Re: [time-nuts] ! PPS Source
Tom, I for one would like to see what you have down that "rabbit hole" regarding the new architecture of the CNS clock, that does not do variable delay line sawtooth corrections. Dana On Sat, Aug 15, 2020 at 1:41 PM Tom Van Baak wrote: > Hi Ed, > > > I'm also aware that some people have come up with external > > sawtooth correction, using programmable digital delay lines > > and special control circuits. > > For decades fellow time nut Rick Hambly has done hands-on work with GPS > receiver timing, including sawtooth correction. Here's a recent version > of his classic series of presentations: > > > https://www.cnssys.com/files/TOW/High-accuracy_Time_and_Frequency_in_VLBI_2019_sem.pdf > > His "CNS clock" was an example of a GPS/1PPS source that used the delay > technique to reduce sawtooth effects. The paper is worth a look; lots of > good info packed along with plenty of plots and photos. > > Note that his most recent version of the CNS clock no longer uses the > delay technique. I can explain more if you want to go down that rabbit > hole. > > > But, why do all that if you can just fix the clock instead? > > Because it is not possible for you or me to "fix the clock instead". > That's a complex business and design decision inside the GPS chip > itself. Almost every DIY or commercial GPSDO uses an off-the-shelf GPS > receiver chip or PCB, so you work with what you can get. Those GPS > modules all use some tiny carefully-specified free-running internal > oscillator for signal sampling, computing, and output pulse generation. > > Trimble was in a unique position when the GPSDO era began because they > made both GPS chips and a GPSDO product. For them the integrated OCXO > and GPS receiver design was possible. Even hp couldn't do that. > > > I think the clock is a Motorola brand odd looking TCXO, labeled "19096" > > or something like that, probably 19.096 MHz, as I recall. > > That sounds right. See http://www.leapsecond.com/pages/vp/sawtooth.htm > and note Dr Clark's mention of 9.54 MHz, which would be half your 19.096. > > > 1. If the GPS RX module's internal clock is synthesized (to the same > > nominal frequency) from the 10 MHz output of the GPSDO, can that > > alone eliminate or substantially reduce the sawtooth effect? > > Yes. But it would also create a list of other technical concerns. I'd > probably rather have to deal with sawtooth than having to deal with DDS > artifacts or phase noise requirements. And now your GPS chip won't even > boot or work right if the user's 10 MHz isn't there, or isn't accurate > enough, or is still warming up, or if the user's 10 MHz has too much > phase noise or instability. > > > 2. Does the T-bolt actually do this, and if so, is that all it takes? > > Yes, we think so. But remember they had control over everything; the GPS > chip, the firmware, and the specification of the OCXO. > > > If either answer is yes, then I would think the GPS RX makers would > have provisions > > for external clock reference, at least for certain high-grade timing > type models. > > Yes, very high-grade timing receivers require, or allow, external > frequency inputs. I've seen some in operation at national timing labs. > When I heard they cost $20k I decided sawtooth correction wasn't so bad > after all. ;-) > > It seems sawtooth bothers you. There are sawtooth effects in many parts > of technology; from printer dots to PC clocks; from calendars to leap > seconds. The goal isn't always to eliminate but rather to fully > understand the effects. The fact that GPS timing receivers output > quantization corrections should be seen as a good thing; it gives the > user maximum information. > > /tvb > > > On 8/13/2020 3:35 PM, ed breya wrote: > > I have often wondered about all this sawtooth correction stuff, and I > > think I've asked here too, but never got a definitive answer. Every > > time this comes up, there are all sorts of explanations of the > > characteristic, and inevitably someone mentions the T-bolt having its > > internal GPS clock synchronized with the desired ideal 10 MHz output, > > either eliminating or greatly reducing the sawtooth effect. But, to my > > knowledge, nobody has said for sure if this is indeed the case, or > > whether that's all it takes to achieve perfection, or if more magic is > > needed besides synchronizing. I'm also aware that some people have > > come up with external sawtooth correction, using programmable digital > > delay lines and special control circuits. But, why do all that if you > > can just fix the clock instead? > > > > Many years ago I looked at my Motorola Oncore VP (or whatever model is > > used in the HPZ3801A), to see what its clock was, and if it was > > reasonably possible to synthesize it from the 10 MHz. I think the > > clock is a Motorola brand odd looking TCXO, labeled "19096" or > > something like that, probably 19.096 MHz, as I recall. That's about as > > far as I got. Since then I've just wondered what would happen
Re: [time-nuts] ! PPS Source
