs and the pros/cons of 10-kHz vs 1-Hz time pulses with interest. As I understand it, there are a couple of reasons why one needs a time-interval / phase measurement implemented outside the MCU: 1) Time resolution inside the MCU is limited by its clock period, which is much too coarse. The GPSDO would ping-pong within a huge dead zone. 2) Software tends to inject non-determinism into the timing. Are there others? I have no background or experience with PLLs/DLLs, so I'm really just feeling my way blindly here. That being said, I find myself wondering as follows: Suppose that we count OCXO cycles (at, say, 10 MHz) using one of the MCU's timer/counter peripherals, and periodically sample the counter value with an interrupt triggered on the rising edge of the GPS 1pps. Assume that this interrupt is the highest priority in the system, so that our measurement is fully deterministic, having only the +/- one cycle ambiguity inherent in the counting. Also assume that we keep the counter running continuously. At this point the time measurement is quite crude, with 100-ns resolution. But because we keep the counter running, the unknown residuals will keep accumulating, and we should be able to average out this quantization noise in the long run. That is, we can measure any T-second period to within 100 ns, so the resolution on a per-second basis becomes 100 ns / T. Is there any reason why this sort of processing cannot attain equivalent performance to the more conventional analog phase-detection approach? Thanks, Mark KJ6PC ___ 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] GPSDO with all-digital phase/time measurement?
Hi Mark, Not to worry. It's not really too coarse. Your method is more than enough for millisecond or microsecond timing. Consider that everything about time frequency is merely an exponent; avoid fuzzy words like coarse or fine. Your approach will work just fine; many a GPSDO has been designed that way. If you measure your oscillator to a second using WWVB and make adjustments, or if you measure your oscillator to a millisecond using NTP and make adjustments, or if you measure your oscillator to a microsecond using GPS and make adjustments, it is really all the same thing. The exponent is different but the concept is the same. The final result depends on the time quality of the external 1PPS, the frequency quality of your oscillator, the resolution quality of your measurement, and the granularity of your adjustment. So every GPSDO works, the question is simply how well does it work? And does it meet your needs? For many applications 1 ms or 1 us timing accuracy is more than enough. For only a few applications does 100 ns or 50 ns matter. It gets much, much harder as you get into the double and single ns digit range. Several members on this list have developed GPSDO that are based on a MCU timestamping of 1PPS pulses. It works fine. Perfecting the tuning constants takes some time, and is somewhat dependent on your GPS receiver, your antenna, your location. Your oscillator, environment, and MCU also play a key part. But the art is to be able to *measure* all of this, to document it, to experiment, and to possibly improve on it over time. /tvb - Original Message - From: Mark Haun hau...@keteu.org To: time-nuts@febo.com Sent: Wednesday, February 26, 2014 1:51 PM Subject: [time-nuts] GPSDO with all-digital phase/time measurement? Hi everyone, I'm new to the list, and have been reading the recent threads on Arduino-based GPSDOs and the pros/cons of 10-kHz vs 1-Hz time pulses with interest. As I understand it, there are a couple of reasons why one needs a time-interval / phase measurement implemented outside the MCU: 1) Time resolution inside the MCU is limited by its clock period, which is much too coarse. The GPSDO would ping-pong within a huge dead zone. 2) Software tends to inject non-determinism into the timing. Are there others? I have no background or experience with PLLs/DLLs, so I'm really just feeling my way blindly here. That being said, I find myself wondering as follows: Suppose that we count OCXO cycles (at, say, 10 MHz) using one of the MCU's timer/counter peripherals, and periodically sample the counter value with an interrupt triggered on the rising edge of the GPS 1pps. Assume that this interrupt is the highest priority in the system, so that our measurement is fully deterministic, having only the +/- one cycle ambiguity inherent in the counting. Also assume that we keep the counter running continuously. At this point the time measurement is quite crude, with 100-ns resolution. But because we keep the counter running, the unknown residuals will keep accumulating, and we should be able to average out this quantization noise in the long run. That is, we can measure any T-second period to within 100 ns, so the resolution on a per-second basis becomes 100 ns / T. Is there any reason why this sort of processing cannot attain equivalent performance to the more conventional analog phase-detection approach? Thanks, Mark KJ6PC ___ 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] GPSDO with all-digital phase/time measurement?
At this point the time measurement is quite crude, with 100-ns resolution. But because we keep the counter running, the unknown residuals will keep accumulating, and we should be able to average out this quantization noise What you are saying is With a long enough gate time you can measure frequency to any desired level of accuracy. For that to be true the frequency must remain stable over the long period you are averaging. For example let's say you average over 10 seconds. What happens of the nominal 10MHz oscillator ran at 10.1 Mhz for five seconds and then 9.9 Mhz for 5 seconds. You'd measure 10Mhz and be happy. So there is the problem. With long period averaging, you can only do it for so long as you trust the clock to not change. All the long gate time tells you is the AVERAGE so it only works of the change over the period is smooth and monotonic. If you want very good accuracy, say one part in 10^14 you can't get there by waiting days and weeks because the frequency will move while you are measuring it. The quicker way is to measure the phase. You can still do this digitally. Make a one bit measurement. zero if the phase leads, 1 if it lags and then over time you want the average to be 0.5 This works much faster because your controller does not have to wait for an entire cycle of error to accumulate. Your counting system would have to wait for a one cycle error and then make a very large and infrequent correction. It could work for some use cases. -- Chris Albertson Redondo Beach, California ___ 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] GPSDO with all-digital phase/time measurement?
