[time-nuts] Z3805A cooling requirements?

Magnus Danielson magnus at rubidium.dyndns.org
Sat Dec 22 13:22:21 UTC 2012

Hi Volker,

On 12/22/2012 04:55 AM, Volker Esper wrote:
> Although I am dog-tired it gives me no peace...
> I come to the following conclusion:
> - The long term Allan deviation gets worse, as long, as the effect of
> EFC compensating is in the range of tau
> - But: It gets back to its normal value after that
> - The short term deviation, however, increases slowly, but it doesn't
> settle. It's increasing more and more.
> So it is the short term stability, that is affected, rather than the
> long term stability.
> Am I right or wrong?
> Perhaps I'm to tired to decide.

You short term will rise with 1/tau in ADEV is due to white noise. This 
noise may be due to the oscillator or (many times) your measurement rig. 
This is expected from theory (see Allan Deviation on Wikipedia).

A free-running OCXO will have a D*tau/sqrt(2) rise due to linear drift. 
If it is beeing steered it would captured by the control loop and 
depending on it's properties there will be some marginal errors due to it.

A free-running OCXO will also experience temperature shifts and that 
will create a ripple effect in the ADEV/MDEV. If it is being steered the 
PLL loop would have issues fighting it which could leave traces in the 

For systematic effects like these, the ADEV/MDEV/TDEV plot isn't the 
ideal tool. phase/frequency/drift plots and FT of them might be more useful.

However, as you might have realized, a single event of temperature event 
like this averages out on the large scale of things. ADEV and friends 
isn't a good tool for transient properties, as it is a statistical tool 
to establish noise levels of various types. For that purpose single 
freak events of systematic effect needs to be averaged out or even taken 

Doing your plots to show how they vary over time illustrates this in a 
good way. The "event" is not part of the long-term noise properties.


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