Hi Keep in mind that a GPS receiver is *very* sensitive to close in phase noise on the clock source. If you dig into the data sheets on the chip sets, they are quite explicit about this. Any approach you use to steer the TCXO will need to be very quiet. Next, the internal firmware *assumes* that the oscillator does not move (much). It needs to take care of things like doppler on the each of the sat’s it is tracking. Doing so with a clock that moves too much likely will not work well. One of the issues with the early clock sources for these devices was perturbations in the frequency vs temperature curve. They spent a lot of time re-designing crystals to “clean them up”. The issue was simply that a perturbation moved the frequency faster than the firmware could keep up. Lots of fun ….. Bob > On Aug 17, 2020, at 4:53 PM, ed breya wrote: > > Thanks all, for the info on this issue. It does indeed look doable to > experiment with phase-locking the GPS RX module's internal clock to the > ultimate 10 MHz GPSDO output. That is, doable, but not necessarily easy or > without problems. > > I dug up my old notes and info from about ten years ago, and found I had > studied it quite extensively, and had some various schemes sketched out > already - I forgot about all this. In the notes I found some pages from US > Patents 4,785,463, and 5,745,741, which are pertinent to the Motorola Oncore > RX units. I also found page 26 of a document "Remote Frequency Calibration: > The NIST Frequency Measurement and Analysis Service," which talks about it > too. > > The internal clock appears to be 19.096 MHz nominal, +/- 2 ppm, from a TCXO, > that may or may not have voltage tuning too, depending on the model. The '463 > patent appears to be about the original overall design, while the '741 is > about reducing the sawtooth error by doing coarse digital corrections in the > counter system, while also fine tuning the VCTXO, to get an integer clock > frequency under all conditions. It mentions the Oncore model as prior art, > and the resulting sawtooth error. > > As I understand it, the overall process disclosed is to allow for the TCXO to > drift to any frequency in range, but automatically tweak it slightly to make > sure it's an integer (Hz ) value, and set the counter system to divide by > that same integer value, so the 1 PPS output is consistent, without sawtooth > error. So, if the clock is nearly exactly right on, the counter divides by > 19,096,000, and it figures out how to fine tune the clock to keep it there. > If the drift goes beyond the fine range to say all the way to the upper > stated limit of +38 Hz, the counter is set to divide by 19,096,038, and the > clock is again tweaked to keep it close to that integer Hz. How it does all > this is disclosed in the patent, but I haven't studied it enough to say any > more. > > It looks like this improvement was in a later model, or was perhaps never > actually used, since this was around the time that Motorola was departing the > GPS business. I assume the older Oncores like mine do not have any of this > improvement, so are subject to both the clock drift and the sawtooth. But, > one thing I got from this, is that if the nominal clock and divide numbers > match, and are fixed at 19,096,000, then replacing the original clock with a > sufficiently clean synthesized external clock should work too. > > I have a number of possible options, depending on the actual original TCXO. > If it's also voltage-tuned, but that isn't utilized, then it can stay, but > needs circuit mods to release and access the tune signal. The clock signal is > accessible at the TCXO, or possibly less risky in 2X form 38.192 MHz from > the downconverter IC. This would be the best option, to make an external PLL > to tweak it. BTW I have a spare GPS RX unit - I would not risk taking the > Z3801A out of commission and messing around with it. > > If the TCXO has no tuning ability, then a new clock signal needs to be made. > Of all the schemes I sketched out back then, the most straightforward seems > to be simply adding 4.096 MHz from a VCXO, PLLed to 10 MHz, and 15 MHz > derived directly from the 10. These can all be scaled up in frequency in > various arrangements, and use standard binary frequency XOs. The next notch > up for instance, is 8.192 MHz + 30 MHz giving twice the clock, and so forth. > Unfortunately, this method is additive in frequency, which I hate - I prefer > to take the difference of two much higher frequencies, which is so much > easier to filter. I don't yet see any ways to do a difference method without > using special frequencies, so for now I assume regular old standard XO > frequencies will have to do, and the filter designs will need to be fancier. > The PLLing seems to be straightforward. I can get a decent 16 kHz comparison > frequency with simple integer dividing, like 4.096 MHz/256 = 10 MHz/625, but > would
Re: [time-nuts] ! PPS Source
