[time-nuts] time-nuts Digest, Vol 43, Issue 35

Tom Van Baak tvb at LeapSecond.com
Fri Feb 15 17:47:06 EST 2008


>> ADEV is a good measure because you can make a stable standard
>> more accurate if you need to, but you cannot make an accurate
>> standard more stable.
>>
>> snip
> 
> Tom,
> 
> I am having a bit of a problem with your statement above.  While it is a nice
> catchy phrase and on the surface certainly seems proper, I think it deserves
> closer inspection.  The question really is the relationship between accuracy and
> stability.

Bill,

I agree. In the next level of close inspection one would mention
the notion that both accuracy and stability are functions of time.

Thus formal specifications of accuracy need to mention over
what duration the time/frequency accuracy is valid, and over
what interval the [frequency] accuracy is being measured, etc.

Similarly, with log-log ADEV plots everywhere, we are all familiar
with the notion that frequency stability is a function of averaging
interval. (Furthermore, as was pointed out already, the variation
of stability at different tau is a major power of the ADEV statistic).

> The basis of my problem is in considering the Quartz or Rubidium, it seems to me
> that stability defines the degree of accuracy that can be obtained.  Sure, you
> can adjust the accuracy of this singular standard, but how long is it going to
> stay there ?  If the accuracy of the adjustment doesn't stay put, then that
> adjustment means little.

Yes, to me that's why you can consider that stability is a sort of
prerequisite to accuracy, rather than the other way around.

> So, the question is how do you make a stand-a-lone standard more accurate then
> its level of stability ?

Doesn't sound possible to me. But I'm open to suggestions.
Can you compute pi to 10 digits on an 8 digit calculator?
Or, measure a length of wire to 30.12345 cm +/- 0.5 cm?
For some tau, can a 1e-9 stable OCXO be accurate to 1e-11?

> 
> Bill....WB6BNQ
> 

/tvb




More information about the time-nuts mailing list