Philip 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 point. Bruce _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
