[time-nuts] Super stable BVA Quartz resonators... BVA??
Bernd T-Online
BNeubig at t-online.de
Sun Dec 9 06:03:59 EST 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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