Bert,
There's a very nice way to separate diurnal thermal sensitivity from long
term ageing. It applies equally well to Rb sources as to OCXOs, which is
where I've used it.
What you do is set up to make frequency observations against a more stable
reference (of course in this case a GPSDO or a similar), and then make
several observations per day for a week or so, preferably at or about the
same time of day each day. Then you plot the results using a spreadsheet, as
ppb offset on the vertical axis and time of day (NOT total elapsed time) on
the horizontal axis.
What will result after several days of observations is either a zig-zag or a
spiral, depending on the relative sizes of the thermal and ageing effects.
You can extract the ageing rate out quite nicely by adding a column with an
estimated ageing rate which you add to the measured offset. If you then plot
this against time of day you should end up with a static rough retraced
circle or line when the estimated offset is correct. The width and height of
the result gives you an estimate of the thermal sensitivity with time of
day.
If you are able also to plot against ambient temperature instead of time of
day, the same process applies, and you can extract the thermal sensitivity.
73,
Murray ZL1BPU
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