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.
Volker
Am 22.12.2012 03:54, schrieb Volker Esper:
Here come the curves...
You'll find two diagrams. The first (named "1_DF9PL...") shows five MDEV
curves (Modified Allan Deviation), each of them measured at different
times. Total time span is 30.5 hours.
At small tau values (up to 1000 s) only a slight increase of sigma over
time can be noticed. However, at a tau of 5000 s or greater you can
watch sigma making a big bump. Ok, that's what we expected before.
In diagram no. 1 it's somewhat fussy to recognize the change of a
particular sigma(tau). Now, that we've got curious, we want to see, how
the sigma(tau) changes over time. So I've been providing a second
diagram ("2_..."), where sigma(tau) is a function of the time.
You can see, for example, the curve of tau=20480s developing a big hump,
and falling back to a proper value after about 1800 minutes. All curves
at a tau greater or equal 2560 do so.
At smaller values the curves are esentially less affected, but - they
are not back at their starting value after 1800 minutes (30 hours)! You
could guess, that the hump moves up to longer times with increasing
sigma - but it doesn't. There is something significantly different below
tau=2560s.
What is it?
Volker
Am 21.12.2012 14:42, schrieb Volker Esper:
(oh, I've got to split the posting)
The picture enclosed can give you a first impression. What we see is
the difference time between the GPS signal and the OCXO (blue)
("PPS-TI"), which is an HP 10811. In red we can see the EFC. The total
span is 24 h.
Before I applied the fan, the noise was at a maximum of about +/- 20 ns.
Some hours after starting the fan the noise is much greater. That should
have a significant impact on the ADEV.
I don't put the ADEV curves here, I make up for it when the EFC
compensation is completely out of the scope, that will be in about 12
hours. I don't have the ADEV at 1 s, but the ADEV at 10 s has been
almost constant. The ADEV at about 1000 s has a nasty bump now.
IMHO that fits to the physical facts: the airflow will surely not affect
the 10 s ADEV since the OCXO tries its best to isolate the oscillator
from short time temperature influences. However, the turbulent air flow
that I applied will influence the longer time ADEV.
Have a nice solstice
Volker
Am 21.12.2012 12:44, schrieb Volker Esper:
Yes, I made such a setup, it's now running 22 hours. I'll post the
results in two hours or so (if nothing evil happens to the earth,
meanwhile).
Volker
Am 21.12.2012 03:35, schrieb [email protected]:
Wish I had more time to play with this setup.
How about fellow time nuts spend some time and present similar test
data on
their OCXO's to compare?
I was interested in the 1s to 100s ADEV, and my runs were from 8
minutes to
20 minutes, certainly enough time to capture data for 1s to 100s ADEV
measurements..
bye,
Said
In a message dated 12/20/2012 14:17:59 Pacific Standard Time,
[email protected] writes:
On 12/20/2012 01:34 PM, Bob Camp wrote:
Hi
Temperature transients are not a good thing for an OCXO. If you
deliberately use the fan to create a transient, then yes the OCXO will
not be
happy. The question it - what happens after the transient has settled
out? The
plot you have still looks a lot like a step function.
I agree. Temperature steps stresses the OCXO oven loop and easily
creates a gradient over the crystal. As the oven loop tracks in, the
frequency returns to around normal. The trouble with forced air over a
crystal is that the metal shield couples very well and acts like a
heat
sink. A think plastic cover over it and forced convection doesn't have
the same effect. There is even being used by at least one vendor.
Works
very well for the extra cents of manufacturing cost.
The HP10811 is recommended to be put in a airflow-quiet corner of the
world. Look at it's mounting in the HP5370A/B for instance.
Cheers,
Magnus
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