Ed, Good work digging deep into that. I remember hearing about someone playing with the Oncore oscillator. It was Robin Giffard, one of the key architects behind the hp SmartClock series (58503A, Z3801A, etc.). A copy of his paper: "Estimation of GPS Ionospheric Delay Using L1 Code and Carrier Phase Observables" https://apps.dtic.mil/sti/pdfs/ADA497270.pdf You'll see the Motorola Oncore VP mentioned in several places. He used a 5061B and a simple M/N PLL to give 3590 / 188 MHz = 19.0957 MHz. I don't recall any other papers describing a similar experiment. In that era the Oncore VP was one of the favorite timing receivers. You would think if there was any merit to the clock hack then lots of people or products would do it. But AFAIK, none did, not even hp. All of these GPSDO had microprocessors and TIC onboard so using the GPS receiver generated sawtooth correction message is all they needed. /tvb On 8/17/2020 1:53 PM, ed breya wrote: Thanks all, for the info on this issue. It does indeed look doable to experiment with phase-locking the GPS RX module's internal clock to the ultimate 10 MHz GPSDO output. That is, doable, but not necessarily easy or without problems. I dug up my old notes and info from about ten years ago, and found I had studied it quite extensively, and had some various schemes sketched out already - I forgot about all this. In the notes I found some pages from US Patents 4,785,463, and 5,745,741, which are pertinent to the Motorola Oncore RX units. I also found page 26 of a document "Remote Frequency Calibration: The NIST Frequency Measurement and Analysis Service," which talks about it too. The internal clock appears to be 19.096 MHz nominal, +/- 2 ppm, from a TCXO, that may or may not have voltage tuning too, depending on the model. The '463 patent appears to be about the original overall design, while the '741 is about reducing the sawtooth error by doing coarse digital corrections in the counter system, while also fine tuning the VCTXO, to get an integer clock frequency under all conditions. It mentions the Oncore model as prior art, and the resulting sawtooth error. As I understand it, the overall process disclosed is to allow for the TCXO to drift to any frequency in range, but automatically tweak it slightly to make sure it's an integer (Hz ) value, and set the counter system to divide by that same integer value, so the 1 PPS output is consistent, without sawtooth error. So, if the clock is nearly exactly right on, the counter divides by 19,096,000, and it figures out how to fine tune the clock to keep it there. If the drift goes beyond the fine range to say all the way to the upper stated limit of +38 Hz, the counter is set to divide by 19,096,038, and the clock is again tweaked to keep it close to that integer Hz. How it does all this is disclosed in the patent, but I haven't studied it enough to say any more. It looks like this improvement was in a later model, or was perhaps never actually used, since this was around the time that Motorola was departing the GPS business. I assume the older Oncores like mine do not have any of this improvement, so are subject to both the clock drift and the sawtooth. But, one thing I got from this, is that if the nominal clock and divide numbers match, and are fixed at 19,096,000, then replacing the original clock with a sufficiently clean synthesized external clock should work too. I have a number of possible options, depending on the actual original TCXO. If it's also voltage-tuned, but that isn't utilized, then it can stay, but needs circuit mods to release and access the tune signal. The clock signal is accessible at the TCXO, or possibly less risky in 2X form 38.192 MHz from the downconverter IC. This would be the best option, to make an external PLL to tweak it. BTW I have a spare GPS RX unit - I would not risk taking the Z3801A out of commission and messing around with it. If the TCXO has no tuning ability, then a new clock signal needs to be made. Of all the schemes I sketched out back then, the most straightforward seems to be simply adding 4.096 MHz from a VCXO, PLLed to 10 MHz, and 15 MHz derived directly from the 10. These can all be scaled up in frequency in various arrangements, and use standard binary frequency XOs. The next notch up for instance, is 8.192 MHz + 30 MHz giving twice the clock, and so forth. Unfortunately, this method is additive in frequency, which I hate - I prefer to take the difference of two much higher frequencies, which is so much easier to filter. I don't yet see any ways to do a difference method without using special frequencies, so for now I assume regular old standard XO frequencies will have to do, and the filter designs will need to be fancier. The PLLing seems to be straightforward. I can get a decent 16 kHz comparison frequency with simple integer dividing, like 4.096 MHz/256 = 10 MHz/625, but would like to get
Re: [time-nuts] ! PPS Source
Thanks all, for the info on this issue. It does indeed look doable to experiment with phase-locking the GPS RX module's internal clock to the ultimate 10 MHz GPSDO output. That is, doable, but not necessarily easy or without problems. I dug up my old notes and info from about ten years ago, and found I had studied it quite extensively, and had some various schemes sketched out already - I forgot about all this. In the notes I found some pages from US Patents 4,785,463, and 5,745,741, which are pertinent to the Motorola Oncore RX units. I also found page 26 of a document "Remote Frequency Calibration: The NIST Frequency Measurement and Analysis Service," which talks about it too. The internal clock appears to be 19.096 MHz nominal, +/- 2 ppm, from a TCXO, that may or may not have voltage tuning too, depending on the model. The '463 patent appears to be about the original overall design, while the '741 is about