[time-nuts] GPS: ADEV or MDEV?

Dr Bruce Griffiths bruce.griffiths at xtra.co.nz
Sat Apr 21 18:30:04 EDT 2007


Hal Murray wrote:
>> 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. 
>>     
>
> What's the time constant on the PLL filter?  How many of the other errors 
> will it mask?
>
> I'd expect it to mask most of the sawtooth errors - all but the hanging 
> bridges that are wider than the PLL time constant.  That's what makes them so 
> interesting.
>
> Has anybody gone hunting for hanging bridges to see what the statistics on 
> their width look like?
>
>
>   
Hal

One has to be careful not to misuse such "statistics" to mask the 
shortcomings of a poorly designed GPSDO.
Hanging bridges, their width and their occurrence frequency are not 
likely to be random, they will tend to be correlated in a complex manner 
with temperature. Only the short term instabilities of the GPS receiver 
oscillator and its ageing corrupting the correlation. Consequently such 
"statistics" will be specific to a particular thermal environment and 
receiver. It would be misleading to apply them to other receivers and 
thermal environments.

Since the fix is a relatively trivial piece of software when the 
sawtooth correction data is available, why not do it?

Bruce



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