[time-nuts] Strange oscillations

[Alberto di Bene]

   Robert Aurand wrote:

Alberto, do you have the spec's for this OCXO?   What are the approximate dimen
sions of the case?    
And just because you don't see evidence of soldering on the pins doesn't mean t
he oscillator did not 
get tested for this condition.  It is common practice in industry to screen OCX
O's for abberant 
behavior by inserting them into test cards using pin sockets. 
  

   Robert,
     instead of attaching huge (well, not so huge...) documents to this
   message,
   I put both a photo and the specs of the OCXO I am using on my Web
   site.
   The photo is here :  [1]http://www.weaksignals.com/images/isotemp.jpg
   and the specs are here :
   [2]http://www.weaksignals.com/bin/isotemp134-10spec.pdf
   [...]

You won't need to measure current to a part in 10e-11 to see the effect.   Most
 likely the frequency
is following the current and the current change might be a few hundred microamp
s.   The current is
changing because the thermistor is sensing some room temperature change and, as
 you have calculated,
the temp change is very small.   IF this is a single oven the thermal gain migh
t be no more than
about 1000 (or even much less).   Minute air currents can easily cause this eff
ect. I eventually
developed my own personal technique for "sneaking up" on oscillators on my work
bench!!


   Yes, this is a rather cheap single oven design. I bought it on eBay as
   its price (25 USD)
   was much less than an 10811. But now I regret somewhat that my
   decision...
   as always, you get what you paid for...

So, do I understand correctly that you have this oscillator locked to something
 when you measure the
AFC??  If so, what?   In industry the usual technique is to ground the tuning v
oltage and measure
frequency against (usually) a Cesium.   This is done in a purpose built "aging
rack" with carefully
controlled temperature.


   The OCXO is locked to a VP Oncore GPS 1pps signal using an
   architecture similar in principle
   to the Shera's one, though different both in hardware and software.
   When tuning the PI
   algorithm in software, I noticed those oscillations which I cannot
   attribute to my PI controller
   for various reasons, the main one being that they do not change much
   when I change the
   PI time constants.
   This night or tomorrow I will measure the oven heather current..
   73  Alberto  I2PHD
   --------------------------

References

   1. http://www.weaksignals.com/images/isotemp.jpg
   2. http://www.weaksignals.com/bin/isotemp134-10spec.pdf