reducing the sawtooth error by doing coarse digital corrections in the counter system, while also fine tuning the VCTXO, to get an integer clock frequency under all conditions. It mentions the Oncore model as prior art, and the resulting sawtooth error. As I understand it, the overall process disclosed is to allow for the TCXO to drift to any frequency in range, but automatically tweak it slightly to make sure it's an integer (Hz ) value, and set the counter system to divide by that same integer value, so the 1 PPS output is consistent, without sawtooth error. So, if the clock is nearly exactly right on, the counter divides by 19,096,000, and it figures out how to fine tune the clock to keep it there. If the drift goes beyond the fine range to say all the way to the upper stated limit of +38 Hz, the counter is set to divide by 19,096,038, and the clock is again tweaked to keep it close to that integer Hz. How it does all this is disclosed in the patent, but I haven't studied it enough to say any more. It looks like this improvement was in a later model, or was perhaps never actually used, since this was around the time that Motorola was departing the GPS business. I assume the older Oncores like mine do not have any of this improvement, so are subject to both the clock drift and the sawtooth. But, one thing I got from this, is that if the nominal clock and divide numbers match, and are fixed at 19,096,000, then replacing the original clock with a sufficiently clean synthesized external clock should work too. I have a number of possible options, depending on the actual original TCXO. If it's also voltage-tuned, but that isn't utilized, then it can stay, but needs circuit mods to release and access the tune signal. The clock signal is accessible at the TCXO, or possibly less risky in 2X form 38.192 MHz from the downconverter IC. This would be the best option, to make an external PLL to tweak it. BTW I have a spare GPS RX unit - I would not risk taking the Z3801A out of commission and messing around with it. If the TCXO has no tuning ability, then a new clock signal needs to be made. Of all the schemes I sketched out back then, the most straightforward seems to be simply adding 4.096 MHz from a VCXO, PLLed to 10 MHz, and 15 MHz derived directly from the 10. These can all be scaled up in frequency in various arrangements, and use standard binary frequency XOs. The next notch up for instance, is 8.192 MHz + 30 MHz giving twice the clock, and so forth. Unfortunately, this method is additive in frequency, which I hate - I prefer to take the difference of two much higher frequencies, which is so much easier to filter. I don't yet see any ways to do a difference method without using special frequencies, so for now I assume regular old standard XO frequencies will have to do, and the filter designs will need to be fancier. The PLLing seems to be straightforward. I can get a decent 16 kHz comparison frequency with simple integer dividing, like 4.096 MHz/256 = 10 MHz/625, but would like to get it as high as possible, without resorting to fancier schemes. Fractional-N is not in the cards here, I don't think. It would be nice if for some reason a 19.096 MHz VCXO module - or even just a crystal - was available for cheap, but I doubt such exists outside of the Oncore line. Maybe a TCXO from a junker would do, but again, it has to have voltage tuning too, and I don't know if any had that. (One of my more far fetched schemes even pictured thermal tuning - TEC heating and cooling the TCXO module - but imagine the nightmare of characterizing the part and the dynamics, and the mechanical and control loop issues.) So anyway, I think I can do it with the additive VCXO combo, but doubt I ever will - there are too many projects to worry about without reviving this one, but it's fun to ponder. BTW one last thing is that in my collection, I have parts of an ancient (ca 1990), huge Trimble L1/L2 GPSDO. I was going to share some info on it, but
[time-nuts] ! PPS Source
The Tbolt won't track sats if the DAC is manually set too far off the value needed for 10 MHz. -- > And now your GPS chip won't even boot or work right if the user's 10 MHz isn't there, or isn't accurate enough, or is still warming up, or if the user's 10 MHz has too much phase noise or instability. ___ 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] ! PPS Source
Hi Ed, > I'm also aware that some people have come up with external > sawtooth correction, using programmable digital delay lines > and special control circuits. For decades fellow time nut Rick Hambly has done hands-on work with GPS receiver timing, including sawtooth correction. Here's a recent version of his classic series of presentations: https://www.cnssys.com/files/TOW/High-accuracy_Time_and_Frequency_in_VLBI_2019_sem.pdf His "CNS clock" was an example of a GPS/1PPS source that used the delay technique to reduce sawtooth effects. The paper is worth a look; lots of good info packed along with plenty of plots and photos. Note that his most recent version of the CNS clock no longer uses the delay technique. I can explain more if you want to go down that rabbit hole. > But, why do all that if you can just fix the clock instead? Because it is not possible for you or me to "fix the