Hi Mark, I'm neither an engineer, nor an expert, but here are my comments. I think that the idea of 100ns/T is wrong. There are several variables that control accuracy, but the time between pulses from your OCXO (assuming no phase or frequency drift) isn't one of them. So, that gives 1/T. Here the problem is that T must get large before your accuracy can be good. You can achieve very good accuracy, but at the cost of waiting thousands of seconds between phase points; i.e. where your 1PPS coincides with the 10 millionth OCXO pulse. The theoretical maximum would be infinity, of course, but your oscillator won't be that stable. Another big problem is the accuracy of the 1PPS pulse. I'm using an Adafruit GPS receiver, and it's listed as accurate to within 10ns. And it is, but you have to be wary of exactly what that means. It doesn't mean +/- 5ns. So, as your 1PPS pulse bobs back and forth, you will often encounter an OCXO pulse up to 10ns early, or up to 10ns late. So, might you count 9,999,999 pulses from the OCXO immediately followed by 10,000,001 pulses. Neither of those, by itself is a signal to change the EFC voltage to your OCXO. In fact, it is normal for your count to alternate between the two for long periods, if you are very very close to exactly 10MHz, just from the quantization error on the 1PPS. It is also normal for 1/T to control the time between phase crossings. So you have to wait for two miscounts in a row in the same direction to make a change. And even then, you can't be 100% sure that it's not due to the quantization errors in your 1PPS signal. The better GPS receivers will output a quantization error value every second. But if you're using the 1/T method, there's nothing you can do with it, so you have to live with whatever quantization errors you get. Anyway, those are my experiences. Bob - AE6RV From: Mark Haun hau...@keteu.org To: time-nuts@febo.com Sent: Wednesday, February 26, 2014 3:51 PM Subject: [time-nuts] GPSDO with all-digital phase/time measurement? Hi everyone, I'm new to the list, and have been reading the recent threads on Arduino-based GPSDOs and the pros/cons of 10-kHz vs 1-Hz time pulses with interest. As I understand it, there are a couple of reasons why one needs a time-interval / phase measurement implemented outside the MCU: 1) Time resolution inside the MCU is limited by its clock period, which is much too coarse. The GPSDO would ping-pong within a huge dead zone. 2) Software tends to inject non-determinism into the timing. snip Thanks, Mark KJ6PC ___ 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] GPSDO with all-digital phase/time measurement?
Tom, I took his 100ns figure to be simply the period of 10MHz. He mentioned using an interrupt driven system, so the counts should not necessarily be limited to 100ns accuracy. At least on the PIC I'm using, the CCP and timer interrupts don't seem to be synchronous with the PIC clock. I could be mistaken. Bob From: Tom Van Baak t...@leapsecond.com To: Discussion of precise time and frequency measurement time-nuts@febo.com Sent: Wednesday, February 26, 2014 11:22 PM Subject: Re: [time-nuts] GPSDO with all-digital phase/time measurement? At this point the time measurement is quite crude, with 100-ns resolution. But because we keep the counter running, the unknown residuals will keep accumulating, and we should be able to average out this quantization noise in the long run. That is, we can measure any T-second period to within 100 ns, so the resolution on a per-second basis becomes 100 ns / T. No. The timing resolution per second is always 100 ns. You're probably thinking about average frequency, in which case dividing by T is sometimes valid, and it looks better and better as time goes by, usually. What saves you here is that your counter noise (100 ns) is likely greater than the quantization noise. So you can pretty much ignore the receiver 1PPS quantization noise. For people with much lower measurement noise (e.g., 1 ns) the quantization noise becomes a more important piece of the error pie. Try not to say average out; that sounds like it goes away over time or gets smaller. You're doing a timing measurement so the 100 ns measurement granularity is always there, on every measurement. Is there any reason why this sort of processing cannot attain equivalent performance to the more conventional analog phase-detection approach? All other factors equal, a GPSDO based on 100 ns measurement resolution can never attain the equivalent of a GPSDO based on 10 ns or 1 ns measurement resolution. Waiting shorter or longer doesn't change the RMS timing accuracy. /tvb ___ 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 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] GPSDO with all-digital phase/time measurement?
b...@evoria.net said: At least on the PIC I'm using, the CCP and timer interrupts don't seem to be synchronous with the PIC clock. I could be mistaken. Unless you have a very strange architecture, it doesn't make sense for an interrupt to not be synchronous with the CPU clock. You are in the middle of an add instruction, and now you want to start an interrupt. What does that mean? I expect there is the standard 2 FF synchronizer on all the input pins. Things like the counter/timers run on the CPU clock, taking their input after the synchronizer. I don't remember seeing a data sheet that comes out and says that, but sometimes you can get some (strong?) hints with things like minimum pulse widths or max clock rate. -- These are my opinions. I hate spam. ___ 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.
