Bruce Griffiths <[EMAIL PROTECTED]> wrote: > Mike
> They actually use an augmented form of GPS common view for which > the GPS PPS signal and its timing variations are largely common to > both locations and thus largely cancel when comparing the > frequencies at the customer site with the standards at NIST. > With a good timing receiver (and antenna location) the sawtooth > corrected PPS signal timing noise can be as low as a few nanosec. Is that like the CNSC02-O1 High Performance PPS system? From their web site, they state: "Provides for dynamic hardware correction of the 1PPS quantization ("sawtooth") error. This reduces the noise on the 1PPS pulses from (typically) +/-27 nsec (15 nsec 1-sigma) to +/- 11 nsec (3 nsec 1-sigma). More importantly, eliminates periods of bias error caused by the quanization error going through a "zero beat" period that can last one to two minutes about every 10 minutes or so." http://www.cnssys.com/cnsclock/CNSClockII.html That paragraph is very interesting. The "zero beat" error is particularly nasty. I tried to get more information, but it wasn't really clear how their correction method fixes this problem. Could you explain it a bit more? I am working on a new technique that might give one or two orders of magnitude improvement locking to the 1PPS signal. It might give sub-nanosecond locking in a very low cost system. But the zero beat is a serious problem, so I am interested to learn as much as possible how other solutions work. Maybe the simplest is to just apply some jitter to the GPS crystal oscillator to keep it off zero beat. > The "all in view" technique will reduce the noise contribution to > the comparison somewhat. That term is a bit confusing. Does it mean using all satellites that are in view, or does it mean the NIST receivers see the same ones that the user sees? > The timestamp resolution of better than 30ps or so ensures that > time stamp quantisation noise is negligible. > It also allows, in principle at least, standalone 3 cornered hat > comparisons of the frequency instabilities of the 3 sources > connected to the customer instrument. Another term I must study and learn how to do: "3 cornered hat". It sounds like a very powerful technique, ideally suited for a fairly simple program and some data logging. > For more detail see: > http://tf.nist.gov/timefreq/service/fms.htm Thanks very much for the link. It is curious they don't seem to spend much effort on correcting the user's frequency errors. They just want to report how much they are off. Why is that? You'd think they would perform a more valuable service by applying advanced techniques to adjust the user's equipment to minimize the error, then report and certify the actual result. Is there some reason they want to leave the user's equipment free-running? > Only carrier phase GPS techniques are potentially capable of > picosecond noise levels. Now we are getting very interesting. How do you do that? > However there are a large number of effects that have to be taken > into account and data reduction and correction is very complex. What if we just want to stabilize an oscillator frequency, without caring much what the exact phase offset is from the USNO? The high carrier frequency should make it much easier to lock a rubidium or crystal oscillator, and it should give much lower phase noise. Of course, auroras and other disturbances would be more significant, but here I'm mainly interested in getting a good lab reference to measure and compare the performance of other commercial oscillators. This is a separate topic, but as long as we are talking about precision signals, do you happen to know what kind of distribution amplifiers are used at the USNO to distribute the signals from their cesium and hydrogen standards? www.usno.navy.mil/ > Bruce Thanks very much for your help. It might seem like a lot of questions, but I'm pretty sure you will be very pleased with the results. I think I can beat just about all the other methods except carrier phase techniques. Best Regards, Mike Monett _______________________________________________ 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.