clock instead". That's a complex business and design decision inside the GPS chip itself. Almost every DIY or commercial GPSDO uses an off-the-shelf GPS receiver chip or PCB, so you work with what you can get. Those GPS modules all use some tiny carefully-specified free-running internal oscillator for signal sampling, computing, and output pulse generation. Trimble was in a unique position when the GPSDO era began because they made both GPS chips and a GPSDO product. For them the integrated OCXO and GPS receiver design was possible. Even hp couldn't do that. > I think the clock is a Motorola brand odd looking TCXO, labeled "19096" > or something like that, probably 19.096 MHz, as I recall. That sounds right. See http://www.leapsecond.com/pages/vp/sawtooth.htm and note Dr Clark's mention of 9.54 MHz, which would be half your 19.096. > 1. If the GPS RX module's internal clock is synthesized (to the same > nominal frequency) from the 10 MHz output of the GPSDO, can that > alone eliminate or substantially reduce the sawtooth effect? Yes. But it would also create a list of other technical concerns. I'd probably rather have to deal with sawtooth than having to deal with DDS artifacts or phase noise requirements. And now your GPS chip won't even boot or work right if the user's 10 MHz isn't there, or isn't accurate enough, or is still warming up, or if the user's 10 MHz has too much phase noise or instability. > 2. Does the T-bolt actually do this, and if so, is that all it takes? Yes, we think so. But remember they had control over everything; the GPS chip, the firmware, and the specification of the OCXO. > If either answer is yes, then I would think the GPS RX makers would have provisions > for external clock reference, at least for certain high-grade timing type models. Yes, very high-grade timing receivers require, or allow, external frequency inputs. I've seen some in operation at national timing labs. When I heard they cost $20k I decided sawtooth correction wasn't so bad after all. ;-) It seems sawtooth bothers you. There are sawtooth effects in many parts of technology; from printer dots to PC clocks; from calendars to leap seconds. The goal isn't always to eliminate but rather to fully understand the effects. The fact that GPS timing receivers output quantization corrections should be seen as a good thing; it gives the user maximum information. /tvb On 8/13/2020 3:35 PM, ed breya wrote: I have often wondered about all this sawtooth correction stuff, and I think I've asked here too, but never got a definitive answer. Every time this comes up, there are all sorts of explanations of the characteristic, and inevitably someone mentions the T-bolt having its internal GPS clock synchronized with the desired ideal 10 MHz output, either eliminating or greatly reducing the sawtooth effect. But, to my knowledge, nobody has said for sure if this is indeed the case, or whether that's all it takes to achieve perfection, or if more magic is needed besides synchronizing. I'm also aware that some people have come up with external sawtooth correction, using programmable digital delay lines and special control circuits. But, why do all that if you can just fix the clock instead? Many years ago I looked at my Motorola Oncore VP (or whatever model is used in the HPZ3801A), to see what its clock was, and if it was reasonably possible to synthesize it from the 10 MHz. I think the clock is a Motorola brand odd looking TCXO, labeled "19096" or something like that, probably 19.096 MHz, as I recall. That's about as far as I got. Since then I've just wondered what would happen if it was synthesized from the 10 MHz, or if it was even worth trying, or if it would result in other problems. Maybe a certain amount of dither is necessary for proper operation. So, here are some questions, that if answered, may go a long way toward possible improvements in our GPS stuff. 1. If the GPS RX module's internal clock is synthesized (to the same nominal frequency) from the 10 MHz
Re: [time-nuts] ! PPS Source
e...@telight.com said: > I'm also aware that some people have come up with external sawtooth > correction, using programmable digital delay lines and special control > circuits. But, why do all that if you can just fix the clock instead? Have you ever worked on a large software project? The API to the GPS module lets you split the software into two parts -- and the GPS part will be written and maintained by somebody else. You don't necessarily need the digital delay line. You can measure the offset between the PPS from the GPS and the PPS from the GPS and do the sawtooth correction in software. -- 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] ! PPS Source
