Hej Magnus Magnus Danielson wrote: > Tom Van Baak skrev: > >> Here are ADEV plots and interesting results from a recent >> experiment on varying the time-constant of a GPSDO: >> >> http://www.leapsecond.com/pages/tbolt-tc/ >> >> There was a thread recently where Warren suggested that the >> loop time constant (TC) of GPSDO was less than ideal. He is >> correct. There are a couple of reasons for this, if I may guess. >> > > I particular liked that you tweaked the damping constant. If it is not > scaled off the normal charts (it doesn't look like it) I think it is > rather low damping factor and this infact does not supprise me at least > to produce a definitive bump. Keeping it at 1.2 is still not "critically > damped" but rather under-damped. > > I would love to see a few variants of measure at say tau = 100s but for > various damping coefficients. By the looks of it, the bump can be > attributed almost completely to resonance in the PLL loop, which is > certainly not what we would like to see... adding to the noise response. > > >> 1) Some GPSDO, like the surplus SmartClock designs from HP, >> were designed to meet spec even when S/A was still in effect. >> With the much greater wander in civilian GPS timing during >> those years, the TC needed to be less than what you can get >> away with today. >> >> 2) If you are a manufacturer and have a GPSDO spec to meet, >> you need to make sure the TC is valid for all OCXO that you ship, >> not just the average one. The way to do this is to be conservative >> and to ship the units with a TC that is short enough that even the >> parts on the lower end of the bell curve still meet your spec. >> >> The other alternative is to individually measure (days, weeks?) >> every OCXO and individually burn a default TC into each unit >> shipped. >> > > With the end result being that noise specs would still vary between > units. Also, if one unit was good at fab and gets pounded during > shipping doesn't help either. > > >> 3) Most commercial GPSDO need to work over a fairly wide >> temperature range. This might require a tighter TC. If the user >> has a more controlled environment they can probably tolerate a >> longer TC that what the manufacturer dare ship as a default. >> >> 4) A GPSDO should still work reliably in the face of phase or >> frequency jumps in the OCXO. Although the timing of these is >> not predictable, their typical magnitude is probably something >> that the designer can learn. The TC needs to be short enough >> so that the GPSDO gracefully handles these jumps. If one is >> too aggressive the GPSDO will wander out of spec instead of >> more closely tracking GPS. >> > > All these 4 points really argue against the principle of choosing one TC > to fit them all. Using suitable heuristics to adapt TC to conditions and > recent history runtime will provide a much more dynamic fashion in which > units would adapt to their performance and their environment. > > >> 5) I'm not sure it's possible to optimize for both time stability >> and frequency stability at the same time. A long TC will help >> avoid too sudden frequency changes in the GPSDO; a short >> TC will help the 1pps stay close to UTC. I suppose a GPSDO >> might be optimized more for one application than the other; >> this would affect the choice of default TC. >> > > Actually no, not really. > > If we remove the issues of hanging bridge, which the ThunderBolt > effectively has since it steers it's timing clock, then another reason > for shifting PPS is due to changing symmetry and when removing or adding > a satellite from the chosen constellation (by tracking set or TRAIM > selective set) the apparent receiver time will jump. In the meanwhile it > may glide as the symmetry changes. Multipath can also aid in shifting > position. So a shorter time constant does not render a closer rendition > of UTC but rather a quicker follow of apparent time position of the > receiver, which isn't quite the same thing. Thus, a longer time > constants acts like an averaging of apparent time position. > > Another factor which plauges most single frequency receivers is that > they can't correct for actual ionospheric delay. They run according to a > parameterized model. There is a deviation between the actual and > apparent delay and it is changing over time. > > Thus, using Rubidium or Caesium to steer local clock will allow for even > greater time constants and thus greater averaging potential, as 1000 s > is still kind of "short". > > The conclusion is that the GPS receivers we use have many inherrent > non-static error sources > > >> 6) Most OCXO demonstrate much better drift rates after they >> have been in operation for weeks or months. The drift rate has >> a some impact on the choice of TC. It's probably not a good >> business model to ship a GPSDO with a TC optimized for how >> the unit might eventually run a year from now. It has to work >> out of the box. So this cause the default TC to be set shorter >> than ideal. >> >> 7) There may also be a SV, sky-view, or latitude dependence. >> Someone enjoying all 32 SV today, with a clear 360 degree >> view of the sky at mid-latitude will probably enjoy slightly better >> performance than someone a few years ago when there were >> less operational SV in orbit, or with mountain, forest, building >> obstructions, or at extreme latitudes. If you have much better >> than average reception you could probably move the TC out >> further. >> > > These two points again suggests a more dynamic approach to setting time > constant. > > There is some old papers discussing straight PLL loops vs. Kalman filter > and Kalman filter (if done properly) will adapt dynamically and they > showed (to some degree) that the Kalman filter approach outperformed the > straight PLL approach. > > >> So for these reasons (more like guesses), it would not surprise >> me if most GPSDO have the TC set on the low side. Let me >> know if you have additional info on this topic. >> > > In general I think you are right. There are many reasons why they go on > the low side. > > >> The good news is that if you, the time-nut, have the gear and >> the time to measure the stability of the OCXO in your GPSDO, >> and know your environment well, then you can probably safely >> lengthen the TC and achieve much better mid-term stability out >> of your GPSDO as shown in the plot above. >> > > Agreed. But also recall that long-term effects like frequency drift is > pretty easy to measure with a GPS. It would not take too much research > to figure out some suitable drift-TC relationship such that you could > either just look the charts to find a good match or even apply them in > real time steering. > > For ThunderBolt owners it is pretty straightforward to adjust the TC and > damping, which is very nice. Use this oppertunity! > >
How exactly can one measure the resultant performance or even select the optimum time constant and damping factor if one doesn't have a quieter reference? Can one really resort to some sort of N cornered hat? > Cheers, > Magnus > > > Bruce _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
