Re: [time-nuts] FW: Question about the PLL of Trimble Thunderbold
> With the idea that, when the adjustment loop is deactivated, an external > signal > can be supplied to the Thunderbolt, and the Time Interval circuit could show > the > difference in between this signal and the feedback of the VCO. I understand what you're trying to do. Your idea would work for most GPSDO because the PCB contains lots of separate parts like: GPS chip, TIC, MCU, DAC, OCXO/EFC. You've probably read how GPSDO work and know this. Note that the GPS chip or module is likely made by a 3rd party and contains a cheap little XO which is used to sample the RF, run the ASIC/MCU, and output a 1PPS. The 1PPS will have clock cycle quantization, so-called sawtooth error. The GPSDO deals with this. But the TBolt is different. Since Trimble is itself a GPS receiver manufacturer they don't need the GPS receiver to be a separate device from the rest of the GPSDO. They are integrated. Since they already have an extremely good OCXO onboard there is no need for the cheap little XO inside the GPS chip. Instead the receiver uses the main OCXO clock. As a result there is no separate receiver 1PPS; there is only the OCXO 1PPS. There is no sawtooth error either. It's a wonderful design and only possible because Trimble is both a GPS receiver company and a GPSDO company. A TBolt can do low-level magic inside that a traditional GPSDO cannot. This is one reason that why we hobbyist like the TBolt. Since part of your project is to deliver a "fake" or external 10 MHz and 1PPS into a GPSDO, the TBolt is probably not the right choice for you. A more traditional GPSDO design would be better. What some of us have done is substitute the TBolt 10 MHz OCXO for a better OCXO or even Rb. But that's a separate issue from your request to substitute the 1PPS as well. It is worth reading the detailed ThunderBoltBook2003.pdf document to further understand the unique TBolt architecture. /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] FW: Question about the PLL of Trimble Thunderbold
Thank you all for your answers, I do have an additional question. Did anybody install an external 1PPS/10MHz input to the Trimble Thunderbolt board ?? With the idea that, when the adjustment loop is deactivated, an external signal can be supplied to the Thunderbolt, and the Time Interval circuit could show the difference in between this signal and the feedback of the VCO. @ Bob kb8tq The aim of this project has no commercial purposes and the project itself is to develop the algorithm which will be in charge of adjusting the clocks. Also is yet to be determined the information that will be exchanged in between nodes in order to achieve as accurate synchronization as possible. >Hi >Unfortunately there are no ?stock? boards to do this sort of thing. If this is >a commercial >requirement, there are companies who do this kind of thing on a custom basis. >Figure on >a few thousand dollars NRE and a minimum order of a few hundred to get somebody >interested. At the ?couple ps? level, the NRE may be a bit above the few >thousand >level. Also expect to supply a full spec requirement when you go shopping ?. >Bob @Attila Kinali Could you please share a link/name of the paper ? All information is welcome ! The method that you've developed, synchronizes 4 local clocks in reference to one, or they keep a certain difference all together in between themselves ?? Which FPGA are you using ? >I have something ready, that can synchronize 4 independent clocks >to eachother at the 180ps level, limited by the FPGA based TDC. >The current incarnation does not allow for an external clock source >to syncrhonize to, but that should be easy to add. That is, if you >don't mind using some half-finished we-have-published-a-paper research >tool. Lets say that the objective is to reach 50ps. Of course is not an easy to achieve goal, but that's the purpose of the project, to try to achieve as best synchronization as possible within an strict time frame. Part of the project will consist in taking into account the propagation delay in between the medium used, be it a cable, fiber or radio link. Still to be determined, but most likely it will be a cable. Nice tips on the cables, I will do a documentation research to learn further. >But going to ps level of synchronization, especially if you mean <10ps, >is not going to be easy. There are not many ways to measure pulses >with this accuracy. If you know what you are doing, about 1-3ps RMS is the >practical limit you can achieve, more likely it'll be in the order of 10-30ps, >for