Hi > On Aug 13, 2020, at 6:35 PM, ed breya wrote: > > I have often wondered about all this sawtooth correction stuff, and I think > I've asked here too, but never got a definitive answer. Every time this comes > up, there are all sorts of explanations of the characteristic, and inevitably > someone mentions the T-bolt having its internal GPS clock synchronized with > the desired ideal 10 MHz output, either eliminating or greatly reducing the > sawtooth effect. But, to my knowledge, nobody has said for sure if this is > indeed the case, or whether that's all it takes to achieve perfection, or if > more magic is needed besides synchronizing. Properly steering the clock that supplies the pps divider is what is needed. There is no *direct* need to use that clock for the GPS. A GPSDO *is* a device that steers the clock that supplies the pps divider. (Yes, there are other ways to get it done) > I'm also aware that some people have come up with external sawtooth > correction, using programmable digital delay lines and special control > circuits. But, why do all that if you can just fix the clock instead? Because you can’t “fix the clock” on a GPS module. That’s what’ this is all about. The GPS *modules* ( = little PCB GPS receivers ) don’t steer their clock. > > Many years ago I looked at my Motorola Oncore VP (or whatever model is used > in the HPZ3801A), to see what its clock was, and if it was reasonably > possible to synthesize it from the 10 MHz. I think the clock is a Motorola > brand odd looking TCXO, labeled "19096" or something like that, probably > 19.096 MHz, as I recall. That's about as far as I got. Since then I've just > wondered what would happen if it was synthesized from the 10 MHz, or if it > was even worth trying, or if it would result in other problems. If you multiply 10 MHz directly to GPS band you may have issues …. > Maybe a certain amount of dither is necessary for proper operation. It’s more an issue of spurs. > > So, here are some questions, that if answered, may go a long way toward > possible improvements in our GPS stuff. > > 1. If the GPS RX module's internal clock is synthesized (to the same nominal > frequency) from the 10 MHz output of the GPSDO, can that alone eliminate or > substantially reduce the sawtooth effect? How are you doing the “synthesized”? Indeed pulse dropping *is* a version of synthesis. If you phase lock the local clock to the GPS code clock, that will duplicate what the TBolt does. > > 2. Does the T-bolt actually do this, and if so, is that all it takes? They phase lock the local 10 MHz OCXO to the GPS code clock. > > If either answer is yes, then I would think the GPS RX makers would have > provisions for external clock reference, at least for certain high-grade > timing type models.m That sounds wonderful. It turns out to cost money. Since you can get the job done just fine with the software correction, why double (or whatever) the price of the module? If I was designing a device (like a GPSDO) the lowest cost device with the best accuracy will be the winner Needless to say, anything that pumps up the price is a major downer. (at least that’s how it worked for the decades I did do this for a living ….). > > 3. Do any GPS RX modules have such provision? No not on the low cost parts. That’s for the simple reason that OEM users of these modules are quite happy doing it with the sawtooth message. It’s dirt cheap to implement and has very few downside issues. Bob > > That's all I can think of for now. > > Ed > > > > ___ > 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] ! PPS Source
I have often wondered about all this sawtooth correction stuff, and I think I've asked here too, but never got a definitive answer. Every time this comes up, there are all sorts of explanations of the characteristic, and inevitably someone mentions the T-bolt having its internal GPS clock synchronized with the desired ideal 10 MHz output, either eliminating or greatly reducing the sawtooth effect. But, to my knowledge, nobody has said for sure if this is indeed the case, or whether that's all it takes to achieve perfection, or if more magic is needed besides synchronizing. I'm also aware that some people have come up with external sawtooth correction, using programmable digital delay lines and special control circuits. But, why do all that if you can just fix the clock instead? Many years ago I looked at my Motorola Oncore VP (or whatever model is used in the HPZ3801A), to see what its clock was, and if it was reasonably possible to synthesize it from the 10 MHz. I think the clock is a Motorola brand odd looking TCXO, labeled "19096" or something like that, probably 19.096 MHz, as I recall. That's about as far as I got. Since then I've just wondered what would happen if it was synthesized from the 10 MHz, or if it was even worth trying, or if it would result in other problems. Maybe a certain amount of dither is necessary for proper operation. So, here are some questions, that if answered, may go a long way toward possible improvements in our GPS stuff. 1. If the GPS RX module's internal clock is synthesized (to the same nominal frequency) from the 10 MHz output of the GPSDO, can that alone eliminate or substantially reduce the sawtooth effect? 2. Does the T-bolt actually do this, and if so, is that all it takes? If either answer is yes, then I would think the GPS RX makers would have provisions for external clock reference, at least for certain high-grade timing type models. 3. Do any GPS RX modules have such provision? That's all I can think of for now. Ed ___ 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] ! PPS Source
