[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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