[time-nuts] ADEV vs MDEV

Bruce Griffiths bruce.griffiths at xtra.co.nz
Sun Feb 7 00:09:16 UTC 2010


As long as one limits oneself to measuring 10811As or similar for short Tau.
However the tight PLL method is useless for comparing GPSDOs or rubidium 
standards for long Tau unless one replaces the VCXO with a sufficiently 
very low ADEV.

The DMTD system doesnt have this limitation as the reference source isnt 
phase locked.
The reference can be a GPSDO or an atomic standard without requiring any 
modifications.

Until you have actually demonstrated that results obtained using the 
tight PLL method agree with the results obtained with accepted methods 
then one has to remain suspicious of the range of validity of the technique.

Bruce

WarrenS wrote:
>
>
>> "An ADEV noise floor of 1E-13 isn't likely when using an HP10811A as 
>> the VCXO for example."
>
> How quickly one forgets and gets lost on these long topics.
>>> "If you accept that the measurement is going to limited by the  
>>> Reference Osc,
>>> for Low COST and SIMPLE,  Can't beat a simple analog version of  
>>> NIST's "Tight Phase-Lock Loop" "
>
> And which method are you saying is NOT limited by the Reference Osc??
> Correct, not going to get to 1e-13 at one sec with a HP10811A,
> nor likely with any other Ref Osc that most Freq nuts have.
> SO Seems like that is GOOD enough noise floor limit to use for a "low 
> cost & simple" configuration.
>
> BTW
> A well setup "Tight Phase-Lock Loop" method will go below that..
> and a good HP 10811A can go below 1e-12 at 0.1 sec. (at a bandwidth of 
> 30 Hz)
>
>
> ws
>
> ***************
> Bruce Griffiths said:
>
>> The noise of the OCXO used as a VCXO will limit the noise floor.
>> An ADEV noise floor of 1E-13 isnt likely when using an HP10811A as the
>> VCXO for example.
>
> Bruce
>
> *****  Original Topic *************
>>> I would appreciate any comments or observations on this topic.
>>> My motivation is to discover the simplest scheme for making
>>> stability measurements at this performance level; this is NOT
>>> even close to the state-of-the-art, but can still be useful.
>>>
>>> Pete Rawson
>
>
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