kb...@n1k.org said: > The TBolt is a GPSDO rather than a GPS module. Itâs a very different beast > than a ZED-F9P. ZED-F9T, or the M-12 the OP is using. At least some GPSDOs contain a GPS module. The Z3801A and friends are an example. Are their any significant advantages to one way or the other? I assume the TBolt approach is harder to do if you don't already have a software team writing GPS code. How many major brands of GPSDO are there? Does anybody have a list? What fraction use a GPS module? -- 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] ! PPS Source
(At least older) Novatel receivers can be set to lock the internal oscillator. /Björn Sent from my iPhone > On 13 Aug 2020, at 18:26, David C. Partridge > wrote: > > IIRC the Thunderbolt DOES lock its internal clock to the GPS > > David > - ___ 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] ! PPS Source
Hi The TBolt is a GPSDO rather than a GPS module. It’s a very different beast than a ZED-F9P. ZED-F9T, or the M-12 the OP is using. Bob > On Aug 13, 2020, at 12:09 PM, David C. Partridge > wrote: > > IIRC the Thunderbolt DOES lock its internal clock to the GPS > > David > -Original Message- > From: time-nuts [mailto:time-nuts-boun...@lists.febo.com] On Behalf Of Bob > kb8tq > Sent: 13 August 2020 14:39 > To: Discussion of precise time and frequency measurement > Subject: Re: [time-nuts] ! PPS Source > > Hi > > Here’s the “whole story”, sorry if it repeats things you already know … > > All GPS modules that I have ever seen use a free running clock. The internal > oscillator is *not* locked to GPS. When they want to generate a 1 pps output > they drop / add cycles from the the internal oscillator to get it “as close > as > possible”. That means that you will always have an error in the PPS. > > Since they *know* this is going on, many devices report this error on a > second by second basis. Since the error looks like a sawtooth if you graph > it, this is often called “sawtooth correction”. This correction also takes > care > of “hanging bridges” where the sawtooth stays to one side or the other of > “correct” for a long time. > > Normally when feeding a PRS-10, the sawtooth correction is not used. That > results in a degraded pps accuracy. The best GPS module to use in this > case is one with a very small sawtooth “window” ( = a fast internal clock). > Right now, the Furuno parts are winning this particular race. > > If you *do* use the sawtooth correction (possibly by feeding a variable > delay line chip), then indeed the F9P and F9T will do a much better job. > > Some numbers: > > Sawtooth on some older modules can be out around +/- 20 ns On newer > parts it might be down around +/-10 ns. On the F9 parts it is +/-4 ns. The > Furuno parts run half that. > > Corrected, on a modern part, and looking at second to second variation, > you can get below 1 ns with various modules. On the F9’s you can get well > below 1 ns. > > = > > All of that is looking at short term variation. Your Rb does not move much > short term (unless the temperature changes …). Its stability and aging likely > are quite good. > > GPS (as received / uncorrected ) swings around a bit during a normal day. > Swings of 10 to 20 ns are pretty normal. > 50 ns is possible under odd > conditions. That’s more than your Rb is likely to move around over a 4 to 12 > hour period. > > If you “follow” GPS with your Rb through a conventional loop, you likely > degrade the stability of the Rb. It takes a fairly fancy loop to do a good job > on an Rb. > > Bob > >> On Aug 12, 2020, at 11:44 PM, Joe Hobart wrote: >> >> I have been using 1 PPS from a Motorola M-12 timing module to steer a SRS >> PRS-10. I recently heard that a U-Blox ZED F9P module receives both L1 and >> L2 >> and can provide much improved positional accuracy. >> >> Would better positions translate into a smoother 1 PPS? Does anyone have >> experience with this U-Blox module? Can this be set up with a fixed >> position as >> a timing module? >> >> Is there a better source of 1 PPS at a reasonable cost? The U-Blox is about >> $200. >> >> Thanks, >> Joe, W7LUX >> >> --- >> This email has been checked for viruses by AVG. >> https://www.avg.com >> >> >> ___ >> 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. ___ 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] ! PPS Source
IIRC the Thunderbolt DOES lock its internal clock to the GPS David -Original Message- From: time-nuts [mailto:time-nuts-boun...@lists.febo.com] On Behalf Of Bob kb8tq Sent: 13 August 2020 14:39 To: Discussion of precise time and frequency measurement Subject: Re: [time-nuts] ! PPS Source Hi Here’s the “whole story”, sorry if it repeats things you already know … All GPS modules that I have ever seen use a free running clock. The internal oscillator is *not* locked to GPS. When they want to generate a 1 pps output they drop / add cycles from the the internal oscillator to get it “as close as possible”. That means that you will always have an error in the PPS. Since they *know* this is going on, many devices report this error on a second by second basis. Since the error looks like a sawtooth if you graph it, this is often called “sawtooth correction”. This correction also takes care of “hanging bridges” where the sawtooth stays to one side or the other of “correct” for a long time. Normally when feeding a PRS-10, the sawtooth correction is not used. That results in a degraded pps accuracy. The best GPS module to use in