[time-nuts] Z3805A cooling requirements?

Volker Esper ailer2 at t-online.de
Sat Dec 22 17:44:57 UTC 2012

Thank you for your statement, I have to think about it.

It seems to be clear, that ADEV/MDEV aren't the convenient tools for 
characterizing such an event. My goal was to answere the question asked 
in the beginning of this thread: Does a constant ventilation of the OCXO 
affect the deviation curves and if so in what manner? More precisely: do 
I have worse deviation due to ventilation?

 > ... phase/frequency/drift plots and FT of them might be more
 > useful.

So let's have a look at the time domain. The picture shows the PPS-TI of 
the Z3805 (blue: time difference between OCXO and GPS; red: EFC).

In the middle of the diagram we recognize the reaction to the 
ventilation. Before that happens, the curve has a somewhat stable 
"high" frequency noise. After starting the ventilation this noise is 
extremely varying in amplitude. IMHO that might mean, that Allan 
deviation will be affected at the short term values of tau. This seems 
to coincide with the MDEV vs time curves of my diagrams.



Am 22.12.2012 14:22, schrieb Magnus Danielson:
> 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
> ADEV/MDEV plot.
> 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
> out.
> 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.
> Cheers,
> Magnus
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