a one-off design. Also keep in mind that ~2mm of cable is already 10ps of >phase shift. Ie you will need to calibrate your cables as well. Cables, >which are of course low dispersion and low temperature coefficient cables. >The dispersion is important so that your pulse remains a sharp pulse. >through the cable and doesn't come out grabled as a weird wave packet, >Quite counter-intuitively, limiting the slew rate might help with this. >The low TC is important if there is any distance between the two >oscillators. Otherwise you can get up to several ps per ?C temperature >change and meter cable length for run of the mill cables. If you have >PTFE cables, you also want to keep them well above 25?C or well below 15?C, >for the same reason. >Attila Kinali @Tom Van Baak >I'm glad you mentioned your requirements. Note that time synchronization at a >>"ps level" is 3 to 4 orders of magnitude beyond what the typical commercial >or >eBay or DIY GPSDO will do. Well, I did a research in order to find suitable boards, and the ones on eBay got my attention, because they were quite affordable and were using used GPSDO's from Trimble or Symmetricon. And I saw some people getting good results with them, so I had to ask. >But as you imply, you're only using GPS to get close (ns levels) and then >using your own two-way communication to narrow it down to ps levels, right? Yes, the GPS is intended to provide a “coarse” adjustment, but even in the case that there's no GPS signal at all, the nodes will need to synchronize in between themselves without an external reference but in respect to a single node. As the method that all the nodes exchange information and correct with each other, would increase the complexity of the project notably. >Yes. Section "A.10.31 Report Packet 0x8F-AC Secondary Timing Packet" says: > "PPS Offset: This field carries the offset of the PPS output relative to > UTC >or GPS as reported by the GPS receiver in nanoseconds. Positive values > indicate >that the ThunderBolt’s PPS is coming out late relative to GPS or > UTC." >The range and precision of that time interval value would look something like >>this: > http://leapsecond.com/pages/tbolt-8d/ >I can send you the raw data if you want to play with it. Attached is a >histogram. Note the standard deviation is about 2 ns. Thanks for the
Re: [time-nuts] FW: Question about the PLL of Trimble Thunderbold
On 10/29/18 12:03 PM, Hal Murray wrote: Cables, which are of course low dispersion and low temperature coefficient cables. If I go looking for good cables, do they specify temperature coefficient? They sure do.. Usually it's specified as a phase shift vs temperature vs frequency. I don't remember ever seeing it when scanning specs, but I probably wasn't looking for it so I could have skimmed over something. What determines the dispersion of a cable? Is it as simple as bigger wire (more copper) is better? the dielectric is the primary determinant of this. Why do I care about the dispersion as long as all cables match? ___ 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] FW: Question about the PLL of Trimble Thunderbold
> Cables, which are of course low dispersion and low temperature coefficient > cables. If I go looking for good cables, do they specify temperature coefficient? I don't remember ever seeing it when scanning specs, but I probably wasn't looking for it so I could have skimmed over something. What determines the dispersion of a cable? Is it as simple as bigger wire (more copper) is better? Why do I care about the dispersion as long as all cables match? -- 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] FW: Question about the PLL of Trimble Thunderbold
On Mon, 29 Oct 2018 06:38:03 + Ferran Valdés wrote: > Yes, a GPSDO is already self adjusting, but for my project, I would like to > either use a GPS or to synchronize one node’s oscillator on another. > > The synchronization goal is in the order of ps level Clock synchronization? Possibly fault tolerant? I think that's my area of expertise :-) I have something ready, that can synchronize 4 independent clocks to eachother at the 180ps level, limited by the FPGA based TDC. The current incarnation does not allow for an external clock source to syncrhonize to, but that should be easy to add. That is, if you don't mind using some half-finished we-have-published-a-paper research tool. But going to ps level of synchronization, especially if you mean <10ps, is not going to be easy. There are not many ways to measure pulses with this accuracy. If you know what you are doing, about 1-3ps RMS