[time-nuts] Designing and building an OCXO and GPSDO

Bruce Griffiths bruce.griffiths at xtra.co.nz
Sun Aug 10 20:51:18 EDT 2008


When using a crystal in an oven you should use a crystal specified for 
oven operation at a specific temperature.
The crystal frequency should be specified for the desired oven temperature.
For example for an AT cut crystal the crystal frequency can be 
approximated by as a cubic function of temperature.
There are usually a couple of stationary points on the curve where the 
slope of frequency versus temperature is zero.
The crystal should be cut so that one of these points coincides with the 
oven temperature as this minimises the effect of small errors in the 
oven temperature set point on the frequency stability.
A crystal specified for non oven operation is usually cut to minimise 
the  frequency variation over the specified range of operating temperaures.
The frequency of such a crystal at one of the turning points may be 
several tens of ppm away from the nominal frequency.
The upper turning point may not even be suitable as it may be too high 
or even within the expected ambient temperature range.

It is also possible to cut the crystal so that the stationary points 
coincide at a point of inflection.
In this case the frequency change corresponding to small deviations from 
this temperature are much smaller than those at the turning points of a 
conventional oven crystal.
However this inflection point will lie close to room temperature for an 
AT cut crystal so that the "oven" will have to be cooled when the 
ambient temperature is above this point and heated when it is below this 


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