Hi Ulrich: Your M12+T plot ends at a little over a day (100k seconds) and the stability is on the order of 4E-13. But Cesium and other oscillators can be better than this. So how do you check them, use longer averaging time?
Have Fun, Brooke Clarke w/Java http://www.PRC68.com w/o Java http://www.pacificsites.com/~brooke/PRC68COM.shtml http://www.precisionclock.com Ulrich Bangert wrote: >Brooks, > > > >>Excel computed that the unaveraged correction data had a >>standard deviation >>of 8.4 nsec, which is consistent with the actual measured 9.5 >>nsec rms >>jitter reported by Rich Hambly (Dec 06, PTTI paper by Clark >>and Hambly, p. >>15). >> >> > >Even if this scientifical improvement has not found its way into Excel: >A certain Mr. Allan has shown that the standard deviaton is NOT the >appropiate measure for noise processes in oscillators. Therefore he had >to find a new statitistics on its own. If you don't own a software to >calculate ADEV and other relevant statistical measures with you may >download one for free from my homepage: > >http://www.ulrich-bangert.de/plotter.zip > > > >>But the question remains "what time averaging is needed to reduce the >>sawtooth/bridge jitter from a typical +/-15 nsec to something >>negligible, >>perhaps +/-1 nsec? >> >> > >Have a look to > >http://www.ulrich-bangert.de/html/photo_gallery_44.html > >If you can read it it will immediatly give you the answer to your >questions: in order to get to a certain precision draw a horizontal line >at this precisision on the vertical axis and at the two crossing points >read the necessary time for SAW corrected and uncorrected data on the >horizontal axis. > >Nevertheless, pardon to contradict you: One simply has NO choice to >average this long or to average that long. You have to set the >regulation loop time constant up to exactly where the OCXO's >tau-sigma-diagram meets the receiver's tau-sigma. Every loop time >constant different from that is a faulty design and nothing else. The >regulation loop dynamics may be improved a bit by pre-averaging the >phase data before they are fed into the loop but not by computing the >arithmetic mean over a time but by a gliding exponential average as is >explained in detail in the PRS-10's handbook. Due to stability reasons >even this time constant of this pre-filter is more or less fixed to abt. >1/3 the main loop's time constant. > >Regards >Ulrich Bangert,DF6JB > > > > >>-----Ursprüngliche Nachricht----- >>Von: [EMAIL PROTECTED] >>[mailto:[EMAIL PROTECTED] Im Auftrag von Brooks Shera >>Gesendet: Donnerstag, 21. Dezember 2006 18:50 >>An: Discussion of precise time and frequency measurement >>Betreff: [time-nuts] GPS orthodontics: sawteeth & hanging >>bridges - theeffect of time averaging >> >> >>Recently there has been some mention of the influence of 1pps >>sawtooth and >>hanging bridges jitter on the performance of a GPSDO. >> >>It would seem to me that the jitter must average to zero in >>the long run, >>for if it did not the 1pps signal would drift away from its >>relation to UTC. >> >>But the question remains "what time averaging is needed to reduce the >>sawtooth/bridge jitter from a typical +/-15 nsec to something >>negligible, >>perhaps +/-1 nsec?" >> >>To explore this I used TAC32 to record the 1 pps sawtooth >>correction message >>from a Motorola M12+ receiver for about 1 hour, during which >>time many >>bridges occurred (1). Excel's statistical toolbox was then >>used to examine >>the data. >> >>Excel computed that the unaveraged correction data had a >>standard deviation >>of 8.4 nsec, which is consistent with the actual measured 9.5 >>nsec rms >>jitter reported by Rich Hambly (Dec 06, PTTI paper by Clark >>and Hambly, p. >>15). >> >>Averaging the sawtooth/bridge correction data for several >>averaging times >>produced the following results (2): >> >>Avg Time Standard Deviation Residual Jitter >>none 8.4 nsec +/- 15 nsec >>30 sec 1.53 +/- 4.3 >>100 sec 0.79 +/- 2.2 >>300 sec 0.33 +/- 0.7 >> >>It is evident that jitter is greatly reduced with a bit of >>time-averaging. >>In addition, the hanging bridges quickly disappeared into the >>residual >>jitter of the smoothed data. >> >>It appears to me that a typical GPSDO, which has an >>integration time in the >>range of 100's to many 1000's of sec is not likely to be >>impaired by the >>sawtooth/bridge noise of a GPS rcvr. A GPS-based clock is a >>different story >>since a precise 1pps timing signal without time averaging would be >>desirable. >> >>In summary, it appears that 1pps sawtooth/bridge noise can be >>ignored for a >>GPSDO. In some designs it may even be helpful by introducing further >>deterministic randomness to the phase measurement process. >> >>Regards, Brooks >> >>(1) the M12+ correction-message resolution is 1 nsec and this >>seems adequate >>for a jitter statistics investigation. But as a check, I >>compared the >>correction message data with the actual 1 pps jitter measured >>with a 5370B >>TIC, a PRS10 and a M12+ . This approach has higher >>resolution but does not >>change the conclusions. >> >>(2) I choose 30 sec as the shortest averaging time because >>30 sec is the >>summation time of the phase-measuring circuit of my GPSDO >>design and hence >>the shortest integration time available. Of course, the PLL filter >>configuration switches can extend the integration to many >>hours if desired. >> >> >> >> >> >> >> >>_______________________________________________ >>time-nuts mailing list >>time-nuts@febo.com >>https://www.febo.com/cgi-> bin/mailman/listinfo/time-nuts >> >> >> > > >_______________________________________________ >time-nuts mailing list >time-nuts@febo.com >https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > > > _______________________________________________ time-nuts mailing list time-nuts@febo.com https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts