[time-nuts] Last Call Group Buy Ublox LEA-6T

Charles Steinmetz csteinmetz at yandex.com
Wed May 18 23:23:26 UTC 2016


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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