this case is one with a very small sawtooth “window” ( = a fast internal clock). Right now, the Furuno parts are winning this particular race. If you *do* use the sawtooth correction (possibly by feeding a variable delay line chip), then indeed the F9P and F9T will do a much better job. Some numbers: Sawtooth on some older modules can be out around +/- 20 ns On newer parts it might be down around +/-10 ns. On the F9 parts it is +/-4 ns. The Furuno parts run half that. Corrected, on a modern part, and looking at second to second variation, you can get below 1 ns with various modules. On the F9’s you can get well below 1 ns. = All of that is looking at short term variation. Your Rb does not move much short term (unless the temperature changes …). Its stability and aging likely are quite good. GPS (as received / uncorrected ) swings around a bit during a normal day. Swings of 10 to 20 ns are pretty normal. > 50 ns is possible under odd conditions. That’s more than your Rb is likely to move around over a 4 to 12 hour period. If you “follow” GPS with your Rb through a conventional loop, you likely degrade the stability of the Rb. It takes a fairly fancy loop to do a good job on an Rb. Bob > On Aug 12, 2020, at 11:44 PM, Joe Hobart wrote: > > I have been using 1 PPS from a Motorola M-12 timing module to steer a SRS > PRS-10. I recently heard that a U-Blox ZED F9P module receives both L1 and L2 > and can provide much improved positional accuracy. > > Would better positions translate into a smoother 1 PPS? Does anyone have > experience with this U-Blox module? Can this be set up with a fixed position > as > a timing module? > > Is there a better source of 1 PPS at a reasonable cost? The U-Blox is about > $200. > > Thanks, > Joe, W7LUX > > --- > This email has been checked for viruses by AVG. > https://www.avg.com > > > ___ > 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] ! PPS Source
Hi Here’s the “whole story”, sorry if it repeats things you already know … All GPS modules that I have ever seen use a free running clock. The internal oscillator is *not* locked to GPS. When they want to generate a 1 pps output they drop / add cycles from the the internal oscillator to get it “as close as possible”. That means that you will always have an error in the PPS. Since they *know* this is going on, many devices report this error on a second by second basis. Since the error looks like a sawtooth if you graph it, this is often called “sawtooth correction”. This correction also takes care of “hanging bridges” where the sawtooth stays to one side or the other of “correct” for a long time. Normally when feeding a PRS-10, the sawtooth correction is not used. That results in a degraded pps accuracy. The best GPS module to use in this case is one with a very small sawtooth “window” ( = a fast internal clock). Right now, the Furuno parts are winning this particular race. If you *do* use the sawtooth correction (possibly by feeding a variable delay line chip), then indeed the F9P and F9T will do a much better job. Some numbers: Sawtooth on some older modules can be out around +/- 20 ns On newer parts it might be down around +/-10 ns. On the F9 parts it is +/-4 ns. The Furuno parts run half that. Corrected, on a modern part, and looking at second to second variation, you can get below 1 ns with various modules. On the F9’s you can get well below 1 ns. = All of that is looking at short term variation. Your Rb does not move much short term (unless the temperature changes …). Its stability and aging likely are quite good. GPS (as received / uncorrected ) swings around a bit during a normal day. Swings of 10 to 20 ns are pretty normal. > 50 ns is possible under odd conditions. That’s more than your Rb is likely to move around over a 4 to 12 hour period. If you “follow” GPS with your Rb through a conventional loop, you likely degrade the stability of the Rb. It takes a fairly fancy loop to do a good job on an Rb. Bob > On Aug 12, 2020, at 11:44 PM, Joe Hobart wrote: > > I have been using 1 PPS from a Motorola M-12 timing module to steer a SRS > PRS-10. I recently heard that a U-Blox ZED F9P module receives both L1 and L2 > and can provide much improved positional accuracy. > > Would better positions translate into a smoother 1 PPS? Does anyone have > experience with this U-Blox module? Can this be set up with a fixed position > as > a timing module? > > Is there a better source of 1 PPS at a reasonable cost? The U-Blox is about > $200. > > Thanks, > Joe, W7LUX > > --- > This email has been checked for viruses by AVG. > https://www.avg.com > > > ___ > 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] ! PPS Source
I have been using 1 PPS from a Motorola M-12 timing module to steer a SRS PRS-10. I recently heard that a U-Blox ZED F9P module receives both L1 and L2 and can provide much improved positional accuracy. Would better positions translate into a smoother 1 PPS? Does anyone have experience with this U-Blox module? Can this be set up with a fixed position as a timing module? Is there a better source of 1 PPS at a reasonable cost? The U-Blox is about $200. Thanks, Joe, W7LUX --- This email has been checked for viruses by AVG. https://www.avg.com ___ 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.