is the practical limit you can achieve, more likely it'll be in the order of 10-30ps, for a one-off design. Also keep in mind that ~2mm of cable is already 10ps of phase shift. Ie you will need to calibrate your cables as well. Cables, which are of course low dispersion and low temperature coefficient cables. The dispersion is important so that your pulse remains a sharp pulse. through the cable and doesn't come out grabled as a weird wave packet, Quite counter-intuitively, limiting the slew rate might help with this. The low TC is important if there is any distance between the two oscillators. Otherwise you can get up to several ps per °C temperature change and meter cable length for run of the mill cables. If you have PTFE cables, you also want to keep them well above 25°C or well below 15°C, for the same reason. Attila Kinali -- The bad part of Zurich is where the degenerates throw DARK chocolate at you. ___ 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] FW: Question about the PLL of Trimble Thunderbold
Ferran, > The synchronization goal is in the order of ps level. I'm glad you mentioned your requirements. Note that time synchronization at a "ps level" is 3 to 4 orders of magnitude beyond what the typical commercial or eBay or DIY GPSDO will do. But as you imply, you're only using GPS to get close (ns levels) and then using your own two-way communication to narrow it down to ps levels, right? > Within the Report Packet 0x8F-AC, the bytes 16-19 indicate “Estimate of > UTC/GPS offset”, is this the time difference ? Yes. Section "A.10.31 Report Packet 0x8F-AC Secondary Timing Packet" says: "PPS Offset: This field carries the offset of the PPS output relative to UTC or GPS as reported by the GPS receiver in nanoseconds. Positive values indicate that the ThunderBolt’s PPS is coming out late relative to GPS or UTC." The range and precision of that time interval value would look something like this: http://leapsecond.com/pages/tbolt-8d/ I can send you the raw data if you want to play with it. Attached is a histogram. Note the standard deviation is about 2 ns. > My idea is to develop the control loop which will be able to synchronize one > oscillator to another. Remember that all the ingredients in your system will then need to be stable to ps levels: the oscillator, the DAC, the 1PPS pulse, the cables, the input signal conditioning, and time or phase comparator, etc. That's a pretty tall order but not impossible. You may want to synchronize using 10 MHz instead of 1PPS to reduce your noise and improve the tight lock. How far apart are your two oscillators? In case it helps your effort, see https://en.wikipedia.org/wiki/The_White_Rabbit_Project and read the PDF's. It's an open h/w project. /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] FW: Question about the PLL of Trimble Thunderbold
Thanks everybody for your answers. @ Bob kb8tq Due to a development time constraint, I am looking for a board which has all the implemented hardware In order to have a good starting point. My aim is to let the oscillator to be disciplined by the GPS in normal operation, and at a given moment, an algorithm to take over the adjusting process without upsetting the PLL. My idea is to develop the control loop which will be able to synchronize one oscillator to another. @ ew A 1 PPS will be exchanged in between nodes (each node would have a GPSDO). @ Tom Van Baak Yes, a GPSDO is already self adjusting, but for my project, I would like to either use a GPS or to synchronize one node’s oscillator on another. The synchronization goal is in the order of ps level. @ Mark Sims I have just taken a brief look at Lady Heater. I will go through the manual and get back to it. But what this program does is similar to what I am intending to do, so that’s quite nice to know that the Trimble Thunderbolt is a suitable board ! I am searching for the time interval, but I have not seen the parameter yet. This is the command to set the DAC value --> 0x8E-A0 | Set/Request DAC values | 0x8F-A0 Within the Report Packet 0x8F-AC, the bytes 16-19 indicate “Estimate of UTC/GPS offset”, is this the time difference ? I have seen that on eBay, there are listed some GPSDO modules, which claim to have a “trimble” or “symmetricon” GPSDO inside, and they provide a hardware platform to get access to the GPSDO parameters, however, it depends on the board which is mounted inside if the adjustment loop can be externally governed. Anybody got any experience with any of those boards? Kind regards, Ferran ___ 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.
