[time-nuts] GPS: ADEV or MDEV?

Peter Schmelcher nebula at telus.net
Fri Apr 13 02:43:44 EDT 2007


> >> Surely it would be much simpler (in principle at least) to just to add
> >>  sufficient Gaussian phase noise to the GPS receiver clock so that
> >> potential coherence problems are virtually eliminated.
>
>
>The noise only really needs to be added to the PPS positioning
>algorithm/hardware.


My two cents. Masking the sawtooth is bad news. If you log the pps output 
from a Z3816A you get the incorrect impression that the OCXO and/or the 
atmosphere is unstable for time spans of tens of minutes. My Z3816A pps 
logs look typical sd=10ns a few 50ns spikes the EFC ad=2 sd=5 etc. But use 
the OXCO to synthesize a low noise LO for a second gps and the pps becomes 
very stable a couple of ns. In the Z3816A the gps LO crystals 0.1 second 
tau and the sawtooth interact and do bad things. It is best to eliminate 
the problem and then to move on to new things. I am attenuating my HP 
hockey puck antenna approximately 10dB to remove SV with range instability 
from the gps pps solution. The current setup acquires SV at about 30 degree 
and can then track them down to about 20 degree. The idea is that transit 
time instability modulates the SV signal requiring a greater tracking 
filter bandwidth which reduces the SNR hopefully below tracking. It is not 
perfect - testing continues.

Peter 





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