[time-nuts] Super stable BVA Quartz resonators... BVA??

Bernd T-Online BNeubig at t-online.de
Sun Dec 9 11:03:59 UTC 2007


Poul-Henning Kamp wrote:
> You got me wondering:  How is the TOT determined ?  Is it per unit
> or is it per batch ?  How precisely does an OCXO hold the temperature
> on the TOT ?
For crystals used in precision OCXO the TOT is measured and recorded per 
unit.
There are two main methods: passive and active
In the passive method the crystal is inserted in a test fixture and its 
frequency and other parameters are measured using a network analyzer 
(state of the art). The crystals with its test fixture is in a precision 
temperature chamber, whose temperature is varied in fine steps in the 
vicinity of the expected TOT. The TOT is then calculated by curve 
fitting of the f(T) data by using a 2nd or 3rd order curve fitting 
algorithm, depending on the temperature span. Accuracy and repeatability 
are in the range of a (few) tenth of a degC.
In the active method the crystal is operating in its oven, whose 
temperature is varied over a certain interval. Evaluation of the TOT by 
similar curve fitting as above. Accuracy and reproducibility is about 
1/100 degC or slightly better.
The temperature accuracy and stability of an oven dpends strongly on its 
construction, and there are several orders of magnitude difference 
betewen a simple low-cost DIL14 size OCXO and a high-end OCXO with 
sub-ppb stability. THe electronic part of the temperature control is not 
the difficult task, the main issue is to have a UNIFORM temperature over 
all frequency determining components, which is accurate and stable - and 
uniformity needs physical size. Regarding stability over time, the most 
  critical components are the temperature sensors, mostly thermistors.

Regards

Bernd
DK1AG






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