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

[EWKehren at aol.com]

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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