[time-nuts] GPS: ADEV or MDEV?

Peter Schmelcher nebula at telus.net
Sun Apr 22 05:39:23 EDT 2007


>I made a hardware sawtooth remover a couple of years ago as an
>experiment, but where I used it I didn't seen much difference. I just
>checked again recently, but the sawtooth errors seem very small
>compared to overall instability, even for just an M12 to Rb at a
>fairly stable temperature. The larger errors of a few ns (over 1000s)
>which are occasionally visible when the M12 is temperature controlled
>seem regularly to be exceeded by other instabilities in the pps
>output.
>
>There are plenty of times when sawtooth removal would be of
>use/interest, but in a typical GPSDO which has a heap of other errors,
>I do wonder what improvements in performance would actually be seen in
>the output from the oscillator - which is all that a lot of people are
>really interrested in.
>
>Angus.


I removed the sawtooth error also as an experiment and still have yet to do 
a closed loop test with my GPSDO. I have a MTI-260 OCXO which auto adjusts 
the oven set point every few days by using a transient temperature impulse 
(I believe). I am waiting for convergence of the oven temperature before I 
change my setup and do a closed loop test - frustration increasing - still 
waiting.....

What got me started down the path of sawtooth elimination was a correlation 
between the stability of the least significant digit of my frequency 
counter an HP 53132A (driven by the Z3816A) and the logged pps to gps 
error. If you think about a control system the better the driving signal 
the smaller the servo loop error which results in a more accurate 
system.  I can see a few nanosecond jump in the sawtooth when a SV is added 
or removed from the pps solution mostly because my antenna is rarely at the 
surveyed location. The survey math was intended for SA removal and is 
probably inappropriate now. The statistical MODE position from a night 
survey and a fudge factor to lower the antenna height would be my bet. In 
truth the calculated antenna height is dynamic from day to night for my gps 
receivers.

Peter 





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