Today GPS receivers are getting better by the year due to the fact that in order to save silicon the chips are getting smaller and the clock frequency goes up reducing saw tooth excursion. If it was not for hanging bridge filtering it on the input of a GPSDO would be simple without info from the receiver. Back to the Motorola days the receiver was a larger contributor to the timing error now it is external to the receiver but removing receiver error, depending on the application does make sense. We have done tests of different units using a Cesium and a HP5372A and the Tbolt stand out with 100 psec +- 500. That is short term 278 samples speaking only for the unit performance, long term other factors external to the unit degrades time by a factor 100. Back to ublox if you use a T the question is why. If it is GPSDO it can use the saw tooth data in the software I am sure commercial units do it. Richard MCC was working on his GPSDO incorporating that info. For pure time application if you want hardware correction a programmable timing element makes sense specially since the cost has come down. DS1124 250 psec. does the job and less than $ 5 works for us. Micrel at 10 psec will take two, again does it make sense? Since we are frequency nuts not time nuts DS1124 is our choice. Chips with larger steps are obsolete and would be significantly more expensive since it takes more esilicon. In our GPSDO performance tests we found no difference between using Tbolt and M6.It does incorporate an adjustable GPS filter on the input. Bert Kehren In a message dated 5/18/2016 8:00:34 P.M. Eastern Daylight Time, [email protected] writes:
Bert wrote: > Our first tests are with a 500 ps > DS chip because I did have them, but 200 ps will work with a T5 or 6. We > are also considering using two Micrel SY 89295UTG in series with 10 ps > resolution, but with the limitation of a single frequency GPS receiver and > ionosphere delay variations one has to ask does it make sense? Think first about what the GPS engine has to work with -- nothing but the timing generated by its current GPS timing solution. It seems very doubtful that sub-nS accuracy is possible. This provisional conclusion is supported if we look at commercial GPSDOs using single-frequency GPS engines and sawtooth correction (or using local oscillators that divide evenly by 100nS, like the Tbolt). The best commercial units seem to place the PPS within 5nS or so on a routine basis. So, even 500pS appears to be considerably better than necessary, given the limitations imposed by the timing engine, atmospheric dispersion, and the GPS system. Best regards, Charles _______________________________________________ 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. _______________________________________________ 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.
