[time-nuts] Last Call Group Buy Ublox LEA-6T
EWKehren at aol.com
EWKehren at aol.com
Thu May 19 09:08:40 EDT 2016
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,
csteinmetz at